सूक्ष्मजीवशास्त्र Microbiology

थोडक्यात सारांश

Microbiology (सूक्ष्मजीवशास्त्र) is the study of living things too small to be seen clearly with the naked eye: bacteria, fungi, algae, protozoa and viruses. This topic covers the scientists who built the subject, the famous experiments that ended the idea of spontaneous generation and proved the germ theory, and the many fields where microbiology is used today.

महत्त्वाचे मुद्दे

• Antonie van Leeuwenhoek (Netherlands) made simple single-lens microscopes and was the first to describe bacteria and protozoa, which he called "animalcules" (1670s). He is called the Father of Microbiology (also of bacteriology and protozoology).
• Robert Hooke described "cells" in cork in his book Micrographia (1665).
• Spontaneous generation (abiogenesis) was the old belief that life arises from non-living matter. Francesco Redi (meat and flies), Lazzaro Spallanzani (boiled sealed broth) and finally Louis Pasteur (swan-neck flask experiment, 1860s) disproved it.
• John Tyndall showed that some microbes survive boiling (heat-resistant forms, later known as endospores) and developed tyndallization (intermittent boiling).
• Louis Pasteur (France) also explained fermentation as the work of microbes, developed pasteurization, and made vaccines against chicken cholera, anthrax and rabies (1885). He is called the Father of Modern Microbiology.
• Robert Koch (Germany) proved that a specific microbe causes a specific disease: anthrax (Bacillus anthracis, 1876), and discovered the causes of tuberculosis (Mycobacterium tuberculosis, 1882) and cholera (Vibrio cholerae, 1883). His lab introduced solid media, agar and pure culture methods.
• Edward Jenner (1796) used cowpox material to protect against smallpox — the start of vaccination.
• Joseph Lister used carbolic acid (phenol) in surgery — the start of antiseptic surgery.
• Alexander Fleming discovered penicillin from the mould Penicillium notatum (1928).
• Dmitri Ivanovsky (1892) and Martinus Beijerinck (1898) showed that tobacco mosaic disease is caused by a filterable agent — the start of virology.
• Sergei Winogradsky and Beijerinck founded soil and environmental microbiology (enrichment culture, chemolithotrophy, nitrogen fixation).

महत्त्वाच्या संज्ञा

• Germ theory of disease – infectious diseases are caused by microorganisms.
• Biogenesis – life comes only from pre-existing life.
• Pasteurization – mild heating of milk or juice to kill pathogens without boiling.
• Pure culture – a culture containing only one kind of microorganism.
• Antiseptic – chemical that kills or inhibits microbes on living tissue.
• Vaccine – preparation of a weakened, killed or part of a microbe that produces immunity.

महत्त्वाची माहिती

Koch's postulates (to prove that a microbe causes a disease):

1. The microbe must be present in every case of the disease and absent in healthy hosts.
2. It must be isolated from the diseased host and grown in pure culture.
3. The pure culture, when put into a healthy susceptible host, must cause the same disease.
4. The same microbe must be re-isolated from the experimentally infected host.

Scope of microbiology (branches): medical, food and dairy, agricultural/soil, industrial, environmental and water, pharmaceutical, genetic engineering and biotechnology, immunology, virology, mycology, phycology.

५ महत्त्वाचे प्रश्न

Q1. Who is called the Father of Microbiology and why?

Antonie van Leeuwenhoek. Using his own simple microscopes he was the first to observe and describe bacteria and protozoa ("animalcules") in the 1670s.

Q2. What is pasteurization?

Heating milk to a set temperature for a fixed time and then cooling it quickly. Example: 63 °C for 30 minutes (holding/LTH method) or 72 °C for 15 seconds (HTST method). It kills pathogens and increases shelf life.

Q3. Describe Pasteur's swan-neck flask experiment.

• Nutrient broth was boiled in flasks whose necks were bent into an S (swan) shape.
• Air could enter, but dust and microbes settled in the bend of the neck.
• The broth stayed clear and free of growth for a long time.
• When the neck was broken or the broth tilted into the bend, microbes grew quickly.
• Conclusion: microbes come from microbes in air and dust, not from the broth itself. This disproved spontaneous generation.

Q4. State Koch's postulates and their limitations.

• Write the four postulates (see Key facts).
• Limitations: some microbes cannot be grown in culture (e.g. Mycobacterium leprae, viruses on ordinary media); healthy carriers exist; some diseases are caused by more than one microbe; no animal model for some human pathogens.

Q5. Explain the scope of microbiology.

• Medicine: diagnosis, vaccines, antibiotics.
• Agriculture: biofertilizers, biopesticides, nitrogen fixation.
• Food and dairy: curd, cheese, bread, fermented foods; food safety.
• Industry: alcohol, organic acids, enzymes, vitamins, antibiotics.
• Environment: sewage treatment, bioremediation, biogas.
• Biotechnology: recombinant insulin, genetic engineering.

परीक्षेसाठी टीप

Learn scientist – discovery – year as a table. Koch's postulates and the swan-neck experiment are asked almost every year.

थोडक्यात सारांश

All living cells are of two basic types. Prokaryotic cells (bacteria and archaea) have no true nucleus and no membrane-bound organelles. Eukaryotic cells (fungi, algae, protozoa, plants and animals) have a true nucleus enclosed by a nuclear membrane and many organelles. This topic compares the two cell types and explains Woese's three-domain idea, which is asked in almost every first-year exam.

महत्त्वाचे मुद्दे

• The word prokaryote means "before nucleus"; eukaryote means "true nucleus". The terms were made popular by Édouard Chatton and later by Stanier and van Niel.
• Prokaryotes: bacteria (including cyanobacteria, actinomycetes, rickettsiae, chlamydiae, mycoplasmas) and archaea.
• Eukaryotic microbes: fungi (moulds and yeasts), algae and protozoa.
• Prokaryotic DNA lies free in the cytoplasm in a region called the nucleoid; it is usually one circular chromosome, without histones (archaea have histone-like proteins).
• Eukaryotic DNA is linear, packed with histones into chromosomes inside a nucleus; division is by mitosis and meiosis.
• Prokaryotes divide by binary fission; they have no mitosis.
• Prokaryotic ribosomes are 70S (50S + 30S); eukaryotic cytoplasmic ribosomes are 80S (60S + 40S). Mitochondria and chloroplasts have 70S ribosomes, which supports the endosymbiotic theory (Lynn Margulis).
• Most bacteria have a peptidoglycan cell wall. Fungal walls are mainly chitin, plant and many algal walls are cellulose; animal cells and most protozoa have no wall.
• Prokaryotic flagella are simple, made of the protein flagellin, and rotate. Eukaryotic flagella and cilia have the 9 + 2 microtubule arrangement and whip or beat.
• Sterols (cholesterol, ergosterol) are present in eukaryotic membranes; absent in most bacteria (exception: Mycoplasma takes sterols from the host).
• Carl Woese (1977, three domains formally named 1990) used 16S/18S rRNA sequences to divide life into Bacteria, Archaea and Eukarya.
• Prokaryotic cells are small (about 0.2–2 µm wide); eukaryotic cells are usually 10–100 µm.

महत्त्वाच्या संज्ञा

• Nucleoid – irregular region in a prokaryotic cell that holds the chromosome; no membrane around it.
• Organelle – membrane-bound structure with a special function (mitochondrion, chloroplast, Golgi body).
• Svedberg unit (S) – unit of sedimentation rate in an ultracentrifuge; shows size of ribosomes.
• Endosymbiotic theory – idea that mitochondria and chloroplasts came from bacteria that lived inside an early eukaryotic cell.
• Histones – basic proteins around which eukaryotic DNA is wound.

महत्त्वाची माहिती

५ महत्त्वाचे प्रश्न

Q1. What is a nucleoid?

It is the region in the cytoplasm of a prokaryotic cell where the circular DNA is found. It has no nuclear membrane or nucleolus.

Q2. Name the three domains of life. Who proposed them?

Bacteria, Archaea and Eukarya. They were proposed by Carl Woese on the basis of ribosomal RNA (16S/18S rRNA) sequence comparison.

Q3. Distinguish between prokaryotic and eukaryotic cells.

• Write the comparison table: nucleus, chromosome, organelles, ribosomes, wall, division, flagella, size.
• Give two examples of each.
• Add a labelled outline sketch of a bacterial cell and a eukaryotic cell if time allows.

Q4. Explain the three-domain system of classification.

• Older system: Whittaker's five kingdoms (1969), based on cell type, nutrition and body plan.
• Woese compared rRNA sequences and found archaea as different from bacteria as from eukaryotes.
• Bacteria: peptidoglycan walls, ester-linked membrane lipids.
• Archaea: no peptidoglycan, ether-linked lipids, many live in extreme habitats (methanogens, halophiles, thermophiles).
• Eukarya: true nucleus; includes protists, fungi, plants, animals.

Q5. Give evidence for the endosymbiotic theory.

• Mitochondria and chloroplasts have their own circular DNA.
• They have 70S ribosomes like bacteria.
• They divide by a fission-like process.
• They have a double membrane; the inner one resembles a bacterial membrane.
• Their rRNA sequences are close to those of bacteria (mitochondria to α-proteobacteria, chloroplasts to cyanobacteria).

परीक्षेसाठी टीप

A neat comparison table with at least 8 points earns full marks in the "distinguish" question. Always write 70S vs 80S and peptidoglycan vs chitin/cellulose.

थोडक्यात सारांश

Actinomycetes and cyanobacteria are two special groups of bacteria. Actinomycetes are Gram-positive bacteria that grow as branched threads like fungi and are famous as antibiotic producers. Cyanobacteria (blue-green algae) are Gram-negative bacteria that carry out plant-like oxygenic photosynthesis and many of them fix nitrogen. Both are prokaryotes, even though they look like fungi or algae.

महत्त्वाचे मुद्दे

• Actinomycetes are Gram-positive, mostly aerobic, filamentous bacteria with a high G + C content in their DNA.
• They form a branched network of thin threads (about 0.5–1 µm wide) called mycelium: substrate mycelium grows into the medium and aerial mycelium grows into the air.
• Many reproduce by chains of conidia (conidiospores) at the tips of aerial hyphae, or by sporangiospores in sporangia. These spores are not heat-resistant like endospores.
• They are common in soil; the earthy smell of wet soil is due to geosmin made by Streptomyces.
• Streptomyces produce a large share of natural antibiotics: streptomycin (S. griseus; Waksman), tetracyclines (S. aureofaciens), chloramphenicol (S. venezuelae), erythromycin (Saccharopolyspora erythraea, earlier S. erythreus).
• Other genera: Nocardia (partly acid-fast, can cause nocardiosis), Actinomyces (anaerobic, causes actinomycosis), Frankia (nitrogen-fixing nodules on Casuarina and Alnus), Micromonospora (gentamicin).
• Cyanobacteria are Gram-negative, photosynthetic prokaryotes. They have chlorophyll a and phycobiliproteins (phycocyanin – blue, phycoerythrin – red) in phycobilisomes on thylakoids. They release O₂ (oxygenic photosynthesis).
• Forms: unicellular (Synechococcus, Gloeocapsa), colonial (Microcystis), filamentous (Oscillatoria, Nostoc, Anabaena, Spirulina).
• Heterocysts are thick-walled cells in some filaments (Nostoc, Anabaena) where nitrogen fixation takes place; they lack photosystem II, so no O₂ is made inside to damage nitrogenase.
• Akinetes are thick-walled resting cells; hormogonia are short motile pieces of filament used for reproduction.
• Uses: biofertilizer for paddy (blue-green algae, Azolla–Anabaena), Spirulina as protein-rich food supplement. Harm: water blooms (Microcystis) produce toxins and lower oxygen.
• Cyanobacteria are believed to be the ancestors of chloroplasts.

महत्त्वाच्या संज्ञा

• Mycelium – mass of branched threads (hyphae).
• Geosmin – compound giving soil its earthy smell; made by Streptomyces.
• Heterocyst – special nitrogen-fixing cell of filamentous cyanobacteria.
• Phycobilisome – granule on thylakoid carrying light-harvesting pigments phycocyanin and phycoerythrin.
• Water bloom – heavy growth of cyanobacteria on the surface of nutrient-rich water.

महत्त्वाची माहिती

५ महत्त्वाचे प्रश्न

Q1. Why are actinomycetes placed among bacteria and not fungi?

They are prokaryotic: no nuclear membrane, 70S ribosomes, peptidoglycan wall, and their threads are about 1 µm thick (much thinner than fungal hyphae). They are also sensitive to antibacterial antibiotics.

Q2. What is a heterocyst?

A large, thick-walled, pale cell in filamentous cyanobacteria like Anabaena and Nostoc. It fixes atmospheric nitrogen and protects nitrogenase from oxygen.

Q3. Describe the general characters and economic importance of actinomycetes.

• Gram-positive, filamentous, branched, high G + C.
• Substrate and aerial mycelium; conidia on aerial hyphae.
• Soil decomposers of cellulose, chitin; produce geosmin.
• Antibiotics: streptomycin, tetracycline, chloramphenicol, erythromycin, gentamicin.
• Frankia fixes nitrogen; Nocardia and Actinomyces cause disease.

Q4. Give the characteristics of cyanobacteria.

• Gram-negative prokaryotes; no chloroplasts but thylakoids with chlorophyll a and phycobilins.
• Oxygenic photosynthesis; store cyanophycin and glycogen.
• Forms and examples; gas vacuoles in planktonic forms.
• Reproduction by fission, fragmentation, hormogonia, akinetes.
• Heterocysts for nitrogen fixation.

Q5. Write the importance of cyanobacteria in agriculture and industry.

• Blue-green algae biofertilizer adds nitrogen to paddy fields.
• Azolla–Anabaena symbiosis used as green manure.
• Spirulina as single cell protein, vitamins, food colour (phycocyanin).
• Improve soil structure and reclaim saline soils.
• Negative: toxic blooms in lakes and drinking-water reservoirs.

परीक्षेसाठी टीप

Remember the pair "actinomycetes = antibiotics, cyanobacteria = nitrogen fixation". Draw a Streptomyces mycelium with spore chains and an Anabaena filament with a heterocyst.

थोडक्यात सारांश

Microbes are too small for the eye, so microscopes are the basic tool of microbiology. This topic covers how a light microscope works, magnification and resolving power, and the main types of microscopes — bright field, dark field, phase contrast, fluorescence and electron microscopes — with their uses.

महत्त्वाचे मुद्दे

• The compound light microscope uses two lens systems: the objective (near the specimen) and the eyepiece/ocular (near the eye).
• Total magnification = magnification of objective × magnification of eyepiece. Example: 100× oil immersion objective with 10× eyepiece = 1000×.
• Resolving power (resolution) is the ability to show two close points as separate. It matters more than magnification.
• The limit of resolution of a good light microscope is about 0.2 µm. Electron microscopes resolve far smaller objects (well below 1 nm in practice for TEM).
• Oil immersion: cedarwood oil or immersion oil has a refractive index (about 1.5) close to glass. It stops light from bending away, increases the numerical aperture and improves resolution with the 100× objective.
• Bright field microscope: dark (stained) object on a bright background; most common, used for stained smears.
• Dark field microscope: a special condenser blocks direct light; only light scattered by the object enters the objective, so the object shines on a dark background. Used for thin, unstained living organisms like Treponema pallidum (syphilis).
• Phase contrast microscope (Frits Zernike, Nobel Prize 1953): converts small differences in refractive index into differences in brightness. Used to see living, unstained cells and internal structures.
• Fluorescence microscope: uses UV or blue light; fluorescent dyes (fluorochromes) or labelled antibodies glow. Used in diagnosis (e.g. auramine staining for TB bacilli, immunofluorescence).
• Electron microscope: uses a beam of electrons and electromagnetic lenses in a vacuum. TEM (transmission) shows internal structure of thin sections; SEM (scanning) shows 3-D surface view. Ernst Ruska and Max Knoll built the first one (1931).

महत्त्वाच्या संज्ञा

• Numerical aperture (NA) – measure of the light-gathering ability of a lens: NA = n sin θ.
• Refractive index – how much a medium bends light.
• Condenser – lens below the stage that focuses light on the specimen.
• Working distance – distance between objective and specimen when in focus; smallest for oil immersion.
• Parfocal – objectives remain nearly in focus when you change from one to another.

सूत्रे

• Total magnification = M(objective) × M(eyepiece)
• d = smallest distance between two points that can be seen as separate
• λ = wavelength of light used (green light about 550 nm)
• NA = numerical aperture of the objective
• Limit of resolution, d = 0.61 λ / NA (Abbe's formula, often written d = λ / 2NA)
• NA = n sin θ, where n = refractive index of medium between lens and specimen, θ = half the angle of the cone of light entering the objective.
• Shorter wavelength or higher NA → smaller d → better resolution.

५ महत्त्वाचे प्रश्न

Q1. Why is immersion oil used with the 100× objective?

Oil has nearly the same refractive index as glass. Light passing from slide to lens does not bend away, so more light enters the objective, NA increases and the image becomes sharp with better resolution.

Q2. Calculate total magnification with a 45× objective and 10× eyepiece.

Total magnification = 45 × 10 = 450×.

Q3. Calculate the limit of resolution when λ = 550 nm and NA = 1.25 (use d = 0.61 λ/NA).

• d = 0.61 × 550 / 1.25
• 0.61 × 550 = 335.5
• 335.5 / 1.25 = 268.4 nm ≈ 0.27 µm
• So two points closer than about 0.27 µm cannot be seen separately.

Q4. Describe the principle and uses of the dark field microscope.

• A dark field stop (opaque disc) in the condenser blocks central light.
• Only oblique light hits the specimen; scattered light reaches the objective.
• The object appears bright on a black background.
• Uses: living unstained bacteria, spirochaetes like Treponema, motility study.

Q5. Compare light microscope and electron microscope.

परीक्षेसाठी टीप

Always write the resolution formula with the meaning of each symbol. Draw a labelled compound microscope: eyepiece, body tube, revolving nosepiece, objectives, stage, condenser, iris diaphragm, mirror/light source, coarse and fine adjustment.

थोडक्यात सारांश

Bacteria are prokaryotic cells: they have no true nucleus and no membrane-bound organelles. This topic covers bacterial shapes and arrangements and each part of the cell — from the capsule and flagella outside to the cell wall, membrane, cytoplasm, nucleoid, plasmids and endospores inside.

महत्त्वाचे मुद्दे

• Shapes: coccus (spherical), bacillus (rod), vibrio (comma), spirillum (rigid spiral), spirochaete (flexible spiral).
• Arrangements of cocci: diplococci (pairs, e.g. Streptococcus pneumoniae), streptococci (chains), staphylococci (grape-like clusters), tetrads, sarcinae (cubes of 8).
• Size: most bacteria are about 0.5–5 µm long.
• Capsule / slime layer: outer layer, mostly polysaccharide. It protects from phagocytosis and drying and helps attachment. Capsulated strains are often more virulent.
• Flagella: long, thin protein (flagellin) threads for movement. Parts: filament, hook, basal body. Arrangements: monotrichous (one), lophotrichous (tuft at one end), amphitrichous (at both ends), peritrichous (all over), atrichous (none).
• Pili and fimbriae: short, hair-like protein structures. Fimbriae help attachment; the sex pilus (F pilus) is used in conjugation.
• Cell wall: rigid, gives shape, made of peptidoglycan (murein) — chains of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) cross-linked by short peptides.
• Gram-positive wall: thick peptidoglycan with teichoic acids; no outer membrane.
• Gram-negative wall: thin peptidoglycan, periplasmic space, and an outer membrane containing lipopolysaccharide (LPS). The lipid A part of LPS is the endotoxin.
• Cell membrane: phospholipid bilayer with proteins; controls transport; site of respiration (bacteria have no mitochondria). Mesosomes are infoldings described in older books.
• Cytoplasm: contains 70S ribosomes (50S + 30S), inclusion bodies (glycogen, volutin/polyphosphate granules, PHB, sulphur granules).
• Nucleoid: a single circular, double-stranded DNA chromosome without a nuclear membrane.
• Plasmids: small, circular, extra-chromosomal DNA that replicate on their own; may carry genes for antibiotic resistance (R plasmids) or fertility (F plasmid).
• Endospore: a tough, dormant, resistant body formed by Bacillus and Clostridium. It contains dipicolinic acid and calcium, and survives boiling, drying and chemicals.

महत्त्वाच्या संज्ञा

• Prokaryote – cell without a true nucleus.
• Peptidoglycan – the mesh-like polymer that makes the bacterial wall rigid.
• Periplasm – space between the inner membrane and outer membrane in Gram-negative bacteria.
• Spheroplast / protoplast – cell that has lost part / all of its wall (e.g. by lysozyme or penicillin).
• Sporulation – process of endospore formation; germination – endospore turning back into a vegetative cell.

महत्त्वाची माहिती

५ महत्त्वाचे प्रश्न

Q1. What is peptidoglycan?

A polymer of alternating NAG and NAM sugars linked into chains, with short peptides on NAM cross-linking the chains. It forms the rigid bacterial cell wall.

Q2. Name the types of flagellar arrangement with examples.

Monotrichous (Vibrio cholerae), lophotrichous (Pseudomonas fluorescens is often cited), amphitrichous (Spirillum), peritrichous (E. coli, Salmonella typhi).

Q3. Compare the cell walls of Gram-positive and Gram-negative bacteria.

• Thickness of peptidoglycan.
• Teichoic acids only in Gram-positive.
• Outer membrane with LPS, porins and lipoproteins only in Gram-negative.
• Periplasmic space in Gram-negative.
• Effect: Gram-positive more sensitive to penicillin and lysozyme; Gram-negative LPS acts as endotoxin.
• Draw both wall diagrams.

Q4. Describe the structure of a bacterial endospore.

• Layers from inside out: core (DNA, ribosomes, dipicolinic acid + Ca²⁺), inner membrane, cortex (special peptidoglycan), spore coat (protein), exosporium (outer thin layer).
• Formed when nutrients run out.
• Very low water, highly resistant to heat, radiation, chemicals.
• Position: central, subterminal, terminal (e.g. drumstick in Clostridium tetani).

Q5. Draw and label a typical bacterial cell.

• Labels: capsule, cell wall, cell membrane, cytoplasm, ribosomes, nucleoid, plasmid, inclusion bodies, pili, flagellum.
• Add one line on the function of each part.

परीक्षेसाठी टीप

A neat labelled diagram of the bacterial cell and of the two cell walls earns full marks. Learn the 70S ribosome and endospore layers.

थोडक्यात सारांश

The cell wall is the rigid layer outside the cell membrane that gives a bacterium its shape and protects it from bursting. Its main material is peptidoglycan (murein). Differences in wall structure divide bacteria into Gram-positive and Gram-negative groups, explain the Gram stain result, and decide which antibiotics work. Acid-fast walls and wall-less forms are also part of this topic.

महत्त्वाचे मुद्दे

• Functions: gives shape, prevents osmotic lysis (the inside of the cell has high osmotic pressure), acts as a sieve, anchors flagella, carries antigens, and is a target of antibiotics.
• Peptidoglycan = long glycan chains of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) joined by β-1,4 bonds, cross-linked by short peptides attached to NAM.
• The tetrapeptide often contains L-alanine, D-glutamic acid, a diamino acid (L-lysine in many Gram-positives, meso-diaminopimelic acid (DAP) in Gram-negatives) and D-alanine. D-amino acids are unusual and protect against ordinary proteases.
• In Staphylococcus aureus the cross-link is a pentaglycine bridge; in E. coli the link is direct (DAP to D-alanine).
• Gram-positive wall: thick (about 20–80 nm), many layers of peptidoglycan, with teichoic acids (wall teichoic acid and lipoteichoic acid). No outer membrane.
• Gram-negative wall: thin peptidoglycan (about 2–7 nm) in the periplasmic space, covered by an outer membrane made of phospholipid, lipopolysaccharide (LPS), porins and Braun's lipoprotein (which anchors the outer membrane to peptidoglycan).
• LPS has three parts: lipid A (the toxic endotoxin part), core polysaccharide, and O-antigen (O-specific side chain).
• Lysozyme (tears, saliva, egg white) breaks the β-1,4 bond between NAM and NAG. Penicillin blocks the transpeptidation (cross-linking) step.
• In hypertonic medium, lysozyme gives a protoplast from Gram-positive cells (wall fully removed) and a spheroplast from Gram-negative cells (outer membrane remains).
• Acid-fast wall (Mycobacterium): peptidoglycan linked to arabinogalactan and long mycolic acids, making it waxy; stained by Ziehl–Neelsen method.
• Mycoplasma have no wall at all; they are pleomorphic and resistant to penicillin. L-forms are wall-deficient variants of normal bacteria.
• Archaea have no peptidoglycan; some have pseudomurein (NAM replaced by N-acetyltalosaminuronic acid, β-1,3 bonds) and are lysozyme-resistant.

महत्त्वाच्या संज्ञा

• Peptidoglycan (murein) – mesh-like polymer of sugars and peptides in bacterial walls.
• Teichoic acid – polymer of glycerol or ribitol phosphate found in Gram-positive walls.
• Periplasmic space – gap between cell membrane and outer membrane in Gram-negatives; holds enzymes and binding proteins.
• Porins – protein channels in the outer membrane that allow small molecules to pass.
• Endotoxin – lipid A of LPS; causes fever and shock when released.

महत्त्वाची माहिती

५ महत्त्वाचे प्रश्न

Q1. What is lipid A?

It is the lipid part of lipopolysaccharide in the outer membrane of Gram-negative bacteria. It is the endotoxin; when released after cell death it causes fever, inflammation and septic shock.

Q2. Differentiate protoplast and spheroplast.

A protoplast is a Gram-positive cell whose wall is fully removed by lysozyme; a spheroplast is a Gram-negative cell that keeps part of its wall and outer membrane. Both are round and burst unless kept in hypertonic solution.

Q3. Describe the chemical structure of peptidoglycan.

• Backbone of NAG–NAM repeating units joined by β-1,4 bonds.
• Tetrapeptide on each NAM: L-Ala, D-Glu, L-Lys or DAP, D-Ala.
• Cross-linking directly (Gram-negative) or through pentaglycine bridges (S. aureus).
• Forms a bag-like sacculus around the cell.
• Lysozyme and penicillin act on this structure (draw a simple diagram).

Q4. Compare the cell walls of Gram-positive and Gram-negative bacteria with diagrams.

• Diagrams of both walls with labels.
• Comparison table (thickness, teichoic acid, outer membrane, LPS, porins, periplasm).
• Explain how these differences cause different Gram stain results.

Q5. Write a note on the acid-fast cell wall and on wall-less bacteria.

• Mycobacterium: mycolic acid, arabinogalactan, waxy, slow growth, resistant to drying and many disinfectants; ZN stain.
• Mycoplasma: no wall, sterols in membrane, smallest free-living cells, fried-egg colonies, not affected by penicillin.
• L-forms: arise when wall synthesis is blocked.

परीक्षेसाठी टीप

Always draw the two wall diagrams side by side and label LPS, porins, periplasm and teichoic acid. The NAG–NAM β-1,4 link is a favourite one-mark question.

थोडक्यात सारांश

An endospore is a tough, dormant body formed inside some Gram-positive bacteria when conditions become poor. It can survive boiling, drying, radiation and many chemicals for years, and can grow back into a normal cell (germination) when conditions improve. Endospores matter in sterilization, food canning and diseases like tetanus, botulism and anthrax.

महत्त्वाचे मुद्दे

• Endospores are made mainly by Bacillus (aerobic) and Clostridium (anaerobic); also Sporosarcina, Desulfotomaculum.
• One vegetative cell forms one endospore, and one spore gives one cell on germination, so sporulation is a survival method, not reproduction.
• Sporulation is triggered by nutrient starvation (lack of carbon or nitrogen); it takes about 6–8 hours in Bacillus subtilis.
• Position of the spore helps identification: central (Bacillus anthracis), subterminal (Clostridium botulinum), terminal (Clostridium tetani – "drumstick" or tennis-racket appearance).
• Structure from outside in: exosporium (thin outer layer, not in all species) → spore coat (protein layers) → cortex (special loosely cross-linked peptidoglycan) → core wall → core (DNA, ribosomes, enzymes, very little water).
• Very low water in the core (dehydration).
• Calcium dipicolinate (about 10% of spore dry weight) stabilizes DNA and proteins.
• Small acid-soluble spore proteins (SASPs) bind and protect DNA from heat, UV and chemicals.
• Thick, impermeable spore coat.
• Almost no metabolic activity.
• Reasons for resistance:
• Stages of sporulation (seven stages): axial filament of DNA → asymmetric septum forms the forespore → engulfment by mother cell → cortex formation → coat formation → maturation (resistance develops) → lysis of mother cell releases the spore.
• Germination has three steps: activation (e.g. mild heat), germination proper (spore swells, loses calcium dipicolinate and resistance, takes up water) and outgrowth (new vegetative cell emerges).
• Endospores are killed by autoclaving (121 °C, 15 psi, 15–20 min) or by tyndallization; boiling alone does not kill them.
• Geobacillus stearothermophilus spores are used as biological indicators to check autoclave efficiency.
• They are stained by the Schaeffer–Fulton method (malachite green with steaming; spores green, cells pink).

महत्त्वाच्या संज्ञा

• Sporulation (sporogenesis) – process of endospore formation.
• Forespore – the smaller compartment that becomes the spore after the asymmetric division.
• Cortex – thick peptidoglycan layer that helps keep the core dehydrated.
• Dipicolinic acid – chemical found only in endospores; with calcium it forms calcium dipicolinate.
• Germination – return of a spore to an active vegetative cell.

महत्त्वाची माहिती

५ महत्त्वाचे प्रश्न

Q1. Why is sporulation not a method of reproduction?

Because one bacterial cell forms only one endospore, and one endospore germinates into only one cell. The number of cells does not increase; the spore only helps the cell survive.

Q2. Name two spore-forming bacteria and the disease each causes.

Clostridium tetani – tetanus; Bacillus anthracis – anthrax. (Also C. botulinum – botulism, C. perfringens – gas gangrene.)

Q3. Describe the structure of a bacterial endospore with a labelled diagram.

• Diagram showing exosporium, coat, cortex, core wall and core.
• Role of each layer in resistance.
• Core contents: DNA with SASPs, ribosomes, calcium dipicolinate.

Q4. Explain the stages of sporulation.

• Starvation signal; DNA forms axial filament.
• Asymmetric septum; forespore and mother cell.
• Engulfment; forespore surrounded by two membranes.
• Cortex laid between the membranes; coat proteins added.
• Maturation and release by lysis of mother cell.
• Add a simple flow diagram.

Q5. Why are endospores highly resistant? What is their practical importance?

• Low water, calcium dipicolinate, SASPs, impermeable coat, dormancy.
• Importance: sterilization standards (autoclave, biological indicators), food canning (C. botulinum is the target), hospital infections, soil survival and dust-borne spread, possible bioterrorism (anthrax).

परीक्षेसाठी टीप

Learn the three spore positions with one example each and the five reasons for resistance. A neat labelled diagram of the spore layers is expected in every long answer.

थोडक्यात सारांश

Most bacteria are colourless and transparent, so we stain them to see their shape, size and arrangement. This topic covers dyes, smear preparation, simple staining, negative staining, and differential stains like the Gram stain and acid-fast (Ziehl–Neelsen) stain, plus special stains for capsules, spores and flagella.

महत्त्वाचे मुद्दे

• Basic dyes have a positively charged coloured ion (e.g. methylene blue, crystal violet, safranin, basic fuchsin). Bacterial surfaces are negatively charged, so basic dyes stain them well.
• Acidic dyes have a negatively charged coloured ion (e.g. nigrosin, eosin, India ink). They are repelled by the cell and colour the background — used in negative staining.
• Smear preparation: a thin film of culture on a clean slide → air dry → heat fix (pass through flame 2–3 times). Heat fixing kills cells and sticks them to the slide.
• Simple staining: one dye only (e.g. methylene blue 1 min). Shows shape and arrangement.
• Negative staining: background is coloured, cells stay clear. No heat fixing, so shape is not distorted. Used to see capsules.
• Gram stain (Hans Christian Gram, 1884) divides bacteria into Gram-positive (violet) and Gram-negative (pink/red).
• Acid-fast stain (Ziehl–Neelsen): for Mycobacterium whose walls are rich in mycolic acid (waxy). Acid-fast bacteria stay red; others take the blue counterstain.
• Endospore stain (Schaeffer–Fulton): malachite green with heat stains spores green; safranin stains vegetative cells red.
• Capsule stain: negative stain with India ink or nigrosin — clear halo around cells (e.g. Anthony's method uses crystal violet and copper sulphate).
• Flagella stain: flagella are too thin for light microscopy; a mordant coats them and makes them thick enough to see (e.g. Leifson's method).
• Metachromatic granules (volutin) are seen with Albert's stain in Corynebacterium diphtheriae.

महत्त्वाच्या संज्ञा

• Mordant – substance that fixes the dye more firmly (e.g. Gram's iodine).
• Decolourizer – removes dye from some cells (alcohol or acetone; acid-alcohol in ZN stain).
• Counterstain – second dye of contrasting colour (safranin in Gram stain; methylene blue in ZN stain).
• Differential stain – a stain that separates bacteria into groups (Gram, acid-fast).
• Chromophore – the coloured part of a dye molecule.

महत्त्वाची माहिती

Gram staining steps:

Ziehl–Neelsen steps: carbol fuchsin with heating (steaming) → decolourize with acid-alcohol (or 20% H₂SO₄) → counterstain with methylene blue. Acid-fast = red; non-acid-fast = blue.

५ महत्त्वाचे प्रश्न

Q1. Why is heat fixing done before staining?

It kills the bacteria, makes them stick to the slide so they are not washed away, and helps the dye penetrate. It is not done for negative staining.

Q2. What is negative staining? Give its advantage.

An acidic dye like nigrosin or India ink colours the background, leaving cells clear. Since no heat is used, the true size and shape are seen; capsules appear as clear zones.

Q3. Explain the principle and procedure of Gram staining.

• Steps: crystal violet → iodine → alcohol → safranin (see table).
• Crystal violet and iodine form a large CV–I complex inside the cell.
• Gram-positive: thick peptidoglycan dehydrates and shrinks with alcohol, pores close and the complex is trapped → violet.
• Gram-negative: alcohol dissolves the outer membrane lipid, the thin peptidoglycan cannot hold the complex → colourless, then pink with safranin.
• Importance: identification, choice of antibiotic.

Q4. Describe the Ziehl–Neelsen stain.

• Used for Mycobacterium tuberculosis and M. leprae.
• Mycolic acid makes the wall waxy and hard to stain; heat helps carbol fuchsin enter.
• Once stained, the cells resist decolourization by acid-alcohol (acid-fast).
• Methylene blue counterstains other cells and background.
• Result: red rods on blue background. Used in sputum examination for TB.

Q5. How are endospores stained?

• Schaeffer–Fulton method: smear flooded with malachite green, steamed for about 5 min.
• Wash with water (decolourizes vegetative cells).
• Counterstain with safranin.
• Spores green, vegetative cells pink/red. Example: Bacillus, Clostridium.

परीक्षेसाठी टीप

Learn the four Gram reagents in order with their roles (primary stain, mordant, decolourizer, counterstain). Write the result table in every staining answer.

थोडक्यात सारांश

Sterilization (निर्जंतुकीकरण) means killing or removing all microbes, including spores. Disinfection kills most harmful microbes but not always spores. This topic covers physical methods (heat, filtration, radiation) and chemical methods (disinfectants and antiseptics), how they act, and how they are checked.

महत्त्वाचे मुद्दे

• Moist heat kills by coagulating (denaturing) proteins; it is more effective than dry heat at the same temperature.
• Autoclave: steam under pressure. Standard cycle: 121 °C at 15 lb/in² (psi) for 15–20 minutes. Kills spores. Used for media, glassware, surgical cloth.
• Boiling (100 °C) kills vegetative cells but not all spores.
• Tyndallization: steaming at 100 °C for about 30 min on three successive days. Spores germinate between heatings and are then killed. Used for media that cannot stand autoclaving.
• Pasteurization: 63 °C for 30 min (holding method) or 72 °C for 15 s (HTST). It is not sterilization.
• Dry heat kills by oxidation. Hot air oven: 160 °C for 2 hours (or 170 °C for 1 hour; 180 °C for 30 min). Used for glassware, metal instruments, oils and powders.
• Incineration burns waste; flaming/red heat sterilizes inoculating loops.
• Filtration removes microbes from heat-sensitive liquids (serum, antibiotics, vitamins). Membrane filters of 0.22 µm pore size hold back bacteria. HEPA filters clean air in laminar air flow hoods.
• Radiation: UV light (about 260 nm is most germicidal) damages DNA by forming thymine dimers; poor penetration, used for surfaces and air. Ionizing radiation (gamma rays from cobalt-60) penetrates deeply; used for disposable syringes and plasticware.
• Chemical agents: alcohols (70% ethanol), phenol and its derivatives, halogens (chlorine, iodine), heavy metals (mercury, silver), aldehydes (formaldehyde, glutaraldehyde), gases (ethylene oxide), quaternary ammonium compounds (detergents), hydrogen peroxide.
• 70% alcohol works better than absolute alcohol because water is needed to denature proteins.

महत्त्वाच्या संज्ञा

• Sterilization – removal or killing of all forms of life, including spores.
• Disinfection – killing pathogens on non-living objects.
• Antiseptic – chemical safe on living tissue (e.g. iodine tincture, Dettol).
• Sanitization – lowering microbes to safe public-health levels.
• -cidal / -static – kills (bactericidal) / stops growth (bacteriostatic).
• Thermal death time (TDT) – shortest time to kill all cells at a given temperature.
• D-value (decimal reduction time) – time at a fixed temperature to kill 90% of the population.

सूत्रे

• Phenol coefficient = (highest dilution of test disinfectant that kills the test organism in 10 min but not 5 min) ÷ (the same dilution of phenol). A value above 1 means the disinfectant is stronger than phenol.
• D-value: after each D minutes, the number of survivors falls to one tenth (90% killed).

५ महत्त्वाचे प्रश्न

Q1. Why is moist heat better than dry heat?

Water helps proteins coagulate at lower temperatures, and steam carries more heat (latent heat) and penetrates better. So moist heat sterilizes at 121 °C in 15 min, while dry heat needs 160 °C for 2 hours.

Q2. What is tyndallization?

Intermittent sterilization: steaming at 100 °C for about 30 minutes on three successive days, with incubation in between so that spores germinate and are killed on the next heating.

Q3. Describe the principle, working and uses of the autoclave.

• Principle: water boils at a higher temperature under higher pressure; steam at 15 psi reaches 121 °C.
• Working: water heated in a closed chamber, air removed through the vent, steam pressure built to 15 psi, held for 15–20 min, then pressure released slowly.
• Kills all microbes including spores by denaturing proteins.
• Uses: culture media, glassware, surgical items, lab waste.
• Checked by biological indicator spores of Geobacillus stearothermophilus and chemical indicator tape.

Q4. Explain sterilization by radiation.

• Non-ionizing (UV): forms thymine dimers in DNA; used in lab hoods, operation theatres, water; low penetration.
• Ionizing (gamma, X-rays, electron beams): form free radicals that break DNA; high penetration.
• Uses: cold sterilization of syringes, catheters, gloves, some foods and spices.

Q5. Write about chemical disinfectants and their mode of action.

• Alcohols: denature proteins, dissolve lipids.
• Phenols: damage membranes, denature proteins.
• Halogens: oxidize cell components (chlorine for water, iodine for skin).
• Heavy metals: bind –SH groups of proteins.
• Aldehydes and ethylene oxide: alkylate proteins and nucleic acids; ethylene oxide gas sterilizes heat-sensitive items.
• Detergents (QACs): disrupt cell membranes.

परीक्षेसाठी टीप

Remember the three standard numbers: autoclave 121 °C/15 psi/15 min, hot air oven 160 °C/2 h, membrane filter 0.22 µm.

थोडक्यात सारांश

A microbiology laboratory handles living organisms, some of which can cause disease. Biosafety means the rules, equipment and lab design that protect the worker, other people and the environment from these organisms. This topic covers good laboratory practices, the four biosafety levels (BSL-1 to BSL-4), biosafety cabinets and the safe disposal of biomedical waste.

महत्त्वाचे मुद्दे

• Risk groups: microorganisms are grouped (WHO) from Risk Group 1 (unlikely to cause disease, e.g. Bacillus subtilis) to Risk Group 4 (cause severe, easily spread disease with no cure, e.g. Ebola virus).
• Routes of lab infection: inhalation of aerosols, ingestion (mouth pipetting, eating in lab), accidental injection (needles, broken glass), and contact with eyes or broken skin.
• Wear a lab coat, closed shoes and gloves; tie long hair.
• Never mouth-pipette; use a pipette bulb or pipette aid.
• No eating, drinking, applying cosmetics or storing food in the lab.
• Disinfect the work bench before and after work (e.g. 70% ethanol, 1% sodium hypochlorite).
• Flame loops to red-hot; cool before touching cultures.
• Label all cultures; report every spill and injury.
• Wash hands with soap before leaving.
• Good laboratory practices (GLP / basic rules):
• Biosafety levels describe increasing containment (see table).
• Biosafety cabinets (BSC) use HEPA filters (High Efficiency Particulate Air; remove at least 99.97% of particles of 0.3 µm). Class I protects worker and environment, Class II (most common) protects worker, environment and the sample, Class III is a fully sealed glove box for BSL-4 work.
• A laminar air flow (LAF) bench protects only the sample (sterile work, media pouring), not the worker; it is not a biosafety cabinet.
• Spill handling: cover with paper towels, pour disinfectant from the edges inward, wait about 20–30 minutes, then clean and autoclave the waste.
• Biomedical waste in India is handled under the Bio-Medical Waste Management Rules, 2016 with colour-coded bags: yellow (infectious anatomical/soiled waste, incineration), red (contaminated recyclable plastics like tubing and gloves, autoclave then recycle), white translucent puncture-proof container (sharps), blue (glassware). Check your university syllabus for the exact list.
• Used cultures and media are autoclaved before disposal.

महत्त्वाच्या संज्ञा

• Biosafety – protecting people and the environment from biological agents in the lab.
• Biosecurity – preventing theft or misuse of dangerous microorganisms.
• Aerosol – tiny liquid droplets in air that can carry microbes.
• HEPA filter – high-efficiency filter that traps microbes from air.
• Containment – safe methods and equipment used to keep infectious agents within the lab.

महत्त्वाची माहिती

५ महत्त्वाचे प्रश्न

Q1. Why is mouth pipetting prohibited in a microbiology lab?

Because the culture or chemical may be sucked into the mouth by accident and cause infection or poisoning. Pipette bulbs or automatic pipettes are used instead.

Q2. What is a HEPA filter?

A High Efficiency Particulate Air filter that removes at least 99.97% of particles 0.3 µm in size from air. It is used in biosafety cabinets and laminar air flow benches.

Q3. Describe the four biosafety levels with examples.

• Write the table: agents, practices, equipment and facility for each level.
• Mention that each level includes all practices of the level below.
• Give one Indian example of a high-containment lab, e.g. the BSL-4 facility at ICMR–National Institute of Virology, Pune.

Q4. List the general safety rules in a microbiology laboratory.

• Personal protection, no eating/drinking, no mouth pipetting.
• Bench disinfection, proper flaming, labelling.
• Handling spills and injuries; reporting.
• Autoclaving of waste and hand washing.

Q5. Differentiate between a biosafety cabinet and a laminar air flow bench.

• LAF: HEPA-filtered air blows towards the worker; protects only the sample.
• BSC Class II: inward air curtain at front and HEPA-filtered downflow; protects worker, sample and environment.
• LAF must not be used for pathogens.

परीक्षेसाठी टीप

Memorise one example organism for each BSL and the colour code of biomedical waste bags. Short notes on "HEPA filter" and "biosafety cabinet" are common.