Microbiology सूक्ष्मजीवशास्त्र

Quick summary

Every microbial cell is built from a few kinds of large molecules: carbohydrates, proteins, lipids and nucleic acids. Water and minerals make up the rest. Knowing the basic chemistry of these molecules — their units, bonds and roles — helps in understanding cell structure, metabolism and genetics later. This is the "chemical basis of microbiology" unit.

Key points

• A bacterial cell is about 70% water. Of its dry weight, protein is the largest part (roughly half), followed by nucleic acids (RNA more than DNA), lipids and polysaccharides.
• Main elements: C, H, O, N, P, S (CHONPS), plus K, Mg, Ca, Fe and trace elements.
• Chemical bonds: covalent bonds hold atoms within molecules; weak bonds — hydrogen bonds, ionic bonds, hydrophobic and van der Waals interactions — shape proteins and hold the DNA double helix together.
• Water is a polar solvent, has a high specific heat and takes part in hydrolysis and condensation reactions.
• Monosaccharides: glucose, fructose (hexoses); ribose, deoxyribose (pentoses).
• Disaccharides: sucrose (glucose + fructose), lactose (glucose + galactose), maltose (glucose + glucose), joined by glycosidic bonds.
• Polysaccharides: starch, glycogen (storage), cellulose (β-1,4 glucose), chitin (fungal walls), peptidoglycan (bacterial walls), dextran (capsules).
• Carbohydrates (CH₂O)n:
• Proteins: polymers of 20 standard amino acids joined by peptide bonds. Structure levels: primary (sequence), secondary (α-helix, β-sheet), tertiary (3-D folding), quaternary (several chains). Roles: enzymes, transport, flagellin, pili, toxins.
• Amino acids (except glycine) are optically active; proteins use L-amino acids, but bacterial walls also have D-alanine and D-glutamic acid.
• Lipids: fats (glycerol + 3 fatty acids, ester bonds), phospholipids (amphipathic; form the membrane bilayer), sterols (in eukaryotes), waxes such as mycolic acid in Mycobacterium, and poly-β-hydroxybutyrate (PHB) as a storage material in bacteria.
• Nucleic acids: polymers of nucleotides (phosphate + pentose sugar + nitrogen base) joined by phosphodiester bonds. DNA has A, G, C, T and deoxyribose; RNA has A, G, C, U and ribose.
• ATP (adenosine triphosphate) is a nucleotide that carries energy in its high-energy phosphate bonds.
• Denaturation by heat, pH or chemicals destroys protein shape and function — the basis of sterilization by heat and of many disinfectants.

Important terms

• Monomer / polymer – small unit / large chain made of many units.
• Glycosidic bond – link between two sugar units.
• Peptide bond – link between the –COOH of one amino acid and –NH₂ of the next.
• Phosphodiester bond – link between nucleotides in DNA and RNA.
• Amphipathic – having both water-loving and water-fearing parts (e.g. phospholipid).

Key facts

5 important questions

Q1. What is a peptide bond?

A covalent C–N bond formed between the carboxyl group of one amino acid and the amino group of the next, with the loss of one water molecule (condensation).

Q2. Name two storage polymers found in bacteria.

Glycogen (polysaccharide) and poly-β-hydroxybutyrate (lipid-like). Volutin (polyphosphate) is a third.

Q3. Describe the classification and microbial importance of carbohydrates.

• Mono-, di-, oligo- and polysaccharides with examples.
• Energy source (glucose), structural (peptidoglycan, chitin, cellulose), capsule (dextran), storage (glycogen).
• Sugars in culture media and in biochemical tests (fermentation of glucose, lactose).

Q4. Explain the levels of protein structure.

• Primary to quaternary, with the bonds that stabilize each (peptide, hydrogen, disulfide, ionic, hydrophobic).
• Example: haemoglobin for quaternary; enzymes for tertiary.
• Denaturation and its use in sterilization.

Q5. Give the structure and functions of lipids in microorganisms.

• Fats, phospholipids, sterols, waxes.
• Phospholipid bilayer of the cell membrane.
• Lipid A of LPS (endotoxin), mycolic acid of acid-fast walls.
• PHB as reserve food; archaeal ether-linked lipids.

Exam tip

Make a four-row table of unit, bond and example for each biomolecule; it answers many short questions. Remember: DNA has thymine and deoxyribose, RNA has uracil and ribose.

Quick summary

A culture medium is a nutrient preparation used to grow microbes in the lab. This topic covers the ingredients of media, their classification (by consistency, composition and use), and the methods used to get a pure culture — streak plate, pour plate and spread plate — and to preserve it.

Key points

• Basic needs of microbes: water, carbon source, nitrogen source, minerals, growth factors (vitamins), and right pH, temperature and oxygen.
• Common ingredients: peptone (nitrogen), beef extract and yeast extract (vitamins, minerals), NaCl (osmotic balance), sugars (carbon and energy), agar (solidifying agent), water.
• Agar comes from red algae (seaweeds like Gelidium). It melts at about 100 °C and sets at about 40–45 °C; most bacteria cannot digest it. Fanny Hesse suggested its use in Koch's lab. Usual concentration about 1.5–2%.
• By consistency: liquid (broth), semi-solid (about 0.5% agar, for motility), solid (about 1.5–2% agar).
• By composition: synthetic/defined (exact chemical composition known) and complex/non-synthetic (contains extracts, e.g. nutrient broth).
• Selective media allow some microbes and inhibit others (e.g. MacConkey agar with bile salts and crystal violet selects Gram-negative enteric bacteria).
• Differential media show differences between colonies (MacConkey: lactose fermenters pink, non-fermenters colourless).
• Enriched media have extra nutrients like blood or serum for fastidious organisms (blood agar, chocolate agar).
• Enrichment media (liquid) favour growth of one organism over others (e.g. selenite F broth for Salmonella).
• Other types: transport media, anaerobic media (e.g. Robertson's cooked meat medium, thioglycollate broth), assay media.
• Pure culture contains only one kind of microbe. Methods: streak plate, pour plate, spread plate, serial dilution, and micromanipulation (single cell).
• Preservation: periodic subculturing, refrigeration (4 °C), covering with mineral oil, freezing with glycerol, lyophilization (freeze-drying — best long-term).

Important terms

• Colony – visible mass of cells grown from one cell (or a few) on solid medium.
• Inoculation – putting microbes into a medium.
• Aseptic technique – working so that no unwanted microbes enter.
• Fastidious organism – one with complex nutrient needs.
• CFU (colony forming unit) – a viable cell or group that forms one colony.

Formulas

• Viable count (CFU/mL) = number of colonies ÷ (volume plated in mL × dilution).
• Example: 150 colonies from 0.1 mL of a 10⁻⁵ dilution → 150 ÷ (0.1 × 10⁻⁵) = 1.5 × 10⁸ CFU/mL.
• Countable plates usually have 30–300 colonies.

5 important questions

Q1. Why is agar preferred as a solidifying agent?

It is not digested by most bacteria, it stays solid at incubation temperatures (melts near 100 °C), it sets only at about 40–45 °C so heat-sensitive nutrients can be added, and it is transparent.

Q2. Differentiate selective and differential media.

Selective medium inhibits unwanted organisms and allows the desired one to grow (e.g. bile salts in MacConkey). Differential medium allows several organisms to grow but shows them in different ways (e.g. pink vs colourless colonies on MacConkey).

Q3. Describe the streak plate method.

• Sterilize the loop by flaming and cool it.
• Take a loopful of mixed culture and streak on one area of the agar plate.
• Flame the loop, rotate the plate, streak again from the end of the first lines; repeat for 3–4 sectors (quadrant streak).
• The cells are thinned out; in the last sector, single cells grow into separate colonies.
• Incubate, pick an isolated colony and subculture to get a pure culture.

Q4. Explain the pour plate and spread plate methods.

• Pour plate: sample serially diluted; 1 mL mixed with molten cooled agar (45 °C) in a Petri plate; colonies grow inside and on the surface. Used for counting.
• Spread plate: 0.1 mL diluted sample spread on the surface of solid agar with a sterile L-shaped glass spreader; only surface colonies.
• Both give isolated colonies and viable counts.
• Worked example: 150 colonies from 0.1 mL of 10⁻⁵ dilution = 1.5 × 10⁸ CFU/mL.

Q5. Classify culture media with examples.

• Consistency: liquid, semi-solid, solid.
• Composition: synthetic, complex.
• Purpose: basal/general (nutrient agar), enriched (blood agar), selective (MacConkey), differential (MacConkey, blood agar for haemolysis), enrichment (selenite F), transport, anaerobic (Robertson's cooked meat).

Exam tip

Draw the quadrant streak pattern. Always give one example medium for each type.

Quick summary

In microbiology, "growth" means an increase in the number of cells, not the size of one cell. Bacteria divide by binary fission, so their number doubles every generation. This topic covers the growth curve, generation time, ways to measure growth, and how temperature, pH, oxygen and other factors affect growth.

Key points

• Binary fission: the cell copies its DNA, grows longer, forms a septum (cross wall) in the middle and splits into two equal daughter cells.
• Exponential growth: 1 → 2 → 4 → 8 … ; after n generations, N = N₀ × 2ⁿ.
• Generation time (doubling time): time for the population to double. E. coli in rich medium at 37 °C: about 20 minutes. Mycobacterium tuberculosis: about 18–24 hours (much slower).
• Lag phase – no increase in number; cells adapt, make enzymes, grow in size.
• Log (exponential) phase – steady, fastest division; cells most uniform and most sensitive to antibiotics.
• Stationary phase – nutrients run out and wastes collect; new cells = dying cells; secondary metabolites (e.g. antibiotics) and endospores often formed.
• Death (decline) phase – cells die exponentially.
• Growth curve (batch culture) has four phases:
• Continuous culture: fresh medium added and used medium removed at the same rate, keeping cells in log phase. Devices: chemostat (growth controlled by a limiting nutrient) and turbidostat (controlled by turbidity).
• Synchronous culture: all cells divide at the same time (by filtration or temperature shift methods).
• Measuring growth: direct microscopic count (Petroff–Hausser counting chamber), viable plate count (CFU), turbidity (spectrophotometer, optical density), dry weight, and chemical measures (protein, DNA).
• Temperature groups: psychrophiles (optimum below about 15 °C), mesophiles (about 20–45 °C; most pathogens, optimum 37 °C), thermophiles (about 45–80 °C), hyperthermophiles (above 80 °C).
• Oxygen groups: obligate aerobes, obligate anaerobes, facultative anaerobes (e.g. E. coli), microaerophiles, aerotolerant anaerobes.
• pH groups: acidophiles, neutrophiles (most bacteria), alkaliphiles.
• Salt: halophiles need high salt (e.g. Halobacterium); halotolerant bacteria like Staphylococcus aureus survive it.

Important terms

• Generation time (g) – time per doubling.
• Growth rate (k) – number of generations per hour.
• Batch culture – closed system with fixed nutrients.
• Turbidity – cloudiness of a broth due to cells.
• Cardinal temperatures – minimum, optimum and maximum growth temperatures.

Formulas

• N = N₀ × 2ⁿ (N = final number, N₀ = initial number, n = number of generations)
• n = (log N − log N₀) / log 2 = (log N − log N₀) / 0.301
• Generation time g = t / n (t = time of growth)
• Growth rate k = n / t = 1 / g (generations per unit time)

5 important questions

Q1. Define generation time.

The time needed for a bacterial population (or one cell) to double in number. It is calculated as g = t / n.

Q2. A culture starts with 100 cells and grows for 3 generations. How many cells are there?

N = N₀ × 2ⁿ = 100 × 2³ = 100 × 8 = 800 cells.

Q3. A culture grows from 10³ cells to 10⁹ cells in 10 hours. Find n and g.

• log N − log N₀ = 9 − 3 = 6
• n = 6 / 0.301 = 19.93 ≈ 20 generations
• g = t / n = 10 h / 20 = 0.5 h = 30 minutes
• So the generation time is about 30 minutes.

Q4. Describe the bacterial growth curve with a diagram.

• Graph: log of cell number (y-axis) against time (x-axis).
• Lag phase: flat line; adaptation, enzyme synthesis.
• Log phase: straight rising line; constant doubling.
• Stationary phase: flat top; nutrients exhausted, toxic waste, growth = death.
• Death phase: falling line; exponential loss of viable cells.
• Importance: industrial harvesting, antibiotic action, vaccine production.

Q5. Explain the effect of temperature and oxygen on bacterial growth.

• Temperature controls enzyme activity; too high denatures proteins, too low makes membranes rigid.
• Groups: psychrophiles, mesophiles, thermophiles, hyperthermophiles, with one example each (e.g. Thermus aquaticus is a thermophile).
• Oxygen: aerobes need O₂; obligate anaerobes are killed by it (e.g. Clostridium) as they lack catalase/superoxide dismutase; facultative anaerobes grow both ways; microaerophiles need low O₂ (e.g. Campylobacter).
• Growth in thioglycollate broth shows these patterns (top, bottom, throughout).

Exam tip

Practise generation time numericals: always write the formula, then log values, then the answer with units. Draw the growth curve with all four phases labelled.

Quick summary

Classification (वर्गीकरण) arranges living things into groups based on similarities. This topic covers the main systems — Haeckel's three kingdoms, Whittaker's five kingdoms and Woese's three domains — the place of microbes in them, binomial nomenclature, and the basis of bacterial classification as in Bergey's Manual.

Key points

• Taxonomy has three parts: classification (grouping), nomenclature (naming) and identification.
• Taxonomic ranks: Domain → Kingdom → Phylum (Division) → Class → Order → Family → Genus → Species.
• Binomial nomenclature (Carl Linnaeus): every species has two names — genus (capital first letter) and species (small letter), printed in italics or underlined when hand-written. Example: Escherichia coli, short form E. coli.
• Ernst Haeckel (1866) proposed a third kingdom, Protista, for microorganisms, beside plants and animals.
• R. H. Whittaker (1969) proposed five kingdoms: Monera (prokaryotes — bacteria, cyanobacteria), Protista (unicellular eukaryotes — protozoa, some algae), Fungi, Plantae, Animalia. Based on cell type, body organization and mode of nutrition.
• Carl Woese (1977, formalised 1990) used 16S rRNA sequences and proposed three domains: Bacteria, Archaea and Eukarya.
• Archaea are prokaryotes but differ from bacteria: no peptidoglycan in walls, ether-linked membrane lipids, and some features of gene expression close to eukaryotes. Many live in extreme places (methanogens, extreme halophiles, thermoacidophiles).
• Viruses are not placed in any kingdom because they are not cells.
• Bergey's Manual is the standard reference for bacterial classification. Bergey's Manual of Determinative Bacteriology helps identification; Bergey's Manual of Systematic Bacteriology (2nd edition) arranges bacteria mainly by 16S rRNA phylogeny.
• Basis of bacterial classification: morphology (shape, Gram reaction, spores, flagella), cultural characters, biochemical tests (sugar fermentation, IMViC, catalase, oxidase), serology, G + C content of DNA, DNA–DNA hybridization, rRNA sequencing.
• Numerical taxonomy (Adansonian) gives equal weight to many characters and calculates similarity coefficients.
• A bacterial species is often defined by over 70% DNA–DNA reassociation, or over about 97% 16S rRNA similarity as a working guide.

Important terms

• Taxon – any group at any rank (e.g. a genus).
• Strain – a population descended from a single isolate within a species.
• Phylogeny – evolutionary history of organisms.
• Type strain – the reference strain that the species name is attached to.
• Phenetic classification – based on overall similarity of characters.

Key facts

Similarity coefficient (numerical taxonomy): S_J = a / (a + b + c), where a = characters positive in both, b and c = characters positive in only one of the two strains.

5 important questions

Q1. What is binomial nomenclature? Give an example.

A system of giving each species a two-part Latin name — genus and species — introduced by Linnaeus. Example: Bacillus subtilis.

Q2. Name Whittaker's five kingdoms.

Monera, Protista, Fungi, Plantae and Animalia.

Q3. Explain Woese's three-domain classification.

• Based on comparison of 16S (small subunit) rRNA sequences.
• Domains: Bacteria, Archaea, Eukarya.
• Showed that archaea are a separate line, not just odd bacteria.
• Write differences (table above).
• Importance: modern classification is phylogenetic.

Q4. What characters are used to classify bacteria?

• Morphological: shape, size, arrangement, Gram stain, capsule, spores, flagella.
• Cultural: colony type, growth in broth, pigment.
• Biochemical: sugar fermentation, enzymes (catalase, oxidase, urease), IMViC.
• Physiological: temperature, oxygen, pH needs.
• Serological: antigen–antibody reactions.
• Molecular: G + C content, DNA hybridization, 16S rRNA sequencing.

Q5. Compare Whittaker's and Woese's systems.

• Whittaker: five kingdoms, based on cell structure, body form, nutrition; all prokaryotes in Monera.
• Woese: three domains above kingdoms, based on rRNA; prokaryotes split into Bacteria and Archaea.
• Woese's system reflects evolution better.

Exam tip

Write scientific names correctly: genus capitalised, species small, in italics (underline in handwritten answers).

Quick summary

Rickettsiae, chlamydiae and mycoplasmas are small, unusual bacteria that were once thought to lie between bacteria and viruses. Rickettsiae and chlamydiae are obligate intracellular parasites, while mycoplasmas can grow on cell-free media but have no cell wall. They are true bacteria (prokaryotes with both DNA and RNA, 70S ribosomes and sensitivity to some antibiotics), and they cause important human, animal and plant diseases.

Key points

• Small Gram-negative rods or coccobacilli (about 0.3 × 1–2 µm); stain well with Giemsa or Gimenez stain.
• Obligate intracellular; grow in the cytoplasm (and sometimes nucleus) of host cells; cultured in yolk sac of embryonated eggs, cell cultures or animals.
• Spread by arthropod vectors: lice, fleas, ticks, mites.
• Diseases: epidemic typhus (Rickettsia prowazekii, body louse), endemic/murine typhus (R. typhi, rat flea), Rocky Mountain spotted fever (R. rickettsii, tick), scrub typhus (Orientia tsutsugamushi, mite larva – common in India).
• Weil–Felix test: patient serum agglutinates certain Proteus strains (OX-19, OX-2, OX-K) because of shared antigens.
• Treated with doxycycline (tetracyclines) or chloramphenicol.
• Rickettsiae
• Very small Gram-negative bacteria that cannot make enough of their own ATP ("energy parasites").
• Unique developmental cycle with two forms: elementary body (EB) – small, infectious, non-dividing; reticulate body (RB) – larger, non-infectious, divides inside a vacuole called an inclusion. Cycle takes about 48–72 hours.
• Their wall lacks a typical peptidoglycan layer (only traces detected).
• Diseases: trachoma (a leading infectious cause of blindness) and genital infections by Chlamydia trachomatis; pneumonia by Chlamydia pneumoniae; psittacosis by Chlamydia psittaci (from birds).
• Treated with azithromycin or doxycycline.
• Chlamydiae
• Smallest free-living organisms (about 0.2–0.3 µm); no cell wall, so they are pleomorphic and resistant to penicillin; pass through bacterial filters.
• Membrane contains sterols; most need cholesterol in the medium.
• Colonies look like "fried eggs" (dense centre growing into agar, lighter edge).
• Diseases: primary atypical ("walking") pneumonia (Mycoplasma pneumoniae), urogenital infections (Ureaplasma, M. genitalium); bovine pleuropneumonia (M. mycoides, the first discovered – hence "PPLO").
• Plant pathogens of this type are called phytoplasmas (e.g. little leaf of brinjal, sandal spike, grassy shoot of sugarcane).
• Common contaminants of cell cultures.
• Mycoplasmas

Important terms

• Obligate intracellular parasite – organism that can grow only inside living host cells.
• Elementary body – infectious, spore-like form of chlamydiae.
• Reticulate body – dividing intracellular form of chlamydiae.
• PPLO – pleuropneumonia-like organisms; old name for mycoplasmas.
• Pleomorphism – having many shapes.

Key facts

5 important questions

Q1. Why are mycoplasmas resistant to penicillin?

Penicillin acts by blocking cell wall (peptidoglycan) synthesis. Mycoplasmas have no cell wall, so penicillin has no target in them.

Q2. What is the Weil–Felix reaction?

An agglutination test for rickettsial infections. The patient's serum agglutinates non-motile Proteus strains OX-19, OX-2 or OX-K because they share antigens with rickettsiae.

Q3. Describe the developmental cycle of Chlamydia.

• EB attaches and enters the host cell by endocytosis.
• Inside a vacuole (inclusion) EB changes to RB.
• RB divides by binary fission.
• RBs reorganize into new EBs.
• Host cell bursts and EBs are released (draw the cycle).

Q4. Give the general characters and diseases of rickettsiae.

• Size, shape, staining, obligate intracellular growth, cultivation.
• Arthropod transmission.
• Table of diseases, agents and vectors.
• Diagnosis (Weil–Felix, serology, PCR) and treatment.

Q5. Compare rickettsiae, chlamydiae, mycoplasmas and viruses.

• Use the table above and add viruses: no cell structure, one type of nucleic acid, no ribosomes, not killed by antibiotics.
• Conclude that the first three are true bacteria.

Exam tip

For comparison questions always include viruses as a fourth column. Remember "fried egg colony = Mycoplasma" and "EB and RB = Chlamydia".

Quick summary

Fungi are eukaryotic, non-photosynthetic organisms that absorb food from their surroundings. They include moulds (filamentous) and yeasts (single-celled). Fungi are the main decomposers of plant matter, they give us bread, alcohol, antibiotics and enzymes, and some cause diseases of humans and crops. The branch that studies them is mycology.

Key points

• Fungi are eukaryotic, heterotrophic (saprophytic, parasitic or symbiotic) and absorptive in nutrition; they have no chlorophyll.
• Cell wall is mainly chitin with glucans; the membrane sterol is ergosterol (target of antifungal drugs like amphotericin B and azoles).
• Reserve food is glycogen and lipids (not starch).
• Moulds grow as hyphae (thread-like, 2–10 µm wide); a mass of hyphae is a mycelium. Hyphae are septate (with cross walls, e.g. Aspergillus, Penicillium) or aseptate/coenocytic (no cross walls, e.g. Rhizopus, Mucor).
• Yeasts are unicellular, oval cells (about 3–5 µm) that usually reproduce by budding (Saccharomyces cerevisiae) or fission (Schizosaccharomyces). Some form pseudohyphae (chains of elongated buds, e.g. Candida albicans).
• Dimorphic fungi grow as mould at 25 °C (environment) and yeast at 37 °C (in the body), e.g. Histoplasma, Blastomyces.
• Asexual spores: sporangiospores (inside a sporangium, Rhizopus), conidia (free at hyphal tips, Aspergillus, Penicillium), arthrospores, chlamydospores, blastospores (buds).
• Sexual spores decide the main groups: zygospores, ascospores (in an ascus), basidiospores (on a basidium).
• Main groups (traditional): Zygomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes (fungi imperfecti, no known sexual stage).
• Fungi prefer acidic pH (about 5–6), moist conditions and 20–30 °C; Sabouraud dextrose agar (pH about 5.6) is the common medium.
• Useful: baker's and brewer's yeast, penicillin (Penicillium chrysogenum), citric acid (Aspergillus niger), cheese ripening (P. roqueforti, P. camemberti), edible mushrooms, mycorrhiza.
• Harmful: food spoilage, aflatoxin (Aspergillus flavus), plant diseases (rusts, smuts, blights), human mycoses (ringworm, candidiasis).

Important terms

• Hypha – single filament of a mould.
• Mycelium – network of hyphae forming the body of a mould.
• Budding – asexual reproduction where a small outgrowth forms and separates from a yeast cell.
• Conidium – asexual spore formed at the tip of a hypha, not enclosed in a sac.
• Dimorphism – ability to exist in two forms (mould and yeast).

Key facts

5 important questions

Q1. What is ergosterol? Why is it important?

Ergosterol is the main sterol of fungal cell membranes (humans have cholesterol instead). Antifungal drugs such as amphotericin B (binds it) and azoles (block its synthesis) act on it, so they harm fungi more than human cells.

Q2. Differentiate septate and aseptate hyphae.

Septate hyphae have cross walls (septa) dividing them into cells, e.g. Aspergillus. Aseptate (coenocytic) hyphae have no cross walls and many nuclei in a common cytoplasm, e.g. Rhizopus.

Q3. Give the general characteristics of fungi.

• Eukaryotic, heterotrophic, absorptive nutrition.
• Chitin wall, ergosterol, glycogen reserve.
• Thallus: yeast or mycelium; dimorphism.
• Asexual and sexual spores.
• Habitat and growth conditions; Sabouraud agar.

Q4. Describe the structure and reproduction of yeast.

• Oval cell with wall, nucleus, vacuole, mitochondria (draw diagram).
• Budding: nucleus divides, bud enlarges and separates; bud scar left.
• Sexual: two haploid cells fuse; diploid cell forms an ascus with 4 ascospores.
• Uses of Saccharomyces cerevisiae in bread, beer, wine and ethanol.

Q5. Write the economic importance of fungi.

• Industry: alcohol, antibiotics, organic acids, enzymes (amylase, cellulase).
• Food: mushrooms, cheese, fermented foods; single cell protein.
• Agriculture: mycorrhiza, biocontrol (Trichoderma).
• Harmful: spoilage, mycotoxins, plant and human diseases.

Exam tip

Draw a Rhizopus sporangium, an Aspergillus conidial head and a budding yeast cell — these three diagrams cover most fungal questions.

Quick summary

Algae and protozoa are eukaryotic microorganisms usually placed in the kingdom Protista (Protoctista). Algae are photosynthetic, mostly aquatic, and produce a large part of the earth's oxygen. Protozoa are animal-like, heterotrophic, usually motile single cells; most are free-living, but some cause serious diseases such as malaria and amoebiasis. The study of algae is phycology and of protozoa is protozoology.

Key points

• Eukaryotic, photoautotrophic, with chlorophyll a plus accessory pigments; have chloroplasts.
• Body (thallus) is not divided into root, stem and leaf; forms are unicellular (Chlamydomonas, Chlorella), colonial (Volvox), filamentous (Spirogyra) or large seaweeds (Sargassum).
• Cell wall mostly cellulose; diatoms have a silica wall (frustule).
• Reproduce vegetatively, asexually (zoospores) and sexually (isogamy, anisogamy, oogamy).
• General characters of algae:
• Main algal groups by pigment and stored food (see table).
• Importance of algae: primary producers in water; oxygen release; food and supplements (Chlorella); agar and carrageenan from red algae, alginates from brown algae; diatomaceous earth used in filters; biofuel research. Harm: toxic blooms, red tides (dinoflagellates), clogging of filters.
• Eukaryotic, unicellular, no cell wall (a flexible pellicle in some), heterotrophic (holozoic or saprozoic).
• Usually 10–200 µm; have contractile vacuoles (osmoregulation) and food vacuoles.
• Many form cysts (resistant, infective stage) and trophozoites (active feeding stage).
• Reproduce asexually by binary fission, multiple fission (schizogony) or budding; sexually by conjugation (ciliates) or gamete fusion (Plasmodium in mosquito).
• General characters of protozoa:
• Sarcodina – pseudopodia: Amoeba, Entamoeba histolytica (amoebic dysentery).
• Mastigophora – flagella: Euglena, Giardia lamblia (giardiasis), Trypanosoma (sleeping sickness), Leishmania donovani (kala-azar).
• Ciliophora – cilia: Paramecium, Balantidium coli.
• Sporozoa (Apicomplexa) – no locomotory organs in adults: Plasmodium (malaria), Toxoplasma.
• Classification of protozoa by locomotion (traditional):
• Protozoa feed on bacteria and help in sewage treatment (activated sludge); termite gut flagellates digest wood.

Important terms

• Thallus – simple plant-like body without true roots, stem and leaves.
• Frustule – glass-like silica wall of a diatom.
• Pellicle – firm outer covering of some protozoa.
• Trophozoite / cyst – active feeding form / dormant resistant form of a protozoan.
• Pseudopodium – temporary "false foot" used for movement and feeding.

Key facts

5 important questions

Q1. Why are cyanobacteria not called true algae?

They are prokaryotes: no nucleus, no chloroplasts, 70S ribosomes and a peptidoglycan wall. True algae are eukaryotes with chloroplasts.

Q2. Name the source of agar and its use in microbiology.

Agar comes from red algae such as Gelidium and Gracilaria. It is used to solidify culture media because most bacteria cannot digest it and it melts at about 85–100 °C but sets at about 32–40 °C.

Q3. Give the general characters and economic importance of algae.

• Characters: eukaryotic, photosynthetic, thallus forms, pigments, walls, reproduction.
• Table of groups.
• Uses: food, agar, alginate, carrageenan, diatomite, oxygen, biofuel.
• Harm: blooms and toxins.

Q4. Describe the classification of protozoa with examples.

• Four groups based on locomotion with organelle and examples.
• Mention disease-causing members of each.
• Add a simple sketch of Amoeba, Euglena, Paramecium.

Q5. Explain the general characters of protozoa.

• Size, shape, pellicle, organelles (nucleus, contractile vacuole, food vacuole).
• Nutrition types; habitat.
• Encystment and excystment.
• Asexual and sexual reproduction.
• Ecological and medical importance.

Exam tip

Learn one pathogen per protozoan group and the pigment + stored food of each algal group. "Agar from red algae" is a favourite one-mark question.

Quick summary

Viruses (विषाणू) are very small, non-cellular infectious particles that can multiply only inside living host cells. This topic covers their general properties, structure, symmetry, classification, the lytic and lysogenic cycles, and how viruses are grown in the lab.

Key points

• Viruses are obligate intracellular parasites: outside a host they are inert particles; inside they use the host's machinery.
• They contain only one type of nucleic acid — DNA or RNA, never both.
• Size: roughly 20–300 nm; most pass through bacterial filters and are seen only with an electron microscope.
• They have no ribosomes and no energy-producing system, so they do not grow or divide by binary fission.
• Virion: a complete infectious virus particle.
• Structure: nucleic acid core + protein coat called capsid, made of subunits called capsomeres. Nucleic acid + capsid = nucleocapsid. Some viruses have an outer lipid envelope with glycoprotein spikes (e.g. influenza, HIV).
• Symmetry of capsid: helical (e.g. tobacco mosaic virus), icosahedral (20 triangular faces, e.g. adenovirus, poliovirus), complex (e.g. T4 bacteriophage, poxvirus).
• Baltimore classification puts viruses in seven groups based on genome type and how mRNA is made (dsDNA, ssDNA, dsRNA, +ssRNA, −ssRNA, ssRNA with reverse transcriptase, dsDNA with reverse transcriptase).
• Classification is also based on host: animal, plant and bacterial viruses (bacteriophages).
• Lytic cycle: attachment → penetration → synthesis of viral parts → assembly → release by lysis of host cell. Example: T4 phage.
• Lysogenic cycle: viral DNA joins the host chromosome as a prophage and is copied with it; it may later switch to the lytic cycle (induction). Example: lambda (λ) phage.
• Cultivation of viruses: (1) in animals (e.g. mice), (2) in embryonated chicken eggs (influenza vaccine production), (3) in cell/tissue culture (most common). Bacteriophages are grown on bacterial lawns; they form clear zones called plaques.
• Viroids are small naked circular RNA molecules that infect plants (e.g. potato spindle tuber viroid). Prions are infectious proteins without nucleic acid (e.g. mad cow disease, Creutzfeldt–Jakob disease).
• Discovery: Ivanovsky (1892) and Beijerinck (1898) with tobacco mosaic disease; W. M. Stanley crystallized TMV (1935).

Important terms

• Capsid / capsomere – protein coat / its subunits.
• Envelope – lipid membrane taken from the host, with viral proteins.
• Prophage – phage DNA integrated in a bacterial chromosome.
• Plaque – clear area on a bacterial lawn where phages have lysed cells.
• Cytopathic effect (CPE) – visible damage to cultured cells caused by a virus.

Key facts

5 important questions

Q1. Why are viruses called obligate intracellular parasites?

Because they have no metabolic machinery or ribosomes of their own and can multiply only by using a living host cell.

Q2. What are viroids and prions?

Viroids are small, naked, circular RNA molecules without a protein coat that cause plant diseases. Prions are infectious protein particles without nucleic acid that cause brain diseases such as mad cow disease.

Q3. Describe the general structure of a virus with a diagram.

• Nucleic acid core (DNA or RNA).
• Capsid made of capsomeres; nucleocapsid.
• Envelope with spikes in some viruses.
• Symmetry types: helical, icosahedral, complex, each with example.
• Draw: icosahedral virus or T4 phage (head, collar, tail sheath, base plate, tail fibres).

Q4. Explain the lytic and lysogenic cycles.

• Lytic: adsorption, penetration (DNA injection), early proteins, replication, late proteins, assembly, lysis and release.
• Lysogenic: DNA integrates as prophage, replicates with host, host survives; induction (e.g. by UV) starts lytic cycle.
• Examples: T4 (lytic), lambda (temperate).
• Lysogenic conversion: e.g. diphtheria toxin gene carried by a phage.

Q5. How are viruses cultivated?

• Animal inoculation (mice, guinea pigs).
• Embryonated egg: sites — chorioallantoic membrane, amniotic cavity, allantoic cavity, yolk sac.
• Cell culture: primary, diploid and continuous cell lines (e.g. HeLa); detect by CPE.
• Phages: plaque assay on a bacterial lawn.

Exam tip

Always state that viruses have only DNA or RNA. A labelled T4 phage diagram is a common 5-mark question.

Quick summary

Only a small fraction of microorganisms cause disease; most are harmless or useful. Microbes recycle nutrients, make soil fertile, give us food, medicines and industrial chemicals, clean wastewater and live in our gut. Others spoil food, cause diseases of humans, animals and plants, and damage materials. This topic gives an overview of both sides, with Indian examples, as asked in first-year exams.

Key points

• Decomposition of dead matter and recycling of C, N, S and P (biogeochemical cycles).
• Nitrogen fixation by Rhizobium, Azotobacter, cyanobacteria; biofertilizers.
• Biopesticides: Bacillus thuringiensis (Bt toxin), Trichoderma against soil fungi.
• Sewage treatment and bioremediation (oil-eating Pseudomonas).
• Biogas (methane) from cow dung by methanogenic archaea.
• Beneficial — environment and agriculture
• Curd and cheese (Lactobacillus, Lactococcus, Streptococcus thermophilus), bread and alcohol (Saccharomyces cerevisiae), idli–dosa batter fermentation (lactic acid bacteria and yeasts), vinegar (Acetobacter).
• Antibiotics: penicillin (Penicillium), streptomycin (Streptomyces).
• Organic acids (citric acid by Aspergillus niger), enzymes (amylase, protease), vitamins (B₁₂), amino acids (glutamic acid by Corynebacterium glutamicum).
• Vaccines and recombinant products: insulin made in E. coli or yeast.
• Beneficial — food and industry
• Beneficial — health: normal flora of gut makes vitamin K and B vitamins and blocks pathogens; probiotics.
• Harmful — human diseases: bacteria (tuberculosis, typhoid, cholera), viruses (polio, measles, COVID-19), fungi (ringworm, candidiasis), protozoa (malaria, amoebiasis).
• Harmful — animal diseases: anthrax, foot-and-mouth disease, bird flu.
• Harmful — plant diseases: citrus canker (Xanthomonas), bacterial blight of rice, late blight of potato (Phytophthora infestans), rust of wheat (Puccinia), tobacco mosaic (TMV).
• Harmful — food: spoilage (souring of milk, mould on bread) and food poisoning (Staphylococcus aureus toxin, Clostridium botulinum toxin, Salmonella).
• Harmful — materials: biodeterioration of wood, paper, leather, paints; corrosion of iron pipes by sulphate-reducing bacteria (Desulfovibrio).
• Harm and benefit often depend on place: E. coli is normal in the gut but causes urinary tract infection in the bladder.

Important terms

• Pathogen – microbe that causes disease.
• Normal flora – microbes that normally live on or in a healthy body without harm.
• Biofertilizer – living microbial preparation that adds nutrients to soil.
• Biodeterioration – damage to useful materials by microbes.
• Probiotic – live microbes that give a health benefit when taken in enough amount.

Key facts

5 important questions

Q1. Give two examples of microorganisms used as biopesticides.

Bacillus thuringiensis (its crystal protein kills caterpillars) and the fungus Trichoderma (controls soil-borne plant pathogens). Nuclear polyhedrosis viruses are another example.

Q2. What is biodeterioration? Give an example.

The damage of useful materials by microbial activity. Example: fungi rotting wood and paper, or sulphate-reducing bacteria corroding iron pipes.

Q3. Describe the beneficial activities of microorganisms.

• Agriculture: nitrogen fixation, decomposition, biopesticides.
• Food: fermented foods and beverages.
• Industry: antibiotics, acids, enzymes, vitamins, alcohol.
• Environment: sewage treatment, biogas, bioremediation.
• Health: normal flora, vaccines, probiotics, recombinant drugs.

Q4. Explain the harmful effects of microorganisms.

• Diseases of humans, animals and plants with one example each.
• Food spoilage and food poisoning.
• Biodeterioration and corrosion.
• Toxins and allergies (fungal spores).

Q5. "Microorganisms are friends as well as foes." Justify.

• Write a balanced answer using the table.
• Give at least four useful and four harmful examples.
• Conclude that useful roles far outnumber harmful ones.

Exam tip

In "friend and foe" answers, a two-column table with Indian examples (idli, curd, biogas, Bt crops, rice blight) scores well.