Microbiology - Bacteria, Viruses and Fungi

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Prokaryotes (bacteria, archaea): No membrane-bound nucleus; DNA is circular and in the cytoplasm; no membrane-bound organelles; smaller ribosomes (70S); usually 1–10 μm. Eukaryotes (animals, plants, fungi, protists): Have a nucleus and membrane-bound organelles; larger ribosomes (80S).

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Cell wall (peptidoglycan), cell membrane, cytoplasm, circular chromosome (nucleoid region), ribosomes (70S), plasmids (small circular DNA). Optional: capsule (slime layer), flagella (motility), pili (attachment), endospores (survival structures).

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Coccus (spherical): e.g., Staphylococcus. Bacillus (rod-shaped): e.g., E. coli. Spirillum (spiral/helical): e.g., Helicobacter pylori. Vibrio (comma-shaped): e.g., Vibrio cholerae. Arrangements: diplo- (pairs), strepto- (chains), staphylo- (clusters).

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A lab technique distinguishing two major bacterial groups. Gram-positive: Thick peptidoglycan layer retains crystal violet stain → purple. Examples: Staphylococcus, Streptococcus. Gram-negative: Thin peptidoglycan + outer lipopolysaccharide membrane → does not retain stain → pink/red. Examples: E. coli, Salmonella.

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The method of asexual reproduction in bacteria. The circular chromosome replicates → cell elongates → divides into two identical daughter cells. Under optimal conditions, bacteria can divide every 20 minutes — one bacterium can become 16 million in 8 hours.

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Lag phase: Bacteria adapting to environment — no division. Log (exponential) phase: Rapid doubling — fastest growth. Stationary phase: Birth rate = death rate — nutrients limited, waste accumulates. Death (decline) phase: Death rate > birth rate — nutrients exhausted.

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Small, circular, self-replicating DNA molecules in bacteria, separate from the main chromosome. Carry genes for antibiotic resistance, toxin production, and metabolic functions. Can be transferred between bacteria via conjugation. Key tools in genetic engineering.

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An ultramicroscopic infectious agent (~20–300 nm) consisting of a nucleic acid core (DNA or RNA) enclosed in a protein coat (capsid), sometimes surrounded by a lipid envelope. Non-living: cannot reproduce independently, has no metabolism, no cells. Must infect a host cell to replicate.

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DNA viruses (e.g., herpes, hepatitis B, smallpox): Replicate in nucleus; more stable; lower mutation rate. RNA viruses (e.g., influenza, HIV, SARS-CoV-2, polio): Replicate in cytoplasm; less stable; very high mutation rate (RNA polymerase lacks proofreading) → rapid evolution and vaccine challenges.

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The virus immediately hijacks the host cell to replicate: Attachment → Injection of nucleic acid → Replication (host machinery makes viral components) → Assembly → Lysis (host cell bursts, releasing ~100–1000 new viruses). The host cell is destroyed.

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The virus integrates its DNA into the host chromosome as a prophage — lying dormant for many generations. The host cell divides normally, replicating the viral DNA. A trigger (UV light, stress) causes the prophage to excise and enter the lytic cycle. Example: bacteriophage λ (lambda).

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Some viruses (influenza, HIV, herpes, SARS-CoV-2) are surrounded by a lipid bilayer derived from the host cell membrane, embedded with viral glycoproteins. Enveloped viruses are more fragile (destroyed by alcohol, soap — lipids dissolve). Non-enveloped viruses (norovirus) are more resistant to disinfection.

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Human Immunodeficiency Virus — an enveloped RNA retrovirus that infects CD4⁺ T helper cells (a key immune cell). Uses reverse transcriptase to convert RNA → DNA → integrates into host genome. Gradually destroys T cells → immune system collapses → AIDS (Acquired Immunodeficiency Syndrome) — opportunistic infections.

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Retroviruses (e.g., HIV) carry reverse transcriptase — an enzyme that converts their RNA genome into DNA inside the host cell. This DNA integrates into the host chromosome. Unusual because the normal flow is DNA → RNA → protein (central dogma reversed).

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Chemical compounds that kill or inhibit the growth of bacteria. Mechanisms: Cell wall synthesis inhibition (penicillin, cephalosporins — target peptidoglycan). Protein synthesis inhibition (tetracyclines target 30S ribosome; erythromycin targets 50S). DNA replication inhibition (quinolones). Cell membrane disruption (polymyxins). No effect on viruses.

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When bacteria evolve mechanisms to survive antibiotic treatment. Arises through: Mutation of target site. Plasmid transfer of resistance genes (conjugation). Enzyme production (e.g., beta-lactamase breaks down penicillin). Efflux pumps (pump antibiotic out of cell). Accelerated by overuse and misuse of antibiotics.

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Methicillin-Resistant Staphylococcus aureus — a bacterial strain resistant to most beta-lactam antibiotics including methicillin and penicillin. A major hospital-acquired infection. Difficult to treat — requires vancomycin (last-resort antibiotic). Spread by contact. Prevention: hand hygiene.

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Eukaryotic organisms with chitin cell walls (not cellulose like plants or peptidoglycan like bacteria). Heterotrophic — cannot photosynthesize. Reproduce by spores (sexually and asexually). Include moulds (Aspergillus), yeasts (Candida, Saccharomyces), and mushrooms.

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Yeasts: Unicellular fungi, reproduce by budding. Examples: Saccharomyces cerevisiae (bread, beer, wine), Candida albicans (thrush). Moulds: Multicellular fungi forming hyphae (thread-like filaments) that make up a mycelium. Examples: Penicillium (antibiotic source), Aspergillus, Rhizopus (bread mould).

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Break down dead organic matter, releasing nutrients back into the environment as inorganic compounds. Essential for nutrient cycling (carbon cycle, nitrogen cycle). Without decomposers, nutrients would be locked in dead biomass and life could not continue.

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A prion is a misfolded protein — not a pathogen containing nucleic acid. The abnormal prion protein (PrPˢᶜ) causes normal proteins to misfold too, creating a chain reaction. Causes: CJD (Creutzfeldt-Jakob disease), vCJD (variant CJD — linked to BSE/"mad cow disease"), scrapie (sheep). No cure.

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A structured community of microorganisms (usually bacteria) enclosed in a self-produced extracellular polysaccharide matrix, attached to a surface. Examples: dental plaque, slime in drains, infections on medical implants. Biofilms are 1,000× more resistant to antibiotics than free-living bacteria — major clinical challenge.

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Bacteriostatic: Inhibits bacterial growth and reproduction without killing — relies on the immune system to clear bacteria. Examples: tetracyclines, chloramphenicol. Bactericidal: Directly kills bacteria. Examples: penicillin, ciprofloxacin, aminoglycosides.

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Highly resistant dormant structures produced by some bacteria (Bacillus, Clostridium) when nutrients are scarce. Extremely resistant to heat, desiccation, radiation, and chemicals. Can survive for thousands of years. Cause: anthrax (B. anthracis), tetanus (C. tetani), botulism (C. botulinum). Must be killed by autoclaving (121°C steam).

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The theory that many diseases are caused by specific microorganisms (germs). Established by Louis Pasteur (pasteurization, vaccines) and Robert Koch (Koch's postulates — criteria for proving a microbe causes a specific disease). Revolutionized medicine in the 19th century.