Microbiology Lecture: Unusual Bacteria, Growth Curves & Intro to Viruses

Atypical / “Unusual” Bacteria

1. Mycoplasma

• No cell wall → pleomorphic, fragile; cause “walking pneumonia”.

• Transmission: person-to-person via respiratory droplets; not via fomites (requires warm, moist environment).

• Clinical: mild pneumonia; cell-wall-targeting antibiotics (β-lactams) ineffective.

2. Obligate Intracellular Bacteria

(Once mis-classified as viruses)

• Shared traits- Typical Gram-negative architecture but cannot replicate in media – require host ATP/cofactors.

• Term: Obligate intracellular parasites (also used later for viruses).

• Treatment: prolonged antibiotics because drugs must penetrate host cells.

3. Spirochetes (Family Spirochaetaceae)

• Axial filament = spiral bundle of internal flagella within periplasm; clockwise/counter-clockwise rotation ➔ entire cell corkscrews; plus tip enzymes to bore through intact membranes.

4. Archaea (Prokaryotes but not true bacteria)

• Distinctions- Cell wall lacks peptidoglycan.

• Unique lipids, ribosomal proteins, no typical fimbriae/flagella (have hami – grappling hooks).

• Ecology- Extremophiles: thermophiles (hot springs), halophiles (Great Salt Lake), acidophiles (acid-mine drainage), methanogens.

• Importance- Bioremediation: detoxify heavy-metal, acidic sites, oil spills.

• Novel antibiotic source (new bacteriocins unseen by pathogens).

Bacterial Population Growth

Binary Fission Mechanics

1. Genome replication (DNA gyrase/topoisomerase unwind/wind).

2. Chromosome anchors to membrane; cell elongates depositing new membrane between chromosomes.

3. Cross-wall (septum) forms → 2 genetically identical daughters (cloning).

4. Repeat (planes of division determine arrangement).

• Property: In optimal conditions bacteria are effectively immortal—mother becomes daughters.

Generation Time (g)

• Definition: time for one cell to divide into two (doubling time).

• Typical g\approx30\text{ min} (Staph aureus \approx15\,\text{min}; M. tuberculosis \approx18\,\text{h}).

• Calculation example (class demo)- Start: 10 cells → 1 h later: 160 cells.

  • Doublings needed: 10\to20\to40\to80\to160 = 4\text{ doublings}.

  • g = \frac{60\,\text{min}}{4} = 15\,\text{min}.

Batch-Culture / Closed-System Growth Curve

(also applies to infection in tissue)

1. Lag Phase – adjustment; gene regulation, enzyme synthesis; \Delta N\approx0.

2. Exponential (Log) Phase – constant +\text{ve} slope; binary fission outpaces death.

3. Maximum Stationary Phase – carrying capacity reached; reproduction ≈ death; waste ↑ nutrients ↓.

4. Death Phase – exponential decline; death > reproduction.

5. Endpoints- Population Crash: all cells die (desired during antibiotic therapy).

   • Minimum Stationary Phase: small hardy subset persists (endospores, tubercles, tonsillar hideout) ➔ carriers or relapses.

• Clinical links- Take full antibiotic course to push pathogen to crash, not minimum stationary.

• Example hideouts: S. pyogenes in tonsils (recurrent strep), Mycobacterium tuberculosis in lung tubercles.

• Ethanol fermentation stops at \approx12.5\% v/v because yeast reach carrying capacity (self-poison).

Introduction to Virology (Topic 3B Preview)

Fundamental Properties

• Size: smaller than smallest bacteria (see scale – poxviruses largest, picornaviruses \approx30\text{ nm}).

• Structure1. Core (Capsid) – protein shell (capsomers) enclosing genome (DNA or RNA).

1. Optional Envelope – stolen host membrane + viral glycoprotein spikes (e.g., influenza, SARS-CoV-2, HBV).

• Advantage: immune evasion; Disadvantage: fragile (needs close contact, fluid transfer).

1. Complex bacteriophages (e.g., T4) – head–tail–fiber architecture.

• Terminology- Virion = extracellular, metabolically inert particle.

• Obligate intracellular parasite – cannot make ATP nor enzymes for replication; hijacks host machinery.

• Host range: viruses exist for all life forms, incl. bacteria (bacteriophages), fungi, protozoa, plants, animals.

Spike Proteins & Cell Entry

• Spikes = viral glycoproteins (e.g., green/yellow knobs on herpes micrograph) – lock-and-key with host receptors (“doorknobs”).

• Key to tropism (which cells/tissues a virus can infect).

HSV-3 Case Study (Chickenpox & Shingles)

• Primary infection (varicella): 14-day incubation → pruritic vesicular rash trunk > limbs; highly contagious 7 days before rash.

• Virus may enter bloodstream, then dorsal-root ganglia neurons → integrates into host chromosome (latent).

• Reactivation (zoster/shingles): travels down nerve ➔ dermatomal, painful rash in 3 classic bands.

• Implications- A person with shingles can give varicella to seronegative infants/adults.

• Vaccination strategies (live attenuated vs subunit) differ; reactivation risk influences zoster vaccine recommendations.

Cross-Topic Connections & Implications

• Rickettsia/Chlamydia teach principle of obligate intracellular parasitism later applied to viruses.

• Spirochete axial filaments illustrate unique motility – parallels virus reliance on host for motility (via cell processes).

• Archaea as antibiotic sources addresses rising drug resistance seen with R-plasmids discussed in conjugation exercise.

• Growth-curve endpoints (minimum stationary) predict latent viral/reactivation patterns (e.g., HSV-3, HIV reservoirs).

• Ethical/historical tie-ins: Tuskegee syphilis study, Rocky Mountain labs, vaccination debates (attenuation vs subunit), public-health rank – NC #2 syphilis.

Key Equations & Numbers to Memorize

• Generation time: g = \dfrac{\Delta t}{\text{# doublings}}.

• Lyme disease bull’s-eye occurs in \approx40\% cases.

• Alcoholic fermentation ceiling: 12.5\% v/v ethanol.

• HSV-3 incubation: 14\text{ days}; contagious peak 7\text{ days} prior.

• Staph aureus g\approx15\,\text{min}; M. tuberculosis g\approx18\,\text{h}.

• TB test culture impractical due to long g.

Practical / Exam Tips

• For diagram questions: highlight each descriptive term, annotate meaning, then verify every requirement drawn & labelled.

• When asked to calculate doubling time, WRITE the doubling chain to avoid log-calc errors.

• If given a growth-curve graph, be able to- Identify phases.

• Explain nutrient/waste dynamics.

• Predict drug timing (antibiotics most effective in log phase).

• Recognize envelope vs naked virus from description (environmental stability, transmission mode).