Microbiology

Microbiology for the MCAT

Part 1 of 7 — Bacteria: Structure & Classification

Bacterial Cell Structure

Gram Stain Classification

LPS (Lipopolysaccharide) — HIGH YIELD

• Found ONLY in Gram-negative outer membrane
• Released when bacteria lyse, triggering a massive immune response
• Can cause septic shock, fever, disseminated intravascular coagulation (DIC)

Worked Example — Why Bacterial Ribosomes Make Good Drug Targets

Scenario: A passage describes a patient with a Staphylococcus aureus (Gram-positive) infection treated with two antibiotics: penicillin (a $\beta$-lactam) and gentamicin (an aminoglycoside). The question asks why these drugs harm the bacterium but spare the patient's own cells.

Step 1 — Identify the targets. Penicillin inhibits cell wall (peptidoglycan) cross-linking; gentamicin binds the 30S ribosomal subunit and blocks translation.

Step 2 — Find the structural difference exploited. Human cells have no peptidoglycan cell wall, so penicillin has no target in the host. Human ribosomes are 80S (40S + 60S subunits), whereas bacterial ribosomes are 70S (30S + 50S). Aminoglycosides selectively recognize the 30S subunit's rRNA, which differs from the eukaryotic 40S.

Step 3 — State the principle (selective toxicity). A drug is clinically useful when it attacks a structure or pathway that the pathogen has and the host lacks (or that differs enough to allow selective binding). The cell wall and the 70S ribosome are the two classic exploited differences.

MCAT takeaway: When asked "why does this antibiotic spare human cells," scan for a uniquely prokaryotic target — peptidoglycan, 70S ribosome, DNA gyrase, or folate synthesis. Because human ribosomes are 80S, ribosome-targeting antibiotics ($30S$ or $50S$) are selectively toxic.

Bacteria Structure 🎯

Key Takeaways — Part 1

• Gram-positive: thick peptidoglycan, no outer membrane (stains purple)
• Gram-negative: thin peptidoglycan + outer membrane with LPS (stains pink)
• LPS = endotoxin, causing fever and shock
• Bacterial ribosomes = 70S (antibiotics target these); human ribosomes = 80S
• Selective toxicity exploits uniquely prokaryotic targets: cell wall, 70S ribosome, DNA gyrase, folate synthesis

Microbiology for the MCAT

Part 2 of 7 — Bacterial Growth & Metabolism

Bacterial Growth Curve

Binary Fission

$N = N_0 \times 2^n$

Where $N$ = final number, $N_0$ = initial number, $n$ = number of generations. The generation (doubling) time is the time required for one division.

Bacterial Metabolism

Oxygen Requirements

Worked Example — Calculating Growth with Binary Fission

Scenario: A culture starts with $5 \times 10^3$ E. coli cells. The generation (doubling) time is 20 minutes. Assuming uninterrupted exponential growth, how many cells are present after 2 hours?

Step 1 — Find the number of generations. Total time = 2 hours = 120 minutes. With a 20-minute doubling time:

$n = \frac{120 \text{ min}}{20 \text{ min/generation}} = 6 \text{ generations}$

Step 2 — Apply the binary-fission formula. Using $N = N_0 \times 2^n$:

$N = (5 \times 10^3) \times 2^6 = (5 \times 10^3) \times 64 = 3.2 \times 10^5 \text{ cells}$

Step 3 — Sanity check the model. This assumes the log phase persists the entire time. In reality, nutrients deplete and waste accumulates, so the culture eventually enters stationary phase and the true count is lower. The MCAT rewards recognizing that pure exponential growth is an idealization.

MCAT takeaway: Exponential growth uses $N = N_0 \times 2^n$, where $n$ = (elapsed time) / (generation time). Real cultures only follow this during the log phase. Antibiotics that target active division work best precisely here.

Bacterial Growth 🎯

Key Takeaways — Part 2

• Growth curve: Lag → Log → Stationary → Death
• Binary fission: $N = N_0 \times 2^n$, where $n$ = elapsed time / generation time
• Obligate aerobes need O$_2$; obligate anaerobes are killed by O$_2$ (they lack detox enzymes)
• Metabolic classes combine an energy source (photo/chemo) and a carbon source (auto/hetero)
• Antibiotics are most effective during log phase (active division)

Microbiology for the MCAT

Part 3 of 7 — Bacterial Genetics & Antibiotic Resistance

Horizontal Gene Transfer

Unlike vertical transmission (parent to offspring), horizontal gene transfer spreads genes between cells, even across species — a major driver of resistance.

Antibiotic Resistance Mechanisms

MCAT Connection: Antibiotic Targets

Antibiotics DON'T work against viruses — viruses use host machinery!

Worked Example — Tracing How Resistance Spreads

Scenario: A hospital reports an outbreak of Klebsiella carrying a plasmid that encodes both a $\beta$-lactamase and a tetracycline efflux pump. Genetically identical resistance plasmids appear in unrelated E. coli strains on the same ward. A question asks which mechanism most likely spread the resistance and why it threatens multiple drug classes at once.

Step 1 — Recognize the unit of transfer. The genes sit on a plasmid — a self-replicating, mobile piece of DNA separate from the chromosome. Plasmids are the classic vehicles of conjugation.

Step 2 — Choose the transfer mechanism. The same plasmid appearing in a different species points to conjugation: the donor extends a sex pilus, draws the recipient close, and passes a plasmid copy through direct cell-to-cell contact. Transformation (naked DNA uptake) and transduction (phage-mediated) typically move smaller or species-restricted fragments.

Step 3 — Explain the multidrug threat. One plasmid carries two distinct resistance mechanisms: enzymatic destruction ($\beta$-lactamase cleaves the $\beta$-lactam ring of penicillins) and active efflux (a pump expels tetracycline before it can act). A single conjugation event therefore confers resistance to two unrelated drug classes simultaneously.

MCAT takeaway: Conjugation via plasmids is the most efficient route for spreading resistance, especially across species. Watch for plasmids bundling several mechanisms — enzymatic degradation, target modification, efflux, and reduced permeability — onto one mobile element.

Bacterial Genetics 🎯

Key Takeaways — Part 3

• Horizontal gene transfer: Transformation (naked DNA), Transduction (phage), Conjugation (pilus/plasmid)
• Conjugation via plasmids is the most efficient route for spreading resistance, even across species
• Antibiotic resistance: enzyme degradation, target modification, efflux pumps, decreased permeability
• Know antibiotic targets: cell wall, 30S/50S ribosome, DNA gyrase, folate
• Antibiotics target bacteria, NOT viruses (viruses use host machinery)

Microbiology for the MCAT

Part 4 of 7 — Viruses

Virus Structure

• NOT cells — obligate intracellular parasites
• Nucleic acid (DNA or RNA, never both) + protein coat (capsid)
• Some have a lipid envelope (derived from host membrane)

Viral Classification

Replication Cycles

Lytic cycle: Attach → Inject DNA → Replicate → Assemble → Lyse → Release Lysogenic cycle: Viral DNA integrates into host genome (prophage) → replicates with host → can switch to lytic under stress

Baltimore Classification (Important for MCAT)

Worked Example — Why a (+)ssRNA Virus Translates Faster Than a (-)ssRNA Virus

Scenario: A passage compares two RNA viruses: a (+)ssRNA virus (like the common cold coronavirus) and a (-)ssRNA virus (like influenza). Both must make viral proteins after entering a host cell. The question asks which can begin translation immediately and why the other cannot.

Step 1 — Recall what the ribosome reads. Host ribosomes translate only mRNA, which by convention is the (+) sense strand (the same sequence as the mRNA).

Step 2 — Evaluate each genome.

• A (+)ssRNA genome is already mRNA-sense. The ribosome can latch on and translate it the moment it enters the cytoplasm — no intermediate step.
• A (-)ssRNA genome is complementary (antisense) to mRNA. It must first be copied into a (+) strand before any protein can be made.

Step 3 — Identify the required enzyme. Host cells have no enzyme that makes RNA from an RNA template. So a (-)ssRNA virus must package its own RNA-dependent RNA polymerase (RdRp) inside the virion and bring it along; only then can it transcribe its genome into translatable mRNA.

MCAT takeaway: (+)ssRNA = "mRNA-ready," translated immediately. (-)ssRNA must carry its own RdRp into the cell first. Retroviruses (Class VI, e.g., HIV) are a separate case: they package reverse transcriptase to convert RNA → DNA before integrating into the host genome.

Virology 🎯

Key Takeaways — Part 4

• Viruses are obligate intracellular parasites (not alive by themselves) with DNA or RNA, never both
• (+)ssRNA can be directly translated; (-)ssRNA needs a packaged RNA-dependent RNA polymerase
• Retroviruses (HIV): RNA → DNA via reverse transcriptase, then integration
• Lytic = immediate destruction; lysogenic = integration as a prophage, with later switch possible
• Enveloped viruses = fragile; naked viruses = environmental survivors

Microbiology for the MCAT

Part 5 of 7 — Fungi, Parasites & Prions

Fungi

Fungal Reproduction

• Asexual: Budding (yeasts), spore formation
• Sexual: Occurs under stress conditions

Parasitology (Key MCAT Parasites)

Prions

• Misfolded proteins (PrP$^{Sc}$) — NO nucleic acid
• Convert normal PrP$^{C}$ to the misfolded form
• Cannot be sterilized by standard methods (resist heat, UV, chemicals)
• Cause spongiform encephalopathies (BSE, CJD, kuru)

Worked Example — Why Prions Defy Sterilization

Scenario: A passage describes surgical instruments contaminated by a patient with Creutzfeldt-Jakob disease (CJD). Standard autoclaving (which reliably kills bacteria, viruses, and fungal spores) fails to make the instruments safe. A question asks why prions resist this treatment that destroys every other infectious agent.

Step 1 — Identify what the infectious agent is made of. A prion is only a misfolded protein (PrP$^{Sc}$) — it contains no nucleic acid (no DNA or RNA) and no membrane or cell structure.

Step 2 — Recall how standard sterilization works. Autoclaving, UV, and many disinfectants kill pathogens by damaging nucleic acids and disrupting membranes/proteins enough to halt replication. A prion has no genome to damage and no membrane to lyse, so these methods miss their usual targets.

Step 3 — Explain the propagation mechanism. The misfolded PrP$^{Sc}$ acts as a template, forcing normal cellular PrP$^{C}$ to refold into the pathogenic shape — a chain reaction that needs no replication machinery. The aggregated, $\beta$-sheet-rich form is also extraordinarily heat- and protease-stable.

MCAT takeaway: Prions are infectious proteins with no nucleic acid, so genome-targeting sterilization fails. They propagate by templating the misfolding of normal host protein, and require extreme measures (prolonged high-temperature autoclaving with NaOH or incineration) to inactivate.

Fungi & Parasites 🎯

Key Takeaways — Part 5

• Fungi: chitin cell wall, ergosterol in membrane (drug target) — no peptidoglycan, so $\beta$-lactams fail
• Dimorphic fungi switch between yeast and mold forms, often by temperature
• Malaria (Plasmodium) is transmitted by the Anopheles mosquito
• Prions: misfolded proteins with NO nucleic acid that template host protein misfolding
• Antifungals target ergosterol (azoles, amphotericin B) or chitin — not peptidoglycan

Microbiology for the MCAT

Part 6 of 7 — Immune Response to Infection

First Line of Defense (Barriers)

• Skin (physical), mucous membranes, stomach acid, lysozyme (tears/saliva), normal flora

Second Line (Innate Immune Response)

Third Line (Adaptive Immune Response)

MHC Presentation (High Yield)

• MHC class I: on all nucleated cells; presents endogenous (intracellular) antigen to CD8+ cytotoxic T cells
• MHC class II: on antigen-presenting cells (APCs); presents exogenous antigen to CD4+ helper T cells

Vaccination

• Active immunity: Exposure to antigen → immune response → memory (vaccines, natural infection)
• Passive immunity: Receiving pre-formed antibodies (maternal IgG, antiserum)
  • Passive = immediate but temporary. Active = delayed but long-lasting.

Worked Example — Mounting a Response to a Virus-Infected Cell

Scenario: A passage tracks how the immune system eliminates a cell already infected by a virus. The infected cell is displaying viral peptides. A question asks which T cell recognizes it, and on which MHC molecule the peptide is presented.

Step 1 — Classify the antigen source. The virus replicates inside the cell, so viral proteins are endogenous (made within the cytoplasm), not engulfed from outside.

Step 2 — Match antigen source to MHC class. Endogenous antigen is loaded onto MHC class I, which essentially every nucleated cell expresses. (Exogenous antigen taken up by phagocytosis would be presented on MHC class II by APCs instead.)

Step 3 — Match MHC class to the responding T cell. A useful mnemonic is the "rule of 8": MHC class I $\times$ CD8 = 8, and MHC class II $\times$ CD4 = 8. So MHC I presents to CD8+ cytotoxic T cells, which recognize the infected cell and trigger apoptosis (via perforin/granzyme).

Step 4 — Note the innate backup. If the virus downregulates MHC I to hide, NK cells detect the "missing self" and kill the cell without needing antigen specificity.

MCAT takeaway: Endogenous (intracellular) antigen → MHC I → CD8+ cytotoxic T cells (kill the infected cell). Exogenous antigen → MHC II → CD4+ helper T cells. Remember the rule of 8, and that NK cells cover cells that lose MHC I.

Immune Response 🎯

Key Takeaways — Part 6

• Three lines of defense: barriers → innate (neutrophils, complement, NK cells) → adaptive (B and T cells)
• MHC I (endogenous antigen) → CD8+ cytotoxic T cells; MHC II (exogenous antigen) → CD4+ helper T cells (rule of 8)
• Active immunity: long-lasting, requires time. Passive: immediate but temporary.
• Neutrophils = first responders. Macrophages = APCs + phagocytes. NK cells = kill "missing self."
• Vaccines = active immunity (memory cells formed)

Microbiology for the MCAT

Part 7 of 7 — Immune Disorders & Clinical Microbiology

Immune System Disorders

Antibody Classes (HIGH YIELD)

Mnemonic: "Greatest amount, M comes first, A on All surfaces, E for allErgies"

Antibody Structure

• Y-shaped: 2 heavy chains + 2 light chains, joined by disulfide bonds
• Fab region (variable): binds antigen (defines specificity)
• Fc region (constant): determines class and binds immune effectors (complement, phagocytes)

Koch's Postulates

1. Organism found in all cases of disease
2. Organism isolated and grown in pure culture
3. Cultured organism causes disease in a healthy host
4. Organism re-isolated from the new host = original organism

Worked Example — Reading an Antibody Titer (Acute vs. Past Infection)

Scenario: A passage gives serology for a patient: IgM against pathogen X is high, but IgG against pathogen X is low/absent. A second patient has the reverse: low IgM, high IgG. The question asks which patient has a current/recent infection and which has prior exposure or immunity.

Step 1 — Recall the order of the antibody response. On first exposure (primary response), IgM is produced first (it is a pentamer, good at early agglutination). Days later, class switching yields IgG, the dominant long-term antibody that also provides memory.

Step 2 — Interpret patient 1 (high IgM, low IgG). A dominant IgM signal indicates the immune system is in the early phase of a primary response — an acute / recent infection.

Step 3 — Interpret patient 2 (low IgM, high IgG). High IgG with little IgM indicates the response has matured or the exposure was in the past (or via vaccination) — i.e., prior infection / established immunity. A rapid IgG spike on re-exposure reflects memory (secondary response).

Step 4 — Connect to the Fc/Fab logic. Whatever the class, the Fab region's specificity is what binds pathogen X; the Fc (constant) region is what defines the IgM-vs-IgG class being measured.

MCAT takeaway: IgM up first = acute/recent; IgG high (IgM low) = past exposure or immunity. IgM is a pentamer (10 binding sites) excellent at early agglutination; IgG is the most abundant serum antibody, crosses the placenta, and drives the memory (secondary) response.

Immune Disorders 🎯

Microbiology — Complete! ✅

From bacteria to viruses to immune function, microbiology bridges molecular biology with clinical medicine. The MCAT loves testing infectious-disease mechanisms, immune responses, and antibody functions.

Final high-yield recall:

• IgM = first responder (pentamer); IgG = most abundant + crosses placenta + memory; IgA = mucosal; IgE = allergies/parasites
• Antibody specificity comes from the Fab (variable) region; class/effector function from Fc (constant)
• HIV destroys CD4+ T cells; SCID lacks both T and B cells
• Koch's postulates establish microbial causation of disease