General Principles: Prokaryote vs eukaryote, Gram stain & cell walls, growth curves, biofilms & quorum sensing, horizontal gene transfer (transformation, transduction, conjugation).
Bacteriology: Gram-positive cocci/rods, Gram-negative cocci/rods, aerobes/anaerobes, acid-fast organisms, spore formers, key virulence factors (toxins, capsules, adhesins, immune evasion).
Virology: DNA vs RNA, enveloped vs non-enveloped, replication sites, tropism, oncogenic viruses, antigenic drift vs shift, latency and reactivation.
Mycology: Yeasts vs moulds vs dimorphic fungi, opportunists in immunocompromise, superficial/cutaneous vs invasive disease.
Parasitology: Protozoa (GI, blood/tissue), helminths (nematodes, trematodes, cestodes), ectoparasites; life cycles and transmission.
Host Response & Immunity: Innate sensors (TLRs), complement, opsonisation, Th1/Th2/Th17 responses, granulomatous inflammation, immune-mediated pathology.
Laboratory Diagnostics: Microscopy/stains, culture media & ID, susceptibility testing (MIC/zone), antigen/antibody assays, nucleic acid tests, MALDI-TOF, point-of-care tests.
Antimicrobials: Classes & mechanisms, spectrum, PK/PD (time vs concentration-dependent killing), resistance mechanisms, stewardship principles.
Infection Control: Transmission-based precautions, outbreak recognition, device-associated infections, decontamination (incl. spores), vaccination strategies & herd effects.

🧱 Cell wall logic – stain & structure

Gram-positive: Thick peptidoglycan with teichoic acids; retain crystal violet (purple). Sensitive to β-lactams/glycopeptides unless altered PBPs or thickened walls.
Gram-negative: Thin peptidoglycan + outer membrane with LPS (endotoxin; lipid A triggers TLR4 → cytokine storm). Porins gate entry; periplasm harbours β-lactamases.
Acid-fast: Mycolic acids (Nocardia/Mycobacterium) resist Gram stain; Ziehl–Neelsen or auramine O required.
Spores: Bacillus/Clostridium form heat-resistant spores (dormant, metabolically inert). Sterilisation requires steam under pressure (autoclave).

🧪 Virulence factors – how bugs win

Adhesins & biofilms: Pili/fimbriae (E. coli in UTI), polysaccharide matrices (Pseudomonas on devices) resist antibiotics and immunity.
Toxins: A–B exotoxins (diphtheria, cholera, pertussis) hijack host signalling; superantigens (TSST-1) cause cytokine storms; neurotoxins (tetanus, botulinum) alter SNAREs.
Immune evasion: Capsules (S. pneumoniae), antigenic variation (Neisseria pili), intracellular survival (Listeria, Salmonella, TB) within macrophages.

🦠 Virology essentials – families & replication

DNA viruses: Mostly nuclear replication (exception: pox in cytoplasm). Examples: Herpes (latent in ganglia/lymphoid), Adeno (pharyngitis/conjunctivitis), Papilloma (HPV oncogenesis).
RNA viruses: Mostly cytoplasmic (exceptions: influenza in nucleus; retroviruses integrate). Positive-sense act as mRNA (Picorna, Flavi), negative-sense need RNA-dependent RNA polymerase (Orthomyxo, Paramyxo, Rhabdo).
Envelopes: Fragile to detergents/ethanol; non-enveloped survive gut/environment (e.g., norovirus, adenovirus).

🍄 Fungi & 🪱 Parasites – high-yield identities

Yeasts: Candida (pseudohyphae; thrush, candidaemia), Cryptococcus (encapsulated; meningoencephalitis in immunosuppression).
Moulds: Aspergillus (acute-angle septate hyphae; allergic bronchopulmonary aspergillosis, aspergilloma, angioinvasion).
Dermatophytes: Trichophyton/Microsporum/Epidermophyton - skin/nails (“tinea”).
Protozoa: Giardia (steatorrhoea), Entamoeba (dysentery/liver abscess), Plasmodium (cyclical fevers; RBC stage), Toxoplasma (foetal disease, encephalitis).
Helminths: Enterobius (perianal itch), Ascaris (eosinophilia, pulmonary migration), Taenia (cysticercosis with neuro involvement).

📏 Core Tables – Bugs, Traits & First-Line Thinking

Group	Key organisms	Traits & tests	Classic diseases	Initial therapy notes*
Gram+ cocci	Staph aureus; Strep pyogenes (GAS); Strep pneumoniae; Enterococcus	Staph: clusters (catalase+); aureus coagulase+; Strep: chains (catalase−), haemolysis patterns; Enterococcus bile/esculin+	Skin/soft tissue, osteomyelitis, endocarditis, pneumonia, pharyngitis, cellulitis	Fluclox/anti-staph; consider MRSA (vanc/linezolid). Pen V for strep throat; amox for pneumococcus if sensitive.
Gram− rods (Enterics)	E. coli, Klebsiella, Proteus, Salmonella, Shigella, Campylobacter	Lactose fermenters (MacConkey pink); H2S producers (black on TSI); motility/urease vary	UTI, gastroenteritis, biliary infections, typhoid, dysentery	Nitrofurantoin or trimethoprim for simple UTI; broader β-lactam/βLI for complicated; check local resistance.
Non-fermenters	Pseudomonas aeruginosa, Acinetobacter	Oxidase+, fruity odour (Pseudomonas), biofilms, intrinsic resistance	HAP/VAP, CF bronchiectasis infections, burns, device infections	Antipseudomonal β-lactams (piperacillin-tazobactam, ceftazidime), ± aminoglycoside; tailor to susceptibility.
Fastidious/atypical	Mycoplasma, Chlamydia, Legionella	No cell wall (Mycoplasma), intracellular (Chlamydia), special media/urinary antigen (Legionella)	Atypical pneumonia, NGU	Macrolide/doxycycline/resp quinolone (cell wall agents ineffective for Mycoplasma/Chlamydia).
Acid-fast	Mycobacterium tuberculosis complex; NTM	Ziehl–Neelsen/auramine; slow growth, granulomas	TB (pulmonary/extrapulmonary), MAC in immunosuppressed	RIPE (rifampicin, isoniazid, pyrazinamide, ethambutol) with DOT and public health notification.
Neisseria	N. meningitidis; N. gonorrhoeae	Gram− diplococci, oxidase+; fastidious; different sugar use	Meningitis/sepsis; urethritis/PID	Empiric meningitis: cefotaxime/ceftriaxone; STI: ceftriaxone + partner mgmt per guidance.
Anaerobes	Bacteroides fragilis, Clostridioides difficile, Clostridium spp.	Foul odour, gas; spores (Clostridium)	Intra-abdominal abscess, C. diff colitis, tetanus/botulism, gas gangrene	Metronidazole for anaerobes; oral vancomycin/fidaxomicin for C. diff; surgery if necrosis.
DNA viruses	HSV/VZV, HPV, HBV, Adenovirus	Latency (HSV), oncogenic E6/E7 (HPV)	Herpes, shingles, warts/cancer, hepatitis B	Acyclovir/valaciclovir (HSV/VZV); vaccine for HBV/HPV; consider topical/ablative for warts.
RNA viruses	Influenza, RSV, Measles/Mumps/Rubella, SARS-CoV-2, Norovirus	Enveloped respiratory vs hardy enteric; syncytia (paramyxo)	ARIs, bronchiolitis, exanthems, gastroenteritis, COVID-19	Supportive ± neuraminidase inhibitor (flu) early; immunisation prevents MMR; infection control crucial.
Fungi	Candida, Aspergillus, Cryptococcus, Dermatophytes	Yeast vs mould morphology; galactomannan/β-D-glucan assays	Candidaemia, aspergillosis, cryptococcal meningitis, tinea	Echinocandin for candidaemia; triazole for aspergillus; amphotericin B/fluconazole for cryptococcus.
Protozoa	Giardia, Entamoeba, Plasmodium, Toxoplasma	Oocysts/trophozoites; thick/thin films for malaria	Malabsorption, dysentery, malaria, congenital infections	Metronidazole (Giardia/Entamoeba); malaria regimen by species & resistance; spiramycin/pyrimethamine-sulfa for toxo.

*Therapy examples are illustrative; always follow local policies and susceptibility results.

💊 Antimicrobial classes – mechanism & resistance

Cell wall: β-lactams (PBPs; resistance via β-lactamases, PBP changes e.g. mecA in MRSA), glycopeptides (bind D-Ala-D-Ala; VRE makes D-Ala-D-Lac).
Protein synthesis: Aminoglycosides (30S misreading; need O\(_2\)), tetracyclines (30S tRNA block), macrolides & clindamycin (50S translocation; erm methylation → MLSB resistance), linezolid (50S initiation).
DNA/RNA: Fluoroquinolones (gyrA/parC targets; QRDR mutations), rifampicin (RNAP; rapid resistance solo), metronidazole (free radicals in anaerobes).
Metabolic: Trimethoprim–sulfamethoxazole (folate pathway), isoniazid (mycolic acids), dapsone (leprosy), ethambutol (arabinosyl transferase).
Antifungals: Azoles (ergosterol synthesis), echinocandins (β-glucan), amphotericin B (membrane pores), flucytosine (nucleic acid).
Antivirals: NRTIs/NNRTIs/INSTIs/protease inhibitors (HIV), aciclovir class (HSV; requires viral TK), neuraminidase inhibitors (influenza), direct-acting antivirals for HCV.

📈 PK/PD pearls

Time-dependent killing: β-lactams - efficacy ∝ time above MIC; extended/continuous infusions may help.
Concentration-dependent: Aminoglycosides - high peak/MIC ratio; once-daily dosing with post-antibiotic effect.
AUC/MIC: Vancomycin & fluoroquinolones - target exposure over 24 h relative to MIC.

🧭 Pathogen Maps & Diagnostic Algorithms

Scenario	Likely pathogens	First tests	Next steps / Interpretation
Community pneumonia	S. pneumoniae; H. influenzae; atypicals (Mycoplasma, Chlamydia, Legionella); viruses	CXR; sputum Gram & culture; viral PCR; CRP	Urinary antigens (Strep pneumo/Legionella) if severe; consider atypical cover if dry cough/low WCC.
UTI (uncomplicated)	E. coli > others	Urinalysis (nitrite/leukocytes); MSU culture if recurrent/complicated	Nitrites imply Enterobacterales; negative nitrite doesn’t exclude infection (low bladder dwell). Tailor to culture.
Meningitis	N. meningitidis, S. pneumoniae; HSV (encephalitis)	Blood cultures; urgent antibiotics; LP if safe (opening pressure, cell count, glucose, protein), PCR	Bacterial: neutrophils, low glucose, high protein; viral: lymphocytes, normal glucose. Escalate per severity.
Sepsis of unknown origin	Broad (Gram+ cocci, Gram− rods, anaerobes)	2× blood cultures pre-antibiotic; lactate; full septic screen	Source control early; de-escalate when susceptibilities return (“Start smart then focus”).
GI infection	Norovirus/rotavirus; Campylobacter; Salmonella; Shigella; C. difficile (post-antibiotic)	Stool PCR panel/antigen; C. diff toxin + GDH	Blood/mucus/fever → invasive pathogen likely; avoid anti-motility in bloody diarrhoea. Isolate suspected C. diff.
Device infection (line/prosthesis)	Coagulase-negative staph; S. aureus; Gram−; Candida	Blood cultures (peripheral + line); imaging if deep focus	Biofilm risk → remove/replace device where feasible; consider echinocandin if candidaemia.

🧑‍🏫 Exam hack: Name the site, the likely flora, and the host factor (age, device, immunosuppression). That triad drives the short list.

🔬 Gram stain & culture – rapid flow

From sample to species: Assess quality (e.g. sputum squamous cells), Gram stain → early morphology, culture on selective/differential media (blood, chocolate, MacConkey), ID by MALDI-TOF, susceptibilities by disc diffusion/MIC. Remember: anaerobes require special transport/culture, and prior antibiotics may sterilise cultures - use PCR where appropriate.

🧪 Serology & NAATs – when to use

NAAT/PCR: High sensitivity for low-burden or fastidious organisms (STIs, respiratory viruses, TB complex).
Serology: Useful for window periods or past exposure (HBV profile, EBV). Paired sera for rising titres in some infections.
Antigen tests: Rapid but variable accuracy; confirm negatives if clinical suspicion high.

🩺 Clinical Syndromes, Patterns & Pitfalls

Endocarditis: Native valve: viridans strep, S. aureus; prosthetic: coagulase-negative staph early, enterococci. Duke criteria; obtain multiple blood cultures before antibiotics when stable.
Cellulitis vs necrotising infection: Rapid pain, systemic toxicity, crepitus, bullae suggest necrotising fasciitis (polymicrobial or GAS) → urgent surgery + broad spectrum incl. clindamycin (toxin suppression).
TB thinking: Chronic cough, weight loss, night sweats; upper lobe cavitation; risk groups; notify public health; screen contacts; consider extrapulmonary disease.
Meningococcal sepsis: Non-blanching petechiae/purpura, shock, DIC; give immediate therapy; prophylaxis for close contacts per guidance.
Catheter-associated UTI & CRBSI: Often biofilm-related; removing device is key; don’t treat asymptomatic bacteriuria except in pregnancy or prior to urological procedures.

🧯 Resistance patterns – read like a detective

ESBL/AmpC: Resistance to penicillins/cephalosporins (± cephamycins); carbapenems often active (but watch for KPC/NDM).
MRSA: mecA (PBP2a); β-lactams (except ceftaroline) inactive; options include vancomycin/linezolid/daptomycin (not for pneumonia).
VRE: D-Ala-D-Lac; linezolid/daptomycin useful; infection control to prevent spread.
Pseudomonas: Porin loss + efflux; require antipseudomonal agents; avoid narrow Gram-positive drugs.

🧬 Vaccines & herd effects

Vaccines reduce susceptible hosts, lowering effective reproduction number \(R_e = R_0 (1 - v e)\), where \(v\) is coverage and \(e\) effectiveness. Herd immunity threshold \(\approx 1 - \tfrac{1}{R_0}\). Live attenuated (MMR, VZV) induce cellular + humoral responses but contraindicated in severe immunosuppression; inactivated/subunit (influenza, HPV) safer with boosters.

🧼 Infection control – simple wins

Hand hygiene before/after patient contact; alcohol gel ineffective for spores (use soap & water + chlorine for C. diff).
Isolation for droplet/airborne/contact as indicated; cohort during outbreaks.
Stewardship: review at 48–72 h, narrow spectrum, right dose/route/duration; document indication and stop/review date.

🧠 Study & Exam Tips

Triad method: For any case, state host factor (age/comorbidity), exposure/site, syndrome → derive 3–4 likely organisms and appropriate tests.
Mechanism → manifestation: Toxin vs invasion vs immune-mediated damage explains the clinical picture and directs therapy (e.g., clindamycin to suppress toxin in GAS/Clostridia).
Map the drug class to the wall or ribosome: If you can draw where a drug hits, you’ll predict resistance and side-effects.
Data literacy: Interpret culture reports (contaminant vs pathogen), MICs vs breakpoints, and pre-test probability for PCR/antigen tests.
Spaced retrieval: Make one-page bug–drug charts, and rehearse “first-line-if allergic-if severe-IV to PO switch”.

🚀 Rapid drill (answers hidden)

Why do β-lactams fail in MRSA? Ans: Altered PBP (PBP2a) with low β-lactam affinity (mecA).
Which GI protozoan causes steatorrhoea after camping? Ans: Giardia lamblia.
Which fungal assay helps in invasive aspergillosis? Ans: Galactomannan (± β-D-glucan).
Acid-fast bacilli with upper-lobe cavitation: which regimen? Ans: RIPE initially, then continue per sensitivities.
Why add clindamycin in necrotising fasciitis? Ans: Inhibits toxin production and maintains effect in stationary-phase bacteria.

Final thought: Identify the site, the likely flora, and a host factor - then choose tests and empiric therapy you can justify. Micro is pattern-rich; practise those patterns. 💪

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Premedical Microbiology 🧫

🔢 Key Areas of Study

📏 Core Tables – Bugs, Traits & First-Line Thinking

🧭 Pathogen Maps & Diagnostic Algorithms

🩺 Clinical Syndromes, Patterns & Pitfalls

🧠 Study & Exam Tips

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