Master Microbiology
for FMGE

Core Concepts

Microbiology for FMGE focuses on agents causing infectious diseases. Key areas include:

• Bacteriology: Gram-positive (cocci: Staph, Strep; rods: Clostridia, Listeria) vs. Gram-negative (cocci: Neisseria; rods: Enterobacteriaceae, Pseudomonas, Haemophilus, Vibrio). Atypical bacteria (Mycoplasma lacks cell wall, Chlamydia/Rickettsia are obligate intracellular). Mycobacteria are acid-fast. Spore-forming bacteria (Bacillus, Clostridium) are highly resistant. Virulence factors: endotoxins (LPS of Gram-negatives), exotoxins (e.g., botulinum, tetanus, diphtheria toxins), capsules. Antibiotic resistance: Beta-lactamases, altered target sites, efflux pumps.
• Virology: Classification by nucleic acid (DNA/RNA, single/double-stranded) and envelope status. Key families: Herpesviridae (HSV, VZV, CMV, EBV), Hepadnaviridae (HBV), Flaviviridae (HCV, Dengue), Orthomyxoviridae (Influenza), Retroviridae (HIV). Latency (e.g., herpesviruses) and antigenic drift/shift (e.g., influenza) are important concepts.
• Mycology: Yeasts (Candida, Cryptococcus) are unicellular; Molds (Aspergillus, Mucor) are filamentous. Dimorphic fungi (Histoplasma, Blastomyces, Coccidioides, Sporothrix) exist as mold in cold, yeast in heat. Cell wall contains ergosterol (target for antifungals). Opportunistic infections are common in immunocompromised.
• Parasitology: Protozoa (single-celled: Amoeba, Giardia, Plasmodium, Leishmania, Trypanosoma) and Helminths (worms: Nematodes/roundworms, Cestodes/tapeworms, Trematodes/flukes). Understanding life cycles (infective vs. diagnostic stages, intermediate hosts) is crucial.
• Sterilization & Disinfection: Autoclaving (moist heat, gold standard), Dry heat, Filtration, Radiation, Chemical disinfectants (alcohols, chlorhexidine, iodine).

Clinical Presentation

• Systemic: Fever of unknown origin (FUO), Sepsis/Septic shock (tachycardia, hypotension, altered mental status).
• CNS: Meningitis (fever, headache, neck stiffness, photophobia), Encephalitis (altered consciousness, seizures, focal neurological deficits).
• Respiratory: Pneumonia (cough, fever, dyspnea, pleuritic chest pain), Tuberculosis (chronic cough, weight loss, night sweats).
• Gastrointestinal: Diarrhea (acute/chronic, watery/bloody), Dysentery (bloody, mucoid stool with tenesmus), Food poisoning (rapid onset nausea, vomiting, diarrhea).
• Urinary: UTI (dysuria, frequency, urgency, suprapubic pain), Pyelonephritis (flank pain, fever, chills).
• Skin/Soft Tissue: Cellulitis (erythema, warmth, swelling), Abscess, Erysipelas (sharply demarcated erythema).
• STIs: Genital ulcers, urethral/vaginal discharge, lymphadenopathy.
• Opportunistic Infections: In immunocompromised (e.g., HIV, transplant), presentations can be atypical or severe (e.g., Pneumocystis pneumonia, CMV retinitis).

Diagnosis (Gold Standard)

Diagnosis involves a combination of direct and indirect methods:

• Microscopy: Gram stain (bacteria), ZN stain (Mycobacteria, Nocardia), KOH mount (fungi), India ink (Cryptococcus), Wet mount (Trichomonas), Peripheral blood smear (Malaria, Filaria, Leishmania - Giemsa stain).
• Culture: Bacterial (blood, urine, CSF, sputum, pus), Fungal (Sabouraud Dextrose Agar), Viral (cell culture - less common in routine lab, rather PCR). Gold standard for bacterial identification and susceptibility testing.
• Molecular methods: PCR/NAAT (Nucleic Acid Amplification Tests) are highly sensitive and specific, crucial for fast diagnosis of TB (GeneXpert), HIV viral load, HPV, Chlamydia, Gonorrhea, C. difficile.
• Serology: Detection of antibodies (IgM for acute, IgG for past infection/immunity) or antigens. Examples: HIV (ELISA + Western Blot/confirmatory tests), Hepatitis panel (HBsAg, anti-HBs, anti-HBc), Dengue (NS1 antigen, IgM), Syphilis (VDRL/RPR + TPPA/FTA-ABS).
• Imaging: Chest X-ray/CT (pneumonia, TB), MRI (CNS infections), Ultrasound.

Management (First Line)

Management principles are empiric therapy (broad-spectrum based on likely pathogen and site) followed by targeted therapy (narrow-spectrum based on culture and sensitivity).

• Antibacterial: Beta-lactams (penicillins, cephalosporins, carbapenems), Macrolides (azithromycin), Fluoroquinolones (ciprofloxacin, levofloxacin), Aminoglycosides (gentamicin), Tetracyclines (doxycycline), Glycopeptides (vancomycin for MRSA), Metronidazole (anaerobes, some protozoa). Resistance monitoring is vital (e.g., MRSA, ESBL, CRE).
• Antiviral: Acyclovir (HSV, VZV), Ganciclovir (CMV), Oseltamivir (Influenza), HAART (HIV), Direct-acting antivirals (HCV), NRTIs (HBV).
• Antifungal: Azoles (fluconazole, voriconazole), Polyenes (Amphotericin B - systemic, Nystatin - topical), Echinocandins (caspofungin), Terbinafine (dermatophytes).
• Antiparasitic: Metronidazole (Amoebiasis, Giardiasis, Trichomoniasis), Albendazole/Mebendazole (helminths), Praziquantel (trematodes, cestodes), Chloroquine/Artemisinin-based Combination Therapy (ACT) (Malaria).
• Infection Control: Hand hygiene, PPE, sterilization/disinfection, isolation.

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