🦠 NRE Step 1 — Microbiology
NRE Step 1 Microbiology High-Yield Topics 2026 — Organisms, Antibiotics & Clinical Scenarios
The bacteria, viruses, fungi, and parasites that drive NRE clinical decisions — and the antibiotic logic that connects them to treatment.
A 3-year-old child presents with fever, neck stiffness, photophobia, and a non-blanching petechial rash spreading across the trunk. The child’s condition is deteriorating rapidly. What is the most likely causative organism, what is the single most important immediate action before any investigation, and what antibiotic do you give?
This is NRE microbiology. It does not ask you to classify bacteria by Gram stain in isolation. It places a critically ill patient in front of you and asks whether you can identify the organism from clinical clues, order the right investigation, and start the right treatment — in the right sequence.
Microbiology in NRE Step 1 is tested in two ways: directly, as organism identification and antibiotic choice questions, and indirectly, embedded throughout medicine, surgery, paediatrics, and community health questions. Graduates who study microbiology as a list of organisms fail. Those who learn it as a framework for clinical decision-making score well across multiple subjects.
📌 How Microbiology is Weighted in NRE Step 1
Direct microbiology questions contribute approximately 6–10% of NRE Step 1. However, antibiotic choice and infection management is embedded across clinical medicine, surgery, paediatrics, and community health — making microbiology knowledge relevant to an estimated 20–25% of total questions. This article covers both the direct and embedded content.
1. High-Yield Bacteria — Gram-Positive
Gram staining is the first branching point in bacterial identification and the foundation of empirical antibiotic choice. For NRE, every organism you learn must be linked to its clinical syndrome, its key diagnostic feature, and its treatment.
| Organism | Gram Stain / Morphology | Clinical Syndrome | Key Diagnostic Feature | Treatment | Frequency |
|---|---|---|---|---|---|
| Streptococcus pneumoniae | G+ diplococci | Community pneumonia, meningitis, otitis media, sinusitis | Lobar consolidation on CXR; rusty sputum; bile-soluble; optochin sensitive | Amoxicillin (community); ceftriaxone (meningitis) | Very High |
| Staphylococcus aureus | G+ cocci in clusters | Skin/soft tissue infections, osteomyelitis, endocarditis, food poisoning (preformed toxin), toxic shock syndrome | Coagulase positive; golden colonies; catalase positive | Flucloxacillin (MSSA); vancomycin (MRSA) | Very High |
| MRSA | G+ cocci in clusters | Hospital-acquired infections, wound infections, pneumonia in ICU, bacteraemia | Resistant to all beta-lactams; risk factors: hospitalisation, catheters, surgery | Vancomycin or teicoplanin; linezolid | Very High |
| Streptococcus pyogenes (Group A Strep) | G+ cocci in chains | Pharyngitis, cellulitis, impetigo, rheumatic fever, glomerulonephritis (post-strep) | Beta-haemolytic; bacitracin sensitive; ASO titre raised in post-strep complications | Penicillin V (pharyngitis); flucloxacillin (skin) | Very High |
| Streptococcus agalactiae (Group B Strep) | G+ cocci in chains | Neonatal meningitis and sepsis; maternal UTI and chorioamnionitis | Beta-haemolytic; bacitracin resistant; CAMP test positive | IV penicillin (intrapartum prophylaxis if GBS+) | High |
| Clostridium difficile | G+ rod (anaerobe) | Antibiotic-associated diarrhoea; pseudomembranous colitis | After broad-spectrum antibiotics (especially clindamycin, cephalosporins); toxin detected in stool; sigmoidoscopy: yellow-white plaques | Stop offending antibiotic; oral metronidazole (mild-moderate); oral vancomycin (severe) | Very High |
| Clostridium tetani | G+ rod (anaerobe, spore-forming) | Tetanus: trismus (lockjaw), risus sardonicus, opisthotonus, spasms | Puncture wound history; exotoxin blocks inhibitory neurotransmission | Wound debridement + TIG (tetanus immunoglobulin) + metronidazole + diazepam for spasms | High |
| Clostridium perfringens | G+ rod (anaerobe) | Gas gangrene; food poisoning (watery diarrhoea, no vomiting, 8–24 hrs) | Crepitus in tissue; X-ray shows gas in soft tissue planes | Surgical debridement + IV penicillin + metronidazole; hyperbaric O₂ | High |
| Listeria monocytogenes | G+ rod | Meningitis in neonates, elderly, immunocompromised, pregnancy; granulomatosis infantiseptica | Tumbling motility; grows at 4°C (refrigerator); associated with unpasteurised dairy, deli meats | Ampicillin (NOT cephalosporins — intrinsic resistance) | High |
🔑 Neonatal Meningitis Organisms — NRE Always Tests This
The causative organisms differ by age group — a favourite NRE pattern. Neonates (0–3 months): Group B Strep (#1), E. coli, Listeria → treatment: ampicillin + gentamicin. Infants/children (3 months–18 years): Neisseria meningitidis, S. pneumoniae, H. influenzae → ceftriaxone. Adults: S. pneumoniae (#1), N. meningitidis → ceftriaxone + dexamethasone. Elderly/immunocompromised: S. pneumoniae + Listeria → ceftriaxone + ampicillin. The Listeria–ampicillin–elderly triad appears repeatedly as an MCQ.
2. High-Yield Bacteria — Gram-Negative
| Organism | Morphology | Clinical Syndrome | Key Feature | Treatment | Frequency |
|---|---|---|---|---|---|
| Neisseria meningitidis | G− diplococci (intracellular) | Bacterial meningitis; septicaemia; Waterhouse-Friderichsen syndrome (bilateral adrenal haemorrhage) | Non-blanching petechial/purpuric rash; give IM penicillin BEFORE transfer; rifampicin prophylaxis for contacts | IV ceftriaxone; prophylaxis: rifampicin or ciprofloxacin | Very High |
| Neisseria gonorrhoeae | G− diplococci (intracellular) | Urethritis, cervicitis, PID, epididymo-orchitis, septic arthritis, ophthalmia neonatorum | Gram stain of urethral discharge: intracellular diplococci; NAAT most sensitive | Ceftriaxone IM single dose (+ azithromycin for co-treatment of Chlamydia) | Very High |
| Haemophilus influenzae | G− coccobacilli | Epiglottitis (type B — now rare with vaccine); otitis media, sinusitis, exacerbations of COPD | Epiglottitis: “cherry red” epiglottis, drooling, tripod position — do NOT examine throat → risk of complete airway obstruction | Co-amoxiclav or cefuroxime; epiglottitis → secure airway first | High |
| Escherichia coli | G− rod (facultative anaerobe) | UTI (#1 cause), neonatal meningitis, traveller’s diarrhoea (ETEC), HUS (EHEC O157:H7) | Nitrite positive on urine dipstick (urease-producing); HUS: diarrhoea + AKI + thrombocytopaenia — do NOT give antibiotics (increases HUS risk) | Trimethoprim (uncomplicated UTI); co-amoxiclav or cephalosporin (complicated) | Very High |
| Klebsiella pneumoniae | G− rod (encapsulated) | Hospital-acquired pneumonia; UTI (especially in diabetics); liver abscess | “Currant jelly” sputum (bloody, mucoid); upper lobe cavitation; common in alcoholics and diabetics | 3rd-gen cephalosporin or carbapenem (many produce ESBLs) | High |
| Pseudomonas aeruginosa | G− rod (non-fermenter) | Hospital pneumonia (ventilated patients), burn wound infections, otitis externa, UTI in catheterised patients, CF lung infections | Blue-green pigment (pyocyanin); fruity grape-like odour; intrinsically resistant to many antibiotics | Piperacillin-tazobactam or ceftazidime ± aminoglycoside | High |
| Salmonella typhi | G− rod | Typhoid fever: stepwise fever, rose spots, relative bradycardia, splenomegaly, constipation then diarrhoea | Widal test (agglutination — low specificity); blood culture in first week (gold standard); bone marrow culture most sensitive throughout | Ciprofloxacin or ceftriaxone (chloramphenicol in endemic areas) | Very High |
| Shigella spp. | G− rod (non-motile) | Bacillary dysentery: bloody diarrhoea + mucus + tenesmus; low infective dose | Stool microscopy: WBCs; culture gold standard; HUS can complicate S. dysenteriae | Ciprofloxacin or azithromycin; avoid antidiarrhoeals | High |
| Helicobacter pylori | G− spiral rod (urease-positive) | Peptic ulcer disease (#1 cause of duodenal ulcer), gastric carcinoma, MALT lymphoma | CLO (rapid urease) test on biopsy most accurate; urea breath test for diagnosis and confirming eradication; stool antigen test | Triple therapy: PPI + amoxicillin + clarithromycin (7–14 days) | Very High |
| Vibrio cholerae | G− comma-shaped rod | Cholera: profuse rice-water diarrhoea, rapid dehydration, no blood/mucus | Darkfield microscopy: shooting star motility; toxin causes hypersecretion via cAMP | Oral rehydration salts (cornerstone); doxycycline to shorten duration | High |
3. Atypical & Special Bacteria
Atypical Pneumonia
Mycoplasma pneumoniae
- No cell wall → not visible on Gram stain
- Gradual onset, dry cough, young adults
- Cold agglutinins (IgM) → haemolysis
- CXR worse than clinical picture
- Tx: macrolide or doxycycline
- No beta-lactams (no cell wall target)
Atypical Pneumonia
Legionella pneumophila
- Aerosol from water systems, AC units
- Hyponatraemia + confusion + diarrhoea
- Urinary antigen test (rapid, specific)
- Does not grow on standard media
- Tx: macrolide or fluoroquinolone
- High mortality if delayed treatment
STI
Chlamydia trachomatis
- Intracellular; cannot be Gram-stained
- Most common bacterial STI worldwide
- Urethritis, cervicitis — often asymptomatic
- PID, ectopic pregnancy if untreated
- Trachoma (D–K) → blindness
- Tx: doxycycline or azithromycin
STI
Treponema pallidum (Syphilis)
- Primary: painless chancre
- Secondary: maculopapular rash (palms/soles), condylomata lata, fever
- Tertiary: gumma, aortitis, tabes dorsalis, Argyll Robertson pupil
- VDRL/RPR screening; TPHA/FTA-Abs confirmatory
- Tx: benzathine penicillin
Zoonosis
Brucella spp.
- Undulant (intermittent) fever
- Contact with animals or unpasteurised milk
- Hepatosplenomegaly, arthritis, orchitis
- Blood culture gold standard (prolonged incubation)
- Tx: doxycycline + rifampicin (6 weeks)
Food Poisoning
Timing Clues for NRE
- <6 hrs: S. aureus (preformed toxin) or B. cereus (emetic)
- 8–16 hrs: C. perfringens (watery diarrhoea, no vomit)
- 24–72 hrs: Salmonella, Shigella, Campylobacter
- Campylobacter: most common bacterial food poisoning UK; bloody diarrhoea; poultry source; Guillain-Barré association
- Tx: rehydration; antibiotics only if severe/immunocompromised
4. High-Yield Viruses
| Virus | Clinical Syndrome | Key Diagnostic Feature | Treatment / Prevention | Frequency |
|---|---|---|---|---|
| HIV | Acute retroviral syndrome → latency → AIDS (CD4 <200/µL). Opportunistic infections define AIDS: PCP, CMV retinitis, cryptococcal meningitis, toxoplasmosis, MAC | ELISA screening; Western blot confirmatory; CD4 count + viral load for monitoring | ART (highly active antiretroviral therapy) — triple therapy. PCP prophylaxis (co-trimoxazole) when CD4 <200 | Very High |
| Hepatitis B | Acute hepatitis → chronic (if HBsAg persists >6 months) → cirrhosis → HCC | HBsAg = current infection. Anti-HBs = immunity (vaccine or recovery). HBeAg = active replication. HBcAb IgM = acute. Window period: HBsAg negative, anti-HBs negative, HBcAb positive | Tenofovir or entecavir for chronic HBV. Vaccination (3-dose schedule) for prevention. | Very High |
| Hepatitis C | Acute (often asymptomatic) → 80% become chronic → cirrhosis → HCC. No vaccine available. | Anti-HCV ELISA screening; HCV RNA PCR confirmatory (also used to monitor treatment) | Direct-acting antivirals (DAAs — sofosbuvir-based regimens); >95% cure rate | Very High |
| Hepatitis A & E | Both cause acute hepatitis only (no chronic form). HEV: high mortality in pregnancy (20%) | Anti-HAV IgM (acute); faecal-oral transmission; waterborne outbreaks | Supportive only. HAV vaccine available. No HEV vaccine. | High |
| EBV (Epstein-Barr) | Infectious mononucleosis: fever, pharyngitis, cervical lymphadenopathy, splenomegaly; associated with Burkitt lymphoma, nasopharyngeal carcinoma, Hodgkin lymphoma | Monospot test (heterophile antibodies); atypical lymphocytes on blood film; Ampicillin → maculopapular rash (do NOT give in EBV pharyngitis) | Supportive; avoid contact sports (splenomegaly → rupture risk) | Very High |
| CMV | Congenital CMV (#1 congenital viral infection): periventricular calcifications, sensorineural hearing loss, chorioretinitis. CMV retinitis in AIDS (CD4 <50). | Owl eye inclusions on histology; urine/saliva PCR for congenital CMV | Ganciclovir (IV); valganciclovir (oral maintenance) | High |
| Measles | Prodrome: fever, cough, coryza, conjunctivitis. Koplik spots (pathognomonic — white spots on buccal mucosa). Then maculopapular rash (head → body). Complications: pneumonia, encephalitis, SSPE (subacute sclerosing panencephalitis) | Clinical; Koplik spots before rash appear; rising antibody titres | MMR vaccine; Vitamin A in malnourished children (reduces mortality) | Very High |
| Varicella-Zoster (VZV) | Chickenpox: vesicular rash in different stages simultaneously, pruritic, centripetal distribution. Shingles (zoster): dermatomal distribution, pain precedes rash. Ramsay Hunt: geniculate ganglion → facial palsy + ear vesicles | Clinical; Tzanck smear (multinucleated giant cells) | Chickenpox: aciclovir in immunocompromised/adults. Shingles: aciclovir within 72 hrs of rash; reduces post-herpetic neuralgia | Very High |
| Dengue | Breakbone fever: high fever, severe myalgia, retro-orbital pain, maculopapular rash. Dengue haemorrhagic fever: thrombocytopaenia + plasma leakage + bleeding | NS1 antigen (acute <5 days); dengue IgM/IgG; thrombocytopaenia + leucopaenia + raised haematocrit | Supportive: paracetamol only (NOT aspirin/NSAIDs — bleeding risk); IV fluids if DHF; platelets if <10,000 or active bleeding | Very High |
⚠️ Dengue + NSAIDs — A Guaranteed NRE Trap
A patient with dengue fever asks which painkiller to take. The NRE expects you to know that aspirin and all NSAIDs are contraindicated in dengue — they inhibit platelet function and increase the risk of haemorrhage in a patient already at risk of thrombocytopaenia. Paracetamol is the only safe analgesic/antipyretic. This appears as a direct MCQ in almost every NRE session.
5. Fungi & Parasites — Clinically Applied
High-Yield Fungal Infections
| Fungus | Clinical Syndrome | Patient at Risk | Diagnosis | Treatment |
|---|---|---|---|---|
| Candida albicans | Oral thrush, oesophagitis, vaginal candidiasis, systemic candidiasis (IV line–associated) | Immunocompromised, DM, steroid/antibiotic use, neonates, IV drug users | KOH prep: pseudohyphae; culture | Nystatin (topical/oral); fluconazole (systemic); IV caspofungin (refractory) |
| Cryptococcus neoformans | Meningitis in AIDS (CD4 <100): headache, fever, photophobia; CSF: India ink — encapsulated yeast | HIV (CD4 <100), immunocompromised | India ink stain; cryptococcal antigen in CSF/serum; raised opening pressure on LP | IV amphotericin B + flucytosine (induction); fluconazole (maintenance) |
| Aspergillus fumigatus | Aspergilloma (fungus ball in old TB cavity — haemoptysis); invasive aspergillosis (neutropaenic patients); ABPA (allergic — asthma + eosinophilia) | Neutropaenia, steroids, old TB cavity, severe asthma | CXR/CT: “halo sign” or fungus ball; serum galactomannan (invasive); skin prick test (ABPA) | Voriconazole (invasive); itraconazole (ABPA); surgical resection (aspergilloma with haemoptysis) |
| Pneumocystis jirovecii (PCP) | Pneumonia in AIDS (CD4 <200): insidious dyspnoea, dry cough, fever, bilateral interstitial infiltrates; normal CXR possible | HIV (CD4 <200), immunosuppressed | Raised LDH; BAL with methenamine silver stain; bilateral interstitial CXR pattern | High-dose co-trimoxazole; IV pentamidine if allergic; prophylaxis: co-trimoxazole when CD4 <200 |
High-Yield Parasitic Infections
| Parasite | Disease | NRE-Tested Feature | Treatment |
|---|---|---|---|
| Plasmodium falciparum | Malaria — most severe; irregular fever; cerebral malaria; anaemia; blackwater fever (haemoglobinuria) | Thick and thin blood film: banana-shaped gametocytes (falciparum specific); no Schüffner’s dots; multiple rings per RBC; RDT for rapid diagnosis | Artemisinin-based combination therapy (ACT); IV artesunate for severe malaria (NOT quinine first-line now) |
| Plasmodium vivax / ovale | Benign tertian malaria; relapse from liver hypnozoites | Schüffner’s dots in RBCs; enlarged RBCs; 48-hr fever cycle | Chloroquine + primaquine (primaquine eliminates hypnozoites; check G6PD before giving) |
| Entamoeba histolytica | Amoebic dysentery: bloody diarrhoea, flask-shaped ulcers in colon; amoebic liver abscess (right lobe — anchovy sauce pus) | Stool microscopy: trophozoites with ingested RBCs; serology for liver abscess; USS: single right lobe abscess | Metronidazole + diloxanide furoate (eliminate cysts) |
| Toxoplasma gondii | Congenital toxoplasmosis: chorioretinitis, hydrocephalus, intracranial calcifications (diffuse — contrast with CMV periventricular). AIDS: ring-enhancing brain lesions | CT head: multiple ring-enhancing lesions in AIDS; serology (IgM = acute); cats are definitive host | Pyrimethamine + sulfadiazine + folinic acid; prophylaxis: co-trimoxazole (AIDS, CD4 <100) |
| Ascaris lumbricoides | Intestinal obstruction (heavy load); Loeffler’s syndrome (pulmonary migration — eosinophilia + cough) | Stool microscopy: eggs; eosinophilia during larval migration; largest human intestinal worm | Albendazole or mebendazole |
🔑 Congenital Infections — The TORCH Framework
NRE tests congenital infections by clinical picture. Know which organism produces which neonatal features: Toxoplasma → diffuse intracranial calcifications + hydrocephalus + chorioretinitis | Other (syphilis) → saddle nose, Hutchinson’s teeth, periostitis | Rubella → cataracts + patent ductus arteriosus + sensorineural deafness (classic triad) | CMV → periventricular calcifications + sensorineural hearing loss + petechiae | Herpes simplex → vesicular skin lesions + encephalitis + eye involvement. CMV is now the most common congenital infection globally.
6. The Antibiotic Decision Framework for NRE
NRE antibiotic questions follow a consistent logic: identify the most likely organism → choose the antibiotic with the correct spectrum → avoid the drug that is contraindicated for this patient. This table organises the most commonly tested antibiotic decisions.
| Clinical Situation | First-Line Antibiotic | Key Contraindication / Pitfall |
|---|---|---|
| Community-acquired pneumonia (outpatient) | Amoxicillin ± clarithromycin (atypical cover) | Avoid fluoroquinolones first-line (mask TB in endemic areas) |
| Bacterial meningitis (unknown organism, adult) | IV ceftriaxone + IV dexamethasone | Add ampicillin if elderly/immunocompromised (Listeria cover) |
| Uncomplicated UTI (female) | Trimethoprim or nitrofurantoin | Nitrofurantoin contraindicated in eGFR <30; avoid in pregnancy at term |
| Pyelonephritis | Co-amoxiclav or ciprofloxacin (oral); IV ceftriaxone (severe) | Trimethoprim alone insufficient for upper UTI |
| Skin and soft tissue infection (MSSA) | Flucloxacillin | Penicillin V ineffective against S. aureus (penicillinase-resistant required) |
| MRSA infection | Vancomycin IV or teicoplanin | All beta-lactams ineffective against MRSA |
| Pseudomembranous colitis (C. difficile) | Stop offending antibiotic; oral metronidazole (mild); oral vancomycin (severe/recurrent) | Do NOT give anti-motility agents (loperamide) |
| TB (drug-sensitive) | 2RHEZ / 4RH | Ethambutol: check visual acuity; Pyrazinamide: contraindicated in gout/liver disease |
| Gonorrhoea (uncomplicated) | Ceftriaxone IM single dose | Always treat for co-infection with Chlamydia (azithromycin or doxycycline) |
| Syphilis (primary/secondary) | Benzathine penicillin IM | Jarisch-Herxheimer reaction: fever/rigors 2–8 hrs after first dose — warn patient; treat with antipyretics, NOT stopping antibiotics |
| Malaria (severe falciparum) | IV artesunate | Quinine no longer first-line for severe malaria; artesunate superior |
| Amoebic liver abscess | Metronidazole + diloxanide furoate | Drainage only if rupture risk or no response to antibiotics at 72 hrs |
Frequently Asked Questions
How much microbiology is directly tested in NRE Step 1?
Direct microbiology questions — organism identification, Gram stain, culture characteristics — contribute about 6–10% of NRE Step 1. However, infection management is embedded across clinical medicine, surgery, paediatrics, and community health, making microbiology knowledge relevant to an estimated 20–25% of total questions. If you know the clinical syndrome, the organism, and the antibiotic — you are equipped for questions across multiple subjects simultaneously.
Do I need to memorise Gram stain characteristics for NRE?
Yes, but at a functional level — not a laboratory technician level. You need to know: S. pneumoniae is G+ diplococci (affects antibiotic choice), N. meningitidis is G− diplococci (the intracellular feature in urethral discharge means gonorrhoea), Listeria is G+ rod (which means cephalosporins won’t work — use ampicillin). Gram stain knowledge directly drives antibiotic selection decisions in NRE MCQs.
Which microbiology topics most commonly appear in NRE for graduates from China?
Based on the clinical content of NRE and the patient population in Pakistan, the highest-yield areas for China-trained graduates are: hepatitis B serology interpretation (very commonly tested, and Chinese medical schools teach it differently from the NRE framework), dengue management (NSAIDs contraindication trap), typhoid (Widal test vs blood culture — timing matters), anti-TB drug side effects (every attempt), and meningitis organism-by-age-group differentiation. These five areas alone can account for multiple marks in a single NRE attempt.
Is the hepatitis B serology table always in NRE?
Hepatitis B serology is one of the most consistently tested NRE microbiology topics. The exam specifically tests the window period (HBsAg has cleared but anti-HBs has not yet appeared — only HBcAb IgM is positive), the difference between active infection and immunity (anti-HBs), and which marker confirms ongoing viral replication (HBeAg). Memorise the serology table as a complete unit — it saves marks across multiple question variants.
Struggling with Microbiology MCQs in Your NRE Preparation?
Our live NRE Step 1 sessions teach microbiology through clinical scenarios — the same format as the actual exam. Message us on WhatsApp with your exam date and we will tell you exactly where to focus.
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