Medical microbiology: infections and diagnosis

In short

Medical microbiology is the study of micro-organisms that cause human disease — bacteria, viruses, fungi and parasites — how they invade and damage the host, and how an infection is proved in the laboratory. Diagnosis runs on a fixed chain: right specimen, right transport, direct microscopy, culture and identification, then serology or molecular confirmation, with the report interpreted against the clinical picture. It matters because treatment, infection control and notification all follow the laboratory answer: India's tuberculosis (NTEP), HIV (NACO) and vector-borne (NCVBDC) programmes are built on specified diagnostic algorithms. It is examined in every B.Sc/M.Sc Microbiology paper, in CSIR-UGC NET and GATE (XL/BT life-science) biochemistry-and-microbiology sections, and in entry tests for NABL-accredited diagnostic chains (Dr Lal PathLabs, SRL, Metropolis, Thyrocare), hospital microbiology labs, ICMR institutes and IVD companies (Mylab, Molbio, Trivitron, bioMérieux India).

Explained

Host-microbe relationship: the vocabulary examiners test

• Infection - entry, multiplication and establishment of a micro-organism in host tissue; infectious disease - infection that produces signs and symptoms. Most infections are subclinical.
• Colonisation - presence and multiplication without tissue damage (e.g. *Staphylococcus aureus* in 20-30% of nostrils). Reporting a colonising isolate as a pathogen is the commonest clinical error.
• Normal (resident) flora - skin (coagulase-negative staphylococci, *Corynebacterium*), mouth (viridans streptococci), colon (*Bacteroides*, *E. coli*), vagina (*Lactobacillus*, keeping pH ~4). Benefits: nutrient synthesis (vitamin K, B group), competitive exclusion, immune priming. Normally sterile sites: blood, CSF, pleural/peritoneal/synovial fluid, deep tissue, lower respiratory tract (no heavy growth), bladder urine.
• Pathogen types: primary/frank (*M. tuberculosis*, *Salmonella* Typhi), opportunistic (*Candida*, *Pseudomonas*, *Pneumocystis jirovecii* - disease only when immunity or barriers fail), commensal turned pathogen (*E. coli* from gut into bladder).
• Pathogenicity - the ability of a species to cause disease (qualitative); virulence - the degree of that ability (quantitative, measured as ID50/LD50).
• Carrier - harbours and sheds the organism without illness: healthy, incubatory, convalescent, chronic (the classic chronic gall-bladder typhoid carrier; "Typhoid Mary").
• Routes of entry: skin/mucosa, respiratory, alimentary, genitourinary, transplacental (vertical), inoculation/arthropod bite.
• Koch's postulates (1884): (1) organism present in all cases of the disease, (2) isolated in pure culture, (3) reproduces the disease on inoculation into a susceptible host, (4) re-isolated from that host. Exceptions: non-cultivable organisms (*M. leprae*, *Treponema pallidum*), viruses needing living cells, healthy carriers, polymicrobial and immune-mediated disease, no suitable animal model. Molecular Koch's postulates (Falkow) prove virulence by knocking out a gene and restoring it.

Pathogenesis: virulence factors and toxins

• Adhesion - pili/fimbriae (P fimbriae of uropathogenic *E. coli*), adhesins, lipoteichoic acid; without attachment there is no infection.
• Invasion and spread - invasins, actin-based motility (*Listeria*, *Shigella*), spreading factors (hyaluronidase, streptokinase, collagenase).
• Evasion of defences - capsule resists phagocytosis (*S. pneumoniae*, *Klebsiella*, *H. influenzae* type b, *N. meningitidis*); protein A binds IgG Fc; IgA protease; catalase and cord factor; intracellular survival by blocking phagolysosome fusion (*M. tuberculosis*); antigenic variation (influenza, *Neisseria gonorrhoeae*, trypanosomes).
• Biofilm - sessile community in a polysaccharide matrix on catheters, implants and endotracheal tubes; up to 100-1,000× more tolerant to antimicrobials and the reason device infections need device removal.
• Exotoxins - protein, heat-labile, highly potent, antigenic (toxoid vaccines for tetanus and diphtheria). Types: neurotoxins (tetanospasmin blocks GABA/glycine release → rigidity; botulinum blocks acetylcholine → flaccid paralysis), enterotoxins (cholera toxin raises cAMP → watery diarrhoea; *S. aureus* enterotoxin, preformed, acts in 1-6 h), cytotoxins (diphtheria toxin and Shiga toxin inhibit protein synthesis — EF-2 ADP-ribosylation and 28S rRNA cleavage respectively), superantigens (TSST-1, streptococcal pyrogenic exotoxin — non-specific T-cell activation, cytokine storm, toxic shock).
• Endotoxin (LPS) - outer membrane of Gram-negative bacteria; lipid A is the toxic part; heat-stable, poorly antigenic, no toxoid; triggers TLR4 → IL-1, IL-6, TNF-α → fever, hypotension, DIC, septic shock. Detected by the LAL (Limulus amoebocyte lysate) test for pyrogens in injectables.
• Stages of disease: incubation → prodromal → acute/illness → decline → convalescence. Infection patterns: local, focal, systemic, bacteraemia (organisms in blood), septicaemia/sepsis (multiplication with host response), pyaemia, toxaemia, latent (herpes, TB), chronic, reactivation.

Transmission, epidemiology and hospital-acquired infection

• Chain of infection: reservoir → portal of exit → mode of transmission → portal of entry → susceptible host. Breaking any link stops spread.
• Modes: contact (direct/indirect/fomite), droplet (>5 µm, within ~1-2 m), airborne (droplet nuclei, TB, measles, varicella), faeco-oral (water and food — cholera, typhoid, hepatitis A/E), vector-borne (*Aedes aegypti* — dengue, chikungunya; *Anopheles* — malaria; mite — scrub typhus), blood-borne (HIV, HBV, HCV), vertical (rubella, CMV, HIV, syphilis).
• Terms: sporadic, endemic, hyperendemic, outbreak, epidemic, pandemic; zoonosis (rabies, brucellosis, leptospirosis, plague); anthroponosis.
• Healthcare-associated infection (HAI/nosocomial) - acquired ≥48 h after admission, not incubating at entry. Four main types and their devices: CAUTI (urinary catheter), VAP (ventilator), CLABSI (central line), SSI (surgery). Common organisms: *E. coli*, *Klebsiella pneumoniae*, *Acinetobacter baumannii*, *Pseudomonas aeruginosa*, *S. aureus*/MRSA, *Enterococcus*, *Candida auris*, *Clostridioides difficile* after antibiotics.
• Control: hand hygiene (WHO 5 moments, 6 steps), standard plus transmission-based precautions, device bundles, sterilisation and disinfection, isolation, surveillance rates expressed per 1,000 device-days, and antimicrobial stewardship.
• Indian frame: IDSP/IHIP (Integrated Health Information Platform) for outbreak reporting; NCDC and ICMR for national surveillance; state notification lists make TB, cholera, measles, dengue, rabies, H1N1, COVID-19 and others notifiable (TB notification is mandatory for all providers under the Government of India order of 2018 and the NTEP); the Epidemic Diseases Act, 1897 (amended by ordinance in 2020 to protect healthcare workers) and state public-health Acts give outbreak powers. The Clinical Establishments (Registration and Regulation) Act, 2010 governs lab registration in adopting states; NABL accreditation to ISO 15189 is the quality benchmark; laboratory waste follows the Bio-Medical Waste Management Rules, 2016 (as amended).

Specimen collection, transport and direct microscopy

• Golden rules: collect before antibiotics, from the actual site of infection, in adequate volume, in a sterile labelled container, with the request form stating clinical details; transport within 2 h or use a transport medium; refrigerate urine/stool, never refrigerate CSF or blood culture bottles (*N. meningitidis* and *H. influenzae* die).
• Blood culture - the single most important specimen in sepsis: 2-3 sets from separate sites before antibiotics, 8-10 mL per adult bottle, 1:10 blood-to-broth ratio, skin disinfected with 2% chlorhexidine in 70% alcohol or povidone-iodine with full drying time. Automated systems (BacT/ALERT, BACTEC) flag CO₂ production. Repeated growth of coagulase-negative staphylococci from one set only is usually contamination.
• Urine - clean-catch midstream, first morning sample, processed within 2 h or refrigerated at 4 °C up to 24 h; suprapubic or catheter sample where needed; semi-quantitative culture with a calibrated loop (0.001 mL or 0.01 mL).
• Sputum - early morning, deep cough, 2-5 mL; for TB the NTEP collects two samples (spot and morning); Bartlett/Murray-Washington screening: >25 pus cells and <10 squamous cells per low-power field means a lower-respiratory sample.
• CSF - three tubes by lumbar puncture, carried to the lab immediately at room temperature; cell count, protein, glucose, Gram stain, culture, cryptococcal antigen, PCR panel.
• Stool - fresh sample in a wide-mouth container; Cary-Blair for *Salmonella*/*Shigella*/*Campylobacter*, VR fluid or alkaline peptone water for *Vibrio cholerae*; hanging-drop shows darting motility.
• Stains to remember: Gram (crystal violet → iodine mordant → acetone-alcohol decolouriser → safranin; purple = Gram-positive thick peptidoglycan retains the dye-iodine complex, pink = Gram-negative), Ziehl-Neelsen (hot carbol fuchsin, 20% H₂SO₄ + 3% alcohol, methylene blue; acid-fast because of mycolic acid — *Mycobacterium*; 0.5-1% acid for *Nocardia*, modified ZN for *Cryptosporidium*), auramine-O fluorescence (LED-FM) used in NTEP high-load labs, Albert's/Loeffler's for diphtheria metachromatic granules, India ink negative stain for *Cryptococcus* capsule, Giemsa for malaria parasites and *Leishmania* LD bodies, KOH mount and lactophenol cotton blue for fungi, dark-ground microscopy for *Treponema* and leptospires.

Culture, identification and interpretation

• Media classification: basal (nutrient agar/broth), enriched (blood agar, chocolate agar for *H. influenzae* factors X and V), enrichment broth (selenite F, tetrathionate, alkaline peptone water — favours the pathogen over flora), selective (MacConkey, DCA, XLD, TCBS for *V. cholerae*, LJ for *M. tuberculosis*, Thayer-Martin for *Neisseria*), differential/indicator (MacConkey separates lactose fermenters as pink; blood agar shows haemolysis), transport (Cary-Blair, Stuart's, Amies, VTM), anaerobic (Robertson's cooked meat, thioglycollate), sugar-free media for toxigenicity work.
• Incubation: 35-37 °C for 18-24 h aerobically; 5-10% CO₂ for *Pneumococcus* and *Neisseria*; anaerobic jar/chamber for *Bacteroides* and *Clostridium*; 42 °C for *Campylobacter*; 2-8 weeks on LJ for *M. tuberculosis* (2-3 weeks in liquid MGIT).
• Colony morphology and haemolysis: β-haemolysis (*S. pyogenes*, complete clearing), α (*S. pneumoniae*, green), γ (none); swarming *Proteus*; metallic sheen of *E. coli* on EMB; mucoid *Klebsiella*; grape-like odour and pyocyanin of *Pseudomonas*.
• Key biochemical tests: catalase (staphylococci +, streptococci −), coagulase (*S. aureus* +), oxidase (*Pseudomonas*, *Vibrio*, *Neisseria* +; Enterobacteriaceae −), IMViC (*E. coli* ++−− versus *Klebsiella* −−++), TSI and urease (*Proteus* rapid urease +), bile solubility and optochin sensitivity (*S. pneumoniae*), bacitracin (*S. pyogenes*), novobiocin (*S. saprophyticus* resistant), CAMP and hippurate (group B *Streptococcus*).
• Modern identification: automated panels (VITEK 2, BD Phoenix, MicroScan), MALDI-TOF MS — protein fingerprint gives species in minutes from a colony, now in most Indian tertiary labs, 16S rRNA gene sequencing for unidentifiable or non-cultivable isolates, ITS sequencing for fungi.
• Interpretation: quantify (urine CFU/mL, bronchoalveolar lavage ≥10⁴ CFU/mL), judge site sterility, correlate with pus cells, and distinguish contaminant, coloniser and pathogen. Susceptibility testing of the identified isolate is a separate exercise.

Serological and molecular diagnosis

• Antibody detection - useful when the organism is hard to culture, but needs a rising titre (four-fold) in paired sera or a single high titre above the local baseline; IgM suggests recent infection, IgG past infection or vaccination. Examples: Widal for enteric fever (anti-O and anti-H agglutinins; appears only in the second week, confounded by past infection and vaccination in endemic India — cut-offs are local and the test is no substitute for blood culture), VDRL/RPR (non-treponemal, titre falls with treatment) with TPHA/TP-ELISA confirmation for syphilis, ASO titre after streptococcal infection, MAT for leptospirosis, Weil-Felix for rickettsial disease (now largely replaced by scrub typhus IgM ELISA), dengue IgM MAC-ELISA after day 5.
• Antigen detection - faster and positive earlier: dengue NS1 (days 1-5), malaria RDT (HRP-2 for *P. falciparum*, pLDH for *P. vivax*), cryptococcal antigen (CrAg) in CSF/serum, rotavirus and *H. pylori* stool antigen, SARS-CoV-2 RAT, latex agglutination for meningitis panels. The immunoassay principles themselves are covered under the immunology/ELISA note.
• Molecular methods: conventional and real-time PCR (Ct value inversely related to target load), multiplex syndromic panels (meningoencephalitis, respiratory, GI), RT-PCR for RNA viruses, LAMP and CRISPR-based assays as near-patient tests, line probe assay (LPA) for first- and second-line TB drug resistance, Xpert MTB/RIF Ultra (cartridge NAAT, ~2 h, detects *M. tuberculosis* plus *rpoB* rifampicin resistance), TrueNat MTB/Rif (Molbio, Goa — battery-operated chip PCR deployed down to Indian primary health centres), viral load assays for HIV, HBV and HCV, genome sequencing for outbreak tracking (INSACOG for SARS-CoV-2).
• Test performance must be quoted with prevalence: sensitivity and specificity are properties of the test; predictive values depend on how common the disease is.
• Which test when: window of the illness decides it — culture and antigen/NAAT early, antibodies late. Example for enteric fever: blood culture week 1 (~80% yield), Widal/antibody week 2-3, stool and urine culture week 2-4, bone marrow culture highest yield even after antibiotics.

System-wise infections at a glance

• Respiratory: upper — *S. pyogenes* pharyngitis, diphtheria, influenza; lower — community-acquired pneumonia (*S. pneumoniae*, *H. influenzae*, *Mycoplasma*, *Klebsiella*), pulmonary TB (sputum NAAT first under NTEP, then culture/DST), COVID-19, pertussis.
• Gastrointestinal: watery diarrhoea (*V. cholerae* O1 El Tor, ETEC, rotavirus, norovirus), dysentery (*Shigella*, EIEC, *Entamoeba histolytica*), enteric fever (*S.* Typhi/Paratyphi A), food poisoning (*S. aureus*, *B. cereus*, *Clostridium perfringens*, *Salmonella* spp.), *C. difficile* colitis, hepatitis A and E (water-borne, epidemic in India).
• Urinary: uncomplicated UTI mostly *E. coli* (UPEC); also *Klebsiella*, *Proteus* (struvite stones, urease), *Enterococcus*, *S. saprophyticus* in young women; diagnosis by pyuria plus significant colony count.
• CNS: pyogenic meningitis (*S. pneumoniae*, *N. meningitidis*, Hib; *E. coli* and group B *Streptococcus* in neonates), tuberculous and cryptococcal meningitis (lymphocytic, low glucose, high protein), viral encephalitis (Japanese encephalitis, herpes simplex, rabies).
• Bloodstream and systemic: sepsis, infective endocarditis (viridans streptococci, *S. aureus*, HACEK), malaria, dengue, leptospirosis, scrub typhus — the classic Indian "acute febrile illness" differential, worked up as a panel in the monsoon.
• Skin, soft tissue, bone: pyoderma and abscess (*S. aureus*), cellulitis and necrotising fasciitis (*S. pyogenes*), gas gangrene (*C. perfringens*), tetanus, leprosy (slit-skin smear for acid-fast bacilli, bacteriological index), osteomyelitis.
• STI and reproductive tract: syphilis, gonorrhoea, chlamydia (NAAT), trichomoniasis, bacterial vaginosis (Amsel criteria, clue cells), genital herpes, HPV, HIV (NACO three-test strategy with three different antigens/principles for diagnosis; early infant diagnosis by DNA PCR).
• Congenital and perinatal: TORCH group — toxoplasma, rubella, CMV, herpes, plus syphilis, HBV, HIV; IgM in the newborn or persistent IgG beyond 6-9 months indicates true infection.

Key points

• Infection vs disease - establishment and multiplication vs clinical illness; most infections are subclinical.
• Colonisation - growth without damage; a coloniser reported as a pathogen leads to wrong antibiotics.
• Virulence - the measurable degree of pathogenicity, quantified as ID50 or LD50.
• Capsule - chief antiphagocytic factor; the basis of pneumococcal, meningococcal and Hib conjugate vaccines.
• Exotoxin vs endotoxin - protein, heat-labile, toxoid-able, specific action vs LPS lipid A, heat-stable, no toxoid, fever and shock.
• Superantigen - non-specifically activates many T cells (TSST-1) causing toxic shock.
• Biofilm - matrix-embedded growth on devices; needs device removal, not just drugs.
• Koch's postulates - four criteria linking organism and disease; fail for *M. leprae*, viruses and carriers.
• Sterile sites - blood, CSF, joint and deep tissue; any growth there is significant unless contaminated.
• Blood culture - 2-3 sets, 8-10 mL each, 1:10 in broth, before antibiotics; never refrigerate.
• Significant bacteriuria - ≥10⁵ CFU/mL in a midstream sample (Kass); lower counts count in symptomatic women, catheter or suprapubic samples.
• Gram stain - decolourisation step decides the result; thick peptidoglycan retains the dye-iodine complex.
• Acid-fastness - mycolic acid in the cell wall; ZN needs heat, 20% sulphuric acid for mycobacteria.
• Enrichment vs selective - broth that favours the pathogen vs solid medium that suppresses others.
• Antigen early, antibody late - NS1 on days 1-5 of dengue, IgM after day 5; Widal only from the second week.
• NAAT first - Xpert/TrueNat is the NTEP's initial diagnostic test, giving TB and rifampicin resistance together.
• HAI - infection ≥48 h after admission; CAUTI, VAP, CLABSI, SSI; rates reported per 1,000 device-days.
• Notification and quality - TB and listed diseases notifiable through IDSP/IHIP (Nikshay for TB); labs accredited to ISO 15189 by NABL.

Formulas and examples

Example 1 - Urine culture (calibrated loop). A 1 µL (0.001 mL) loop of midstream urine is plated; 85 colonies of *E. coli* grow.

• CFU/mL = 85 ÷ 0.001 = 8.5 × 10⁴ CFU/mL.
• Below the 10⁵ Kass threshold, so in an asymptomatic patient this is doubtful and a repeat clean sample is advised; in a symptomatic young woman with ≥10 pus cells/µL it is reported as significant, because counts of 10²-10⁴ CFU/mL are accepted in acute symptomatic cystitis.

Example 2 - Pour-plate count of a fluid. 1 mL of a 10⁻⁴ dilution of peritoneal fluid gives 62 colonies.

• CFU/mL = 62 × 10⁴ ÷ 1 = 6.2 × 10⁵ CFU/mL — heavy growth from a normally sterile site, so clinically significant.
• Had 0.1 mL of the same dilution been plated and given 62 colonies: 62 × 10⁴ ÷ 0.1 = 6.2 × 10⁶ CFU/mL. Forgetting the volume plated is a 10-fold error.

Example 3 - Sensitivity, specificity and the prevalence trap. A cartridge NAAT is compared with culture on 1,000 sputum samples; 200 are culture-positive, of which the NAAT detects 176; of the 800 culture-negative samples it calls 784 negative.

• TP = 176, FN = 24, TN = 784, FP = 16.
• Sensitivity = 176/200 = 88.0%; Specificity = 784/800 = 98.0%.
• PPV = 176/(176+16) = 176/192 = 91.7%; NPV = 784/(784+24) = 784/808 = 97.0%; Accuracy = (176+784)/1000 = 96.0%.
• Now screen 10,000 people where prevalence is 2% (200 true cases): TP = 0.88 × 200 = 176, FN = 24, TN = 0.98 × 9,800 = 9,604, FP = 196. PPV = 176/(176+196) = 176/372 = 47.3%. Same test, same sensitivity and specificity — but fewer than half the positives are real, which is why mass screening of low-prevalence groups needs a confirmatory test.

Example 4 - Food-poisoning outbreak at a hostel mess. 250 students ate; 68 fell ill within 4 h with vomiting.

• Overall attack rate = 68/250 × 100 = 27.2%.
• Of 180 who ate the chicken salad, 60 fell ill → 33.3%; of 70 who did not, 8 fell ill → 11.4%.
• Relative risk = 33.33/11.43 = 2.92, so the salad is the likely vehicle. The 1-6 h incubation with prominent vomiting points to preformed *S. aureus* enterotoxin; investigate food handlers' nostrils and skin lesions, and report to the district IDSP/IHIP unit.
• If 2 of the 68 died, CFR = 2/68 × 100 = 2.9%.

Example 5 - Herd immunity and PCR load.

• Measles with R₀ ≈ 15: H = 1 − 1/15 = 0.933 → 93.3% immune coverage needed, the reason measles-rubella campaigns target >95% under Mission Indradhanush.
• Two SARS-CoV-2 samples give Ct 22 and Ct 32: ΔCt = 10, fold difference = 2¹⁰ = 1,024×, i.e. about 3 log₁₀ more target in the Ct-22 sample (10/3.32 ≈ 3.0 logs).

Where it is used

• Hospital and diagnostic-chain microbiology labs - bench microbiologist, microbiology technologist and consultant roles at AIIMS, PGIMER, CMC Vellore, Apollo, Fortis, Manipal, and at Dr Lal PathLabs, SRL, Metropolis and Thyrocare; work is organised under NABL/ISO 15189 and the Clinical Establishments Act, 2010.
• National programmes - NTEP laboratory network (designated microscopy centres, TrueNat and Xpert sites, culture and DST labs, Nikshay reporting), NACO integrated counselling and testing centres and viral-load labs, NCVBDC for malaria and dengue, NCDC/IDSP outbreak investigation and VRDLs (ICMR Viral Research and Diagnostic Laboratories in medical colleges).
• Infection-control and quality roles - hospital infection control nurse/officer, HAI surveillance per 1,000 device-days, outbreak investigation, antimicrobial stewardship committees, NABH accreditation support.
• IVD and diagnostics industry - product development, application support and field service for PCR kits, RDTs and culture systems at Molbio (TrueNat), Mylab, Trivitron, Transasia/Erba, bioMérieux India, BD and Thermo Fisher; CDSCO licensing of in-vitro diagnostic kits as medical devices.
• Pharma and vaccine QC - sterility testing, bioburden, environmental monitoring, LAL/pyrogen testing and media fills at Serum Institute, Bharat Biotech, Biological E, Sun Pharma and Cipla, under Indian Pharmacopoeia and Schedule M of the Drugs and Cosmetics Act.
• Public health and research - ICMR institutes (NIV Pune, NIRT Chennai, NICED Kolkata, NIMR), food and water testing laboratories under FSSAI, veterinary and zoonosis surveillance, and sequencing consortia such as INSACOG.

Interview questions

• Q: What is the difference between pathogenicity and virulence? A: Pathogenicity is the qualitative ability of a species to cause disease; virulence is the quantitative degree of that ability, measured as ID50 or LD50.
• Q: Why is a blood-to-broth ratio of about 1:10 insisted on in blood culture? A: Dilution reduces the bactericidal effect of serum, complement, leucocytes and any antibiotic present, while still giving enough blood volume to capture low-level bacteraemia.
• Q: A urine culture reports 10³ CFU/mL of *E. coli*. Is it significant? A: In an asymptomatic patient, no — repeat the sample; in a symptomatic young woman with pyuria, or in a suprapubic/catheter specimen, counts of 10²-10⁴ CFU/mL can be genuine infection.
• Q: Which step of the Gram stain decides the result and why? A: Decolourisation with acetone-alcohol — the thick peptidoglycan of Gram-positive cells holds the crystal violet-iodine complex, while the thin wall and lipid-rich outer membrane of Gram-negative cells let it wash out.
• Q: Why does blood culture beat the Widal test in enteric fever? A: Blood culture gives the organism itself with susceptibility data and is positive in the first week, whereas Widal antibodies appear only in week 2 and are confounded by previous infection and vaccination in an endemic population.
• Q: Why detect dengue NS1 rather than IgM on day 2 of fever? A: NS1 antigen is present from day 1 to about day 5 of viraemia; IgM becomes reliably detectable only after day 5, so an early IgM test gives a false negative.
• Q: An organism cannot be grown in culture. How do you prove it causes the disease? A: Use molecular Koch's postulates and non-culture evidence — NAAT/sequencing from the lesion, antigen detection, serology with paired titres, histopathology, and gene knock-out/restoration where a model exists.
• Q: Can a highly specific test still give many false positives? A: Yes — in a low-prevalence population the few per cent of false positives among a large number of healthy people can outnumber true positives, so PPV falls sharply.
• Q: What does a rifampicin-resistance result from a cartridge NAAT actually detect? A: Mutations in the *rpoB* gene hotspot region; it is a genotypic surrogate and must be confirmed by culture-based or line-probe testing when it conflicts with the clinical picture.
• Q: How do you decide whether coagulase-negative staphylococci in a blood culture are real? A: Growth in multiple sets drawn from different sites, short time to positivity, a plausible focus such as a central line or prosthesis, and clinical signs; a single bottle positive late is usually skin contamination.
• Q: Why must the CSF sample never be refrigerated? A: *Neisseria meningitidis* and *Haemophilus influenzae* are highly susceptible to cold and die quickly, giving a false-negative culture.
• Q: Give one Indian example of decentralised molecular TB diagnosis. A: TrueNat MTB/Rif (Molbio Diagnostics, Goa) — a battery-operated chip-based real-time PCR placed at primary health centres under the NTEP, giving a result in about an hour with rifampicin resistance.

Common mistakes

• Treating any growth as the pathogen — ignoring normal flora, colonisation and contamination, and failing to correlate with pus cells and the clinical picture.
• Collecting the specimen after starting antibiotics, or sending saliva labelled as sputum, and then blaming a negative culture.
• Forgetting the dilution factor or the volume plated in a CFU/mL calculation, or counting plates outside the 30-300 colony range.
• Confusing sensitivity with PPV, and quoting predictive values without stating the prevalence.
• Ordering antibody tests too early (Widal in week 1, dengue IgM on day 2) or reading a single titre as proof without a paired rise.
• Mixing up enrichment broth with selective solid medium, or naming blood agar as "selective" when it is enriched and differential.
• Writing endotoxin as a protein that can be made into a toxoid, or claiming exotoxins are heat-stable.
• Reporting a genotypic resistance or NAAT result as proof of viable organisms — DNA can persist after treatment, so a positive NAAT is not by itself evidence of live, transmissible infection.

Related topics

Antimicrobial susceptibility testing · Immunology and ELISA · Clinical laboratory practice and biosafety · Bacterial structure, growth and physiology · Virology and virus diagnosis · Epidemiology and public health microbiology