Microbiology tests are essential in diagnosing infections by identifying pathogens, their antibiotic resistance, and immune responses. These tests are crucial in infectious diseases, emergency medicine, ICU, surgery, gynecology, pediatrics, and geriatrics.
🦠 MAJOR TYPES OF MICROBIOLOGY TESTS
| Test Category | Examples | Purpose | Commonly Used In |
|---|---|---|---|
| Microscopy & Staining | Gram Stain, Acid-Fast Stain (AFB), Giemsa | Morphological identification | Bacteremia, TB, Malaria |
| Culture & Sensitivity | Blood Culture, Urine Culture, Sputum Culture | Bacterial/Fungal Isolation | Sepsis, UTI, Pneumonia |
| Molecular Tests | PCR (RT-PCR, GeneXpert), NGS | DNA/RNA detection | COVID-19, TB, HIV, Viral Hepatitis |
| Serology | ELISA, Western Blot, Agglutination | Antibody/Antigen detection | HIV, Hepatitis, Dengue |
| Rapid Tests | Malaria RDT, Troponin I (for sepsis) | Quick detection | Emergency/Infectious Disease |
| Antimicrobial Susceptibility | Disk Diffusion, MIC | Drug resistance analysis | Multi-drug resistant (MDR) infections |
🔍 CASE 1: SEPSIS – BLOOD CULTURE & ANTIBIOTIC SENSITIVITY
📌 Clinical Scenario:
A 65-year-old diabetic male presents with high fever, tachycardia, hypotension, confusion, and leukocytosis.
🧪 MICROBIOLOGY REPORT:
| Test | Result | Interpretation |
|---|---|---|
| Blood Culture (Aerobic & Anaerobic) | Klebsiella pneumoniae | Gram-negative sepsis |
| Gram Stain | Gram-negative bacilli | Confirms pathogen |
| Lactate | 4.2 mmol/L | Severe sepsis |
| Procalcitonin | Elevated | Bacterial infection |
| Antibiotic Sensitivity | Resistant: Ceftriaxone, Ciprofloxacin; Sensitive: Meropenem | ESBL-producing strain |
📝 Diagnosis:
- Severe Sepsis due to Multi-drug Resistant (MDR) Klebsiella
✅ Management:
- IV Meropenem (Carbapenem)
- Fluid Resuscitation (30 mL/kg)
- Vasopressors (if persistent hypotension)
- Monitor Organ Failure (SOFA Score)
🔍 CASE 2: TUBERCULOSIS – MOLECULAR & CULTURE TESTING
📌 Clinical Scenario:
A 34-year-old male with chronic cough, weight loss, night sweats, and hemoptysis is suspected of pulmonary TB.
🧪 MICROBIOLOGY REPORT:
| Test | Result | Interpretation |
|---|---|---|
| AFB Smear (Ziehl-Neelsen Stain) | Positive (3+) | Acid-fast bacilli seen |
| GeneXpert MTB/RIF PCR | MTB Detected, Rifampicin Resistance: Detected | Confirms MDR-TB |
| Mycobacterial Culture (Lowenstein-Jensen Medium) | Growth in 3 weeks | Confirms TB |
| Drug Susceptibility Testing (DST) | Resistance to Rifampicin, Isoniazid | MDR-TB detected |
📝 Diagnosis:
- Multi-Drug Resistant Tuberculosis (MDR-TB)
✅ Management:
- Start MDR-TB regimen: Bedaquiline + Linezolid + Levofloxacin
- Monitor Liver Function (LFT) due to hepatotoxic drugs
- Isolate Patient (Airborne Precautions)
🔍 CASE 3: MENINGITIS – CSF MICROBIOLOGY
📌 Clinical Scenario:
A 6-year-old child presents with fever, neck stiffness, altered consciousness, and photophobia.
🧪 MICROBIOLOGY REPORT:
| Test | Result | Interpretation |
|---|---|---|
| CSF Gram Stain | Gram-negative diplococci | Likely Neisseria meningitidis |
| CSF Culture | Neisseria meningitidis | Confirms bacterial meningitis |
| CSF PCR | Positive for N. meningitidis DNA | Rapid confirmation |
| CSF Glucose | Low (30 mg/dL) | Bacterial meningitis |
| CSF Protein | High (250 mg/dL) | Inflammation |
| CSF WBC Count | ↑ 1200/mm³ (Neutrophilic predominance) | Bacterial meningitis |
📝 Diagnosis:
- Meningococcal Meningitis
✅ Management:
- IV Ceftriaxone (Empirical)
- IV Dexamethasone (Reduce Inflammation)
- Post-Exposure Prophylaxis for Contacts (Rifampin/Ciprofloxacin)
🔍 CASE 4: VAGINAL DISCHARGE – STD PANEL
📌 Clinical Scenario:
A 28-year-old female presents with foul-smelling vaginal discharge, dysuria, and lower abdominal pain.
🧪 MICROBIOLOGY REPORT:
| Test | Result | Interpretation |
|---|---|---|
| Wet Mount (Microscopy) | Motile Trichomonads seen | Trichomonas vaginalis |
| Gram Stain | Gram-negative intracellular diplococci | Suggests Neisseria gonorrhoeae |
| Chlamydia PCR | Positive | Confirms Chlamydia trachomatis |
| HIV & Syphilis Serology | Negative | No co-infections |
📝 Diagnosis:
- Co-Infection: Gonorrhea + Chlamydia + Trichomoniasis
✅ Management:
- Ceftriaxone IM + Azithromycin PO (Gonorrhea & Chlamydia)
- Metronidazole PO (Trichomoniasis)
- Partner Treatment & Safe Sex Counseling
📌 CLINICAL PEARLS FOR MICROBIOLOGY TESTING
- Blood Cultures – Always obtain before starting antibiotics in sepsis.
- PCR Tests – Faster than culture, useful in TB, COVID-19, STDs, and meningitis.
- CSF Analysis – Differentiates bacterial vs viral meningitis.
- Antibiotic Susceptibility Testing – Prevents misuse of broad-spectrum antibiotics.
- STI Testing – Always test for multiple infections together (e.g., gonorrhea, chlamydia, syphilis, HIV).