Unit 5: Microbe-host interactions, microbial diseases and clinical microbiology - Subjective Questions

Host-microbe interactions describe the diverse relationships that develop when microorganisms colonize or infect a host. These interactions range along a spectrum from beneficial to harmful.

Major types of host-microbe relationships:

• Mutualism: Both host and microbe benefit. Example: Gut bacteria (e.g., Bacteroides) aid digestion and produce vitamin K, while receiving nutrients and habitat.
• Commensalism: The microbe benefits while the host is neither harmed nor benefited. Example: Staphylococcus epidermidis on skin.
• Parasitism: The microbe benefits at the expense of the host, causing damage or disease. Example: Mycobacterium tuberculosis.
• Amensalism: One organism is harmed while the other is unaffected (e.g., through antibiotic production).

Key concepts:

• Normal microbiota (flora): Resident microbes that colonize body surfaces without causing disease and provide colonization resistance.
• Colonization vs. Infection: Colonization is mere presence/multiplication; infection implies host damage.
• Opportunistic pathogens: Normally harmless microbes that cause disease when host defenses are compromised.

The balance between microbial virulence and host immunity determines the outcome of any interaction, ranging from health to disease.

Pathogenicity is the ability of a microorganism to cause disease in a host.

Virulence is the degree or quantitative measure of pathogenicity, often expressed as (lethal dose for 50% of hosts) or (infectious dose for 50% of hosts).

Major virulence factors:

• Adhesins: Fimbriae, pili, and surface proteins that mediate attachment to host cells (e.g., pili of Neisseria gonorrhoeae).
• Invasins: Enzymes and proteins that promote spread through tissues (e.g., hyaluronidase, collagenase).
• Capsules: Anti-phagocytic polysaccharide layers (e.g., Streptococcus pneumoniae).
• Exotoxins: Secreted proteins with specific toxic effects (e.g., diphtheria toxin, cholera toxin).
• Endotoxin (LPS): Lipid A component of Gram-negative outer membrane, causing fever and shock.
• Antigenic variation & immune evasion: Changing surface antigens to escape immunity.
• Iron acquisition systems: Siderophores that scavenge host iron.

The combined action of these factors allows the pathogen to establish infection, damage the host, and evade immune clearance.

Exotoxins and endotoxins are two major categories of bacterial toxins that differ significantly in structure and action.

Key point: Exotoxins can be inactivated to make vaccines (toxoids), whereas endotoxins cannot, which is important in vaccine design.

Tuberculosis (TB) is a chronic infectious disease and a classic example of a bacterial infection.

Causative agent: Mycobacterium tuberculosis — an acid-fast, aerobic, non-motile bacillus with a waxy, mycolic acid-rich cell wall.

Pathogenesis:

• Transmitted via airborne droplet nuclei (coughing, sneezing).
• Inhaled bacilli reach the alveoli and are engulfed by alveolar macrophages but resist intracellular killing.
• Bacteria multiply, forming a primary lesion (Ghon focus).
• Cell-mediated immunity leads to formation of granulomas (tubercles) with central caseous necrosis.
• Can remain latent or reactivate, causing pulmonary or disseminated (miliary) TB.

Laboratory diagnosis:

• Microscopy: Ziehl-Neelsen (acid-fast) staining of sputum shows red bacilli.
• Culture: Lowenstein-Jensen (L-J) medium; slow growth (2–6 weeks).
• Rapid methods: GeneXpert (MTB/RIF PCR), liquid culture (MGIT).
• Tuberculin skin test (Mantoux) and IGRA for immune response.
• Chest X-ray as supportive evidence.

Treatment: Multidrug regimen (Isoniazid, Rifampicin, Pyrazinamide, Ethambutol) to prevent drug resistance.

Influenza (flu) is a highly contagious respiratory viral infection and a key example of a viral disease.

Causative agent: Influenza virus — an enveloped, single-stranded, negative-sense, segmented RNA virus of the family Orthomyxoviridae (types A, B, C).

Structure:

• 8 RNA segments enclosed in a helical nucleocapsid.
• Two key surface glycoproteins:
  • Hemagglutinin (HA): attachment to sialic acid receptors.
  • Neuraminidase (NA): release of progeny virions.

Replication: HA binds host receptors → endocytosis → uncoating → RNA replication in nucleus → assembly → NA-mediated release.

Antigenic variation:

• Antigenic drift: Minor point mutations in HA/NA → seasonal epidemics.
• Antigenic shift: Major reassortment of genome segments → new subtype → pandemics (e.g., H1N1).

Diagnosis:

• Rapid antigen detection tests (RIDTs) for HA/NA.
• RT-PCR — gold standard for typing.
• Viral culture in embryonated eggs or cell lines.
• Serology (rise in antibody titre).

Prevention: Annual reformulated vaccines and antiviral drugs (oseltamivir, zanamivir).

Candidiasis is one of the most common fungal (opportunistic) infections in humans.

Causative agent: Candida albicans — a dimorphic yeast that is part of the normal flora of skin, gut, and mucous membranes, becoming pathogenic when host defenses are weakened.

Predisposing factors:

• Immunosuppression (HIV/AIDS, chemotherapy)
• Prolonged antibiotic use (disturbs normal flora)
• Diabetes mellitus, pregnancy
• Indwelling catheters and prosthetics

Clinical forms:

• Oral thrush: White creamy patches on the tongue and oral mucosa.
• Vaginal candidiasis (vulvovaginitis): Itching and thick white discharge.
• Cutaneous candidiasis: Intertriginous skin folds.
• Systemic/invasive candidiasis: Bloodstream and organ involvement in immunocompromised patients.

Laboratory diagnosis:

• Microscopy: KOH mount or Gram stain shows budding yeast cells and pseudohyphae.
• Culture: Sabouraud Dextrose Agar (SDA) — creamy white colonies.
• Germ tube test: Positive for C. albicans.
• CHROMagar and molecular methods (PCR) for species identification.

Treatment: Azoles (fluconazole), echinocandins, or amphotericin B for severe cases.

Malaria is a major protozoal disease transmitted by mosquitoes and remains a leading global health problem.

Causative agent: Plasmodium species — P. falciparum (most lethal), P. vivax, P. ovale, P. malariae, P. knowlesi.

Vector: Female Anopheles mosquito.

Life cycle (two hosts):

Human (asexual — schizogony):

• Exo-erythrocytic (liver) phase: Sporozoites injected during a bite invade hepatocytes → schizonts → merozoites.
• Erythrocytic phase: Merozoites invade RBCs → ring → trophozoite → schizont → rupture releases merozoites (causes periodic fever).
• Some form gametocytes.

Mosquito (sexual — sporogony):

• Gametocytes taken up → fertilization → oocyst → sporozoites migrate to salivary glands.

Clinical features: Periodic fever with chills and rigors, anemia, splenomegaly; cerebral malaria in P. falciparum.

Diagnosis:

• Peripheral blood smear (Giemsa stain): Thick smear for detection, thin smear for species identification — the gold standard.
• Rapid Diagnostic Tests (RDTs): Detect parasite antigens (HRP-2, pLDH).
• PCR: For species confirmation and low parasitemia.

Treatment: Artemisinin-based combination therapy (ACT); chloroquine where susceptible.

Normal microbiota (flora) refers to the community of microorganisms that permanently or temporarily inhabit body surfaces (skin, mouth, gut, respiratory and genitourinary tracts) without causing disease under normal conditions.

Beneficial roles:

• Colonization resistance: Competes with pathogens for nutrients and attachment sites, preventing their establishment.
• Nutritional benefits: Gut bacteria synthesize vitamin K and some B-complex vitamins; aid digestion and fermentation.
• Immune development: Stimulate maturation of the immune system (GALT) and promote tolerance.
• Production of antimicrobial substances: Bacteriocins and acids (lowered pH) inhibit pathogens.
• Metabolic functions: Break down bile acids and metabolize drugs.

Disease from disturbances (dysbiosis):

• Antibiotic-associated diarrhea: Loss of normal flora allows overgrowth of Clostridioides difficile.
• Candidiasis: Overgrowth of Candida after antibiotic use.
• Opportunistic infections: Normal flora entering sterile sites (e.g., E. coli causing UTI, S. epidermidis on catheters).

Conclusion: A balanced microbiota is essential for health, and its disruption predisposes the host to infection and disease.

Identification of microorganisms from clinical specimens is central to clinical microbiology and guides diagnosis and treatment.

General steps:

1. Specimen collection: Aseptically collect the appropriate specimen (blood, urine, sputum, CSF, pus, stool) at the right time, before antibiotics if possible.
2. Transport and storage: Use suitable transport media (e.g., Stuart's, Amies) and process promptly.
3. Direct microscopy:
   • Gram staining (differentiates Gram +ve/−ve).
   • Acid-fast staining (for Mycobacterium).
   • Wet mounts (motility, parasites, fungi).
4. Culture: Inoculate onto appropriate media:
   • Enriched (blood agar, chocolate agar)
   • Selective/differential (MacConkey, mannitol salt agar)
5. Colony morphology & isolation: Study colony characteristics; obtain pure culture.
6. Biochemical tests: Catalase, oxidase, coagulase, indole, urease, sugar fermentation, etc.
7. Serological identification: Detect antigens/antibodies (agglutination, ELISA).
8. Antimicrobial susceptibility testing (AST): Disk diffusion (Kirby–Bauer), MIC determination.
9. Advanced/rapid methods: Automated systems (VITEK), MALDI-TOF, molecular PCR.

Conclusion: A systematic combination of these methods ensures accurate identification and appropriate therapy.

Proper specimen collection and transport is the foundation of accurate microbiological diagnosis; errors at this stage compromise all subsequent tests.

Importance:

• Ensures the true pathogen is recovered and prevents false results.
• Maintains viability of fastidious/anaerobic organisms.
• Avoids contamination with normal flora.

Precautions during collection:

• Collect from the actual site of infection.
• Collect before antimicrobial therapy whenever possible.
• Use aseptic technique and sterile containers.
• Collect an adequate quantity at the appropriate time (e.g., blood during fever spike).
• Properly label with patient details and time.

Precautions during transport:

• Transport promptly (ideally within 1–2 hours).
• Use appropriate transport media (e.g., Cary-Blair for stool, Stuart's/Amies for swabs).
• Maintain correct temperature (some refrigerated; CSF/gonococci kept warm).
• Use anaerobic transport systems for anaerobes.
• Follow biosafety guidelines to prevent leakage and infection.

Conclusion: Adherence to these principles ensures reliable and clinically meaningful microbiological results.

Rapid identification methods reduce turnaround time compared to conventional culture and biochemical tests, allowing faster diagnosis and treatment.

Major rapid methods:

• Automated identification systems:
  • VITEK, MicroScan, BD Phoenix — use miniaturized biochemical panels read automatically for ID and susceptibility.
• MALDI-TOF Mass Spectrometry: Identifies organisms by their unique protein spectral fingerprint within minutes.
• Immunological/antigen detection:
  • Latex agglutination for bacterial capsular antigens.
  • ELISA for antigens/antibodies.
  • Immunochromatographic (lateral flow) tests for rapid bedside detection.
• Rapid enzyme tests: Detect specific enzymes (e.g., rapid urease test for H. pylori, spot indole test).
• Chromogenic media: Produce colored colonies for specific organisms (e.g., CHROMagar).
• Molecular rapid tests: GeneXpert (real-time PCR) for TB and resistance genes.
• Automated blood culture systems: BACTEC, BacT/ALERT detect microbial growth via CO₂ sensors.

Advantages: Speed, sensitivity, standardization, and early targeted therapy.

Conclusion: Rapid methods are transforming diagnostic microbiology by shortening the time from specimen to result.

MALDI-TOF MS (Matrix-Assisted Laser Desorption/Ionization – Time-Of-Flight Mass Spectrometry) is a rapid, powerful tool for microbial identification.

Principle:

• A microbial sample is mixed with a matrix and placed on a target plate.
• A laser ionizes the sample, releasing charged molecules (mainly ribosomal proteins).
• Ions are accelerated in an electric field; their time-of-flight (TOF) to the detector depends on their mass-to-charge ratio ():
  
• The resulting mass spectrum is a unique protein fingerprint.
• Software compares the spectrum against a reference database to identify the organism.

Applications:

• Rapid species-level identification of bacteria, yeasts, and molds.
• Direct identification from positive blood cultures.
• Detection of certain resistance markers and typing.
• Epidemiological strain typing.

Advantages:

• Speed: Results in minutes.
• Accuracy and low cost per test after setup.
• Minimal sample and reagents.

Limitations: High initial equipment cost, requires database, difficulty with closely related species.

Molecular methods identify microorganisms based on their nucleic acids (DNA/RNA) rather than phenotypic traits, offering high sensitivity and specificity.

Major molecular techniques:

• Polymerase Chain Reaction (PCR): Amplifies target DNA sequences.
  • Multiplex PCR: Detects multiple pathogens simultaneously.
  • Real-time (qPCR): Quantifies and detects in real time (e.g., GeneXpert MTB/RIF).
  • RT-PCR: For RNA viruses (e.g., influenza, SARS-CoV-2).
• Nucleic acid hybridization probes: Complementary labeled probes detect specific sequences (e.g., FISH).
• 16S rRNA gene sequencing: Universal marker for bacterial identification and phylogeny; 18S rRNA/ITS for fungi.
• DNA sequencing / Next-Generation Sequencing (NGS): Whole-genome analysis, metagenomics, and resistance detection.
• Restriction Fragment Length Polymorphism (RFLP) and PFGE: For strain typing.
• Nucleic Acid Amplification Tests (NAATs): For fastidious/non-culturable organisms (e.g., Chlamydia).

Advantages:

• Detects non-culturable and slow-growing organisms.
• Rapid and highly sensitive/specific.
• Detects resistance genes directly.

Limitations: High cost, need for trained staff, risk of contamination, and detection of dead organisms (positive without viability).

PCR is a molecular technique for the in-vitro amplification of specific DNA sequences, widely used in diagnostic microbiology.

Principle: Repeated cycles of heating and cooling using a thermostable DNA polymerase (Taq), primers, and dNTPs exponentially amplify a target DNA region.

Steps (each cycle):

1. Denaturation (~94–95°C): Double-stranded DNA separates into single strands.
2. Annealing (~50–60°C): Primers bind to complementary target sequences.
3. Extension (~72°C): Taq polymerase synthesizes new strands.

After cycles, the target is amplified approximately -fold:

Types:

• Conventional PCR (end-point detection on gel)
• Real-time (quantitative) PCR with fluorescent detection
• RT-PCR for RNA templates
• Multiplex PCR for multiple targets

Applications in microbiology:

• Detection of non-culturable/slow-growing pathogens (e.g., M. tuberculosis).
• Diagnosis of viral infections (HIV viral load, SARS-CoV-2).
• Detection of antibiotic resistance genes (e.g., mecA, rpoB).
• Genotyping and epidemiological studies.

Advantages: Rapid, sensitive, specific. Limitations: Contamination risk, cannot distinguish live from dead organisms.

Microorganisms produce characteristic metabolic products through their enzymatic activities. Detecting these products helps identify and differentiate species.

Basis: Different microbes possess different enzyme systems, leading to distinct end-products from substrates such as sugars, amino acids, and other compounds.

Important biochemical tests based on metabolic products:

• Sugar fermentation tests: Detect acid and gas production (e.g., glucose, lactose, sucrose fermentation).
• IMViC tests:
  • Indole test: Tryptophan → indole (detected with Kovac's reagent).
  • Methyl Red (MR): Detects stable acid end-products of mixed-acid fermentation.
  • Voges-Proskauer (VP): Detects acetoin (butanediol fermentation).
  • Citrate utilization: Ability to use citrate as sole carbon source.
• Catalase test: Detects breakdown of into water and oxygen.
• Oxidase test: Detects cytochrome c oxidase.
• Urease test: Detects urea hydrolysis to ammonia.
• H₂S production: Detected on TSI/SIM media.
• Gas-Liquid Chromatography (GLC): Analyzes volatile fatty acids — used for identifying anaerobes.

Applications:

• Differentiation of Enterobacteriaceae (IMViC).
• Identification of anaerobes by metabolic end-product profiles.

Conclusion: Metabolic product analysis remains a cornerstone of conventional and rapid microbial identification.

The four major groups of microbial diseases differ in their causative agents and diagnostic approaches.

Key points:

• Bacteria are readily cultured; viruses require cell culture or molecular detection.
• Fungi are identified by morphology and special media.
• Protozoa are largely diagnosed by microscopy of blood/tissue.

Conclusion: Diagnostic strategy must be tailored to the biology of each microbial group.

Opportunistic infection is an infection caused by microorganisms that are normally harmless (often part of the normal flora or environment) but cause disease when the host's defenses are weakened.

Predisposing factors:

• Immunosuppression:
  • HIV/AIDS (e.g., Pneumocystis jirovecii pneumonia, oral candidiasis)
  • Chemotherapy/steroids
  • Organ transplantation (immunosuppressive drugs)
• Disruption of normal flora: Prolonged antibiotic use → Clostridioides difficile colitis, Candida overgrowth.
• Breach of physical barriers: Burns, wounds, surgery, catheters → Pseudomonas, Staphylococcus infections.
• Underlying diseases: Diabetes mellitus predisposes to fungal and bacterial infections.
• Extremes of age: Neonates and the elderly with immature/waning immunity.
• Indwelling medical devices: Biofilm-associated infections (catheters, prosthetics).

Common opportunistic pathogens: Candida albicans, Pseudomonas aeruginosa, Pneumocystis jirovecii, Cryptococcus neoformans, cytomegalovirus.

Conclusion: Opportunistic infections highlight the crucial role of an intact immune system and normal flora in maintaining health.

For a microbe to cause disease, it must gain entry, establish itself, and damage the host. This is the basis of microbial pathogenesis.

Portals of entry:

• Respiratory tract: Inhalation of droplets (e.g., M. tuberculosis, influenza).
• Gastrointestinal tract: Ingestion of contaminated food/water (e.g., Salmonella, Vibrio cholerae).
• Skin and mucous membranes: Cuts, wounds, bites (e.g., Clostridium tetani, rabies virus).
• Genitourinary tract: Sexual contact (e.g., Neisseria gonorrhoeae, HIV).
• Parenteral route: Injections, blood transfusion.

Mechanisms of pathogenesis:

1. Adherence: Attachment via fimbriae, adhesins to host cells.
2. Colonization & invasion: Multiplication and penetration using invasins/enzymes.
3. Immune evasion: Capsules, antigenic variation, intracellular survival.
4. Toxin production:
   • Exotoxins: Specific tissue damage.
   • Endotoxins: Fever, inflammation, shock.
5. Host tissue damage: Direct cytopathic effects or immune-mediated damage.

Portal of exit: Respiratory secretions, feces, blood, etc., enabling transmission.

Conclusion: The interplay of entry route, virulence factors, and host defenses determines the disease outcome.

Culture media are nutrient preparations used to grow, isolate, and identify microorganisms in the laboratory.

Basic requirements: Sources of carbon, nitrogen, energy, minerals, growth factors, appropriate pH, and moisture.

Types based on function:

Distinguishing points:

• Enriched = adds nutrients for fastidious organisms.
• Selective = inhibits unwanted organisms.
• Differential = distinguishes organisms by appearance.

Note: MacConkey agar is both selective and differential.

Conclusion: Choosing the correct medium is essential for successful isolation and identification of pathogens.

Modern molecular and rapid methods have transformed diagnostic microbiology but do not entirely replace conventional culture. A balanced comparison is important.

Advantages of molecular/rapid methods:

• Speed: Results in minutes to hours vs. days for culture.
• High sensitivity & specificity: Detect very low numbers of organisms.
• Detection of non-culturable/slow-growing organisms (e.g., M. tuberculosis, viruses).
• Direct resistance detection via genes (e.g., mecA, rpoB).
• Automation & standardization reduce human error (e.g., VITEK, MALDI-TOF).

Limitations of molecular/rapid methods:

• High cost of equipment and reagents.
• Require skilled personnel and infrastructure.
• Contamination risk leading to false positives.
• Detect dead organisms — positive result without viability.
• Limited to known targets (primers/probes needed).
• Cannot always provide antimicrobial susceptibility profile phenotypically.

Advantages of conventional culture:

• Provides viable isolate for full susceptibility testing.
• Low cost and widely available.
• Allows quantitation and typing.

Limitations of culture: Slow, misses fastidious/non-culturable organisms, affected by prior antibiotics.

Conclusion: A combination of conventional and rapid/molecular methods provides the most reliable and clinically useful diagnosis.