---

BFUHS BSc Nursing 3rd Semester - Applied Microbiology

UNIT 3: PATHOGENIC ORGANISMS - Complete Notes

---

UNIT 3 SYLLABUS OVERVIEW

1. Gram-positive cocci: Staphylococcus, Streptococcus (including S. pneumoniae)
2. Gram-negative rods: Salmonella typhi, E. coli, Vibrio cholerae
3. Mycobacteria: M. tuberculosis, M. leprae
4. Spirochetes
5. Rickettsiae, Chlamydia, Mycoplasma
6. Viruses (basic)

---

1. STAPHYLOCOCCUS AUREUS

Morphology & Biology

• Gram-positive cocci arranged in clusters (grape-like bunches)
• Catalase-positive, coagulase-positive
• Non-motile, non-sporing, facultative anaerobe
• Grows on ordinary media; golden-yellow pigment on nutrient agar

Virulence Factors

Epidemiology

• Normal flora on skin and nasal mucosa
• Spreads by direct contact, fomites (bedlinen, clothing)
• Survives on dry surfaces for long periods (thick peptidoglycan wall)
• MRSA (Methicillin-resistant S. aureus) is a major hospital and community threat

Diseases Caused

• Pyogenic (pus-forming): Impetigo, folliculitis, furuncles (boils), carbuncles, wound infections, osteomyelitis
• Toxin-mediated: Food poisoning, Toxic Shock Syndrome (TSS), Scalded Skin Syndrome (Ritter disease)
• Systemic: Bacteremia, endocarditis, pneumonia, meningitis (via shunts)

Nursing Relevance - Risk Groups

• Infants: Scalded skin syndrome
• Young children: Impetigo
• Post-surgical patients / IV catheter patients: Bacteremia, endocarditis
• Immunocompromised: Systemic infections

Lab Diagnosis

• Gram stain: Gram+ cocci in clusters
• Culture: Mannitol salt agar (selective), blood agar (golden colonies with beta-hemolysis)
• Coagulase test (tube/slide) - definitive for S. aureus

Treatment

• MSSA: Penicillinase-resistant penicillin (cloxacillin, flucloxacillin)
• MRSA: Vancomycin (drug of choice IV), Linezolid, TMP-SMX, Doxycycline (oral)
• Localized infections: Incision and drainage

---

2. STREPTOCOCCUS (including S. pneumoniae)

Classification

Streptococcus pyogenes (Group A Beta-Hemolytic Strep)

• Gram-positive cocci in chains
• Produces streptolysin O (ASO titer elevated in diagnosis) and streptolysin S
• Diseases: Pharyngitis ("strep throat"), scarlet fever, erysipelas, cellulitis, impetigo
• Post-streptococcal complications: Rheumatic fever, acute glomerulonephritis
• Treatment: Penicillin (drug of choice)

Streptococcus pneumoniae (Pneumococcus)

• Elongated gram-positive cocci in pairs (diplococci) and short chains
• Polysaccharide capsule - major virulence factor (anti-phagocytic)
• Alpha-hemolytic; bile-soluble; optochin-sensitive

• Polysaccharide capsule: Prevents phagocytosis
• Pneumolysin: Damages host tissues, stimulates inflammation
• IgA protease: Degrades secretory IgA on mucosal surfaces
• Teichoic acid (C polysaccharide): Triggers inflammation

• Colonizes nasopharynx/oropharynx (endogenous spread)
• Risk groups: Young children, elderly, patients with sickle cell disease, asplenic patients, post-viral respiratory illness
• Person-to-person spread via respiratory droplets is rare

• Pneumonia (most common cause of community-acquired pneumonia)
• Meningitis (most common cause in adults)
• Otitis media, sinusitis
• Bacteremia/sepsis

• Gram stain: Lancet-shaped diplococci
• Culture on blood agar: Alpha-hemolytic, mucoid colonies
• Optochin sensitivity test (sensitive = pneumococcus)
• Bile solubility test (positive = pneumococcus)
• Quellung reaction (capsule swells with type-specific antibody)

• Penicillin for susceptible strains
• Vancomycin + Ceftriaxone (empirical therapy)
• Vaccines: 13-valent conjugate vaccine (children under 2), 23-valent polysaccharide vaccine (adults at risk)

---

3. MYCOBACTERIUM TUBERCULOSIS

Morphology & Biology

• Weakly gram-positive, strongly acid-fast bacilli (AFB)
• Slender rods, non-motile, non-sporing, aerobic
• Lipid-rich cell wall (mycolic acid) - makes it:
  • Resistant to Gram stain (use Ziehl-Neelsen stain)
  • Resistant to disinfectants and detergents
  • Resistant to many antibiotics
  • Resistant to host immune killing
• Obligate intracellular pathogen - grows inside alveolar macrophages
• Slow growing: 4-6 weeks on Lowenstein-Jensen (LJ) medium

Virulence

• Cord factor (trehalose dimycolate): Inhibits phagosome-lysosome fusion
• Sulfatides: Prevent lysosomal attack
• Disease is primarily from the host immune response (granuloma formation), not direct toxin

Epidemiology

• Humans are the only reservoir
• Transmission: Person-to-person via infectious aerosols (droplet nuclei)
• One of the most common infectious diseases worldwide - 10.4 million new cases/year, 1.6 million deaths/year
• India has the highest TB burden globally
• Risk groups: HIV/immunocompromised patients, malnourished, homeless, substance abusers, diabetics, crowded living conditions

Pathogenesis

1. Inhaled bacilli reach alveoli
2. Engulfed by macrophages - survive and replicate intracellularly
3. Primary complex (Ghon complex): Focus in lung + affected lymph nodes
4. Cell-mediated immunity develops (2-6 weeks) - granuloma formation
5. Most cases remain latent (90%); 10% progress to active disease
6. Reactivation TB: When immunity falls (HIV, malnutrition, steroids)

Clinical Features

Lab Diagnosis

• Sputum microscopy: Ziehl-Neelsen (ZN) stain - AFB appear red on blue background
• Culture: LJ medium (4-6 weeks); BACTEC system (faster)
• Tuberculin skin test (Mantoux): Purified Protein Derivative (PPD) injected intradermally; induration >10 mm positive (>5 mm in HIV)
• IGRA (Interferon-gamma release assay): More specific than Mantoux
• Nucleic acid amplification: Gene Xpert MTB/RIF (rapid; detects rifampicin resistance)

Treatment (RIPE Regimen)

• R - Rifampicin
• I - Isoniazid (INH)
• P - Pyrazinamide
• E - Ethambutol

• Rifampicin + Isoniazid

• BCG vaccine (Bacillus Calmette-Guérin): Given at birth in India; protects against severe forms
• INH prophylaxis for 6-9 months for close contacts

---

4. MYCOBACTERIUM LEPRAE (Leprosy)

Key Features

• Acid-fast rods (weakly)
• Cannot be cultured in artificial media (only in armadillo footpad or mouse footpad)
• Very slow growing (doubling time ~13 days)
• Temperature optimum: 27-30°C (grows in cool body parts - skin, peripheral nerves)

Transmission

• Prolonged close contact with untreated leprosy patients
• Via nasal secretions (aerosol) or skin contact
• Long incubation: 2-10 years

Types of Leprosy

Diagnosis

• Slit skin smear: ZN stain - AFB in globi (lepromatous form)
• Lepromin (Mitsuda) test: Measures cell-mediated immunity (not for diagnosis, for prognosis)

Treatment (MDT - Multi-Drug Therapy, WHO)

---

5. SALMONELLA TYPHI (Typhoid Fever)

Morphology

• Gram-negative rods (Enterobacteriaceae)
• Motile (peritrichous flagella), non-capsulated
• Produces H₂S, does not ferment lactose

Pathogenesis

1. Ingested via contaminated food/water
2. Bacteria attach to M cells (Peyer's patches) of small intestine
3. Survive inside macrophages, transported to bloodstream (bacteremia)
4. Multiply in liver, spleen, gallbladder (chronic carriers possible)
5. Re-enter intestine via bile - causes ulceration of Peyer's patches

Epidemiology

• Source: Only humans (fecal-oral route)
• Vehicle: Contaminated food, water; chronic carriers (especially via gallbladder)
• 27 million cases/year worldwide; highest in developing countries
• No animal reservoir

Clinical Features (Widal Test)

Lab Diagnosis

• Blood culture: Gold standard (positive in Week 1)
• Stool/urine culture: Positive from Week 2 onwards
• Widal test: Antibody agglutination (O and H antigens); rising titers significant
• Modern: ELISA, PCR

Treatment

• Ceftriaxone (drug of choice for hospitalized patients)
• Azithromycin (for uncomplicated cases)
• Fluoroquinolones (ciprofloxacin; resistance increasing)
• Chloramphenicol (historical drug of choice; now resistance widespread)
• Prevention: Safe water, sanitation, typhoid vaccine (Ty21a oral or Vi polysaccharide injectable)

---

6. VIBRIO CHOLERAE (Cholera)

Morphology

• Gram-negative, comma-shaped (curved) rods
• Highly motile (single polar flagellum - "darting motility")
• Oxidase-positive
• Grows on TCBS (Thiosulfate Citrate Bile Sucrose) agar - yellow colonies

Virulence - Cholera Toxin

• CT (Choleragen) - AB₅ toxin
  • B subunit: Binds to GM₁ ganglioside receptors on intestinal epithelium
  • A subunit: Activates adenylate cyclase → ↑↑ cAMP → massive Cl⁻ secretion + Na⁺ and water follow
  • Result: "Rice-water" diarrhea (up to 20 litres/day)

Epidemiology

• Fecal-oral route; contaminated water and food (shellfish, fish)
• Biotype El Tor is the current pandemic strain (7th pandemic)
• Endemic in parts of Asia, Africa, South America

Clinical Features

• Incubation: Few hours to 5 days
• Sudden onset profuse watery "rice-water" stools
• Vomiting without nausea
• Severe dehydration, hypovolemic shock, metabolic acidosis
• "Washerwoman's hands" - skin wrinkling
• No fever (enterotoxin-mediated, not invasive)

Treatment

• ORS (Oral Rehydration Solution) - cornerstone of treatment
• IV fluids (Ringer's lactate) for severe dehydration
• Doxycycline - shortens duration; tetracycline alternatives
• Notification: Cholera is a notifiable disease

---

7. CLOSTRIDIUM (Gram-positive Spore-forming Anaerobes)

Clostridium tetani (Tetanus)

• Drum-stick appearance (terminal spore)
• Tetanospasmin toxin: Blocks inhibitory neurotransmitter (GABA, glycine) → spastic paralysis, trismus (lock jaw), risus sardonicus
• Prevention: TT (Tetanus Toxoid) vaccine
• Treatment: Antitoxin (TIG - Tetanus Immunoglobulin), metronidazole, wound care, diazepam

Clostridium perfringens (Gas Gangrene)

• Lecithinase (alpha toxin) causes gas gangrene in wounds
• Also causes food poisoning

Clostridium botulinum (Botulism)

• Produces botulinum toxin (most potent biological toxin)
• Blocks acetylcholine release at NMJ → flaccid paralysis
• Food-borne (canned foods), infant, wound botulism

---

8. SPIROCHETES

Treponema pallidum (Syphilis)

• Very thin, cannot be seen on routine staining - use dark-field microscopy
• Transmitted sexually (or congenitally)

• Non-treponemal tests: VDRL, RPR (screening; false positives possible)
• Treponemal tests: FTA-ABS, TPHA (confirmatory)

Leptospira (Leptospirosis)

• Zoonosis (rats, cattle, dogs)
• Weil's disease: Jaundice + renal failure + hemorrhage
• Treatment: Doxycycline, penicillin

Borrelia burgdorferi (Lyme Disease)

• Transmitted by tick (Ixodes tick)
• Erythema migrans (bull's-eye rash), arthritis, cardiac and neurological involvement
• Treatment: Doxycycline

---

9. RICKETTSIAE

Key Features

• Obligate intracellular parasites (cannot survive outside host cells)
• Gram-negative, very small
• Transmitted by arthropod vectors (lice, ticks, fleas, mites)
• Cannot be cultured on ordinary media; require cell cultures

Important Species

Weil-Felix Reaction

• Serological test using Proteus agglutination (cross-reactive antigens)
• OX-19, OX-2, OX-K agglutinins used diagnostically

Treatment

• Doxycycline (drug of choice for all rickettsial infections)
• Chloramphenicol (alternative)

---

10. CHLAMYDIA

Key Features

• Obligate intracellular parasites
• Two forms:
  • Elementary body (EB): Infectious, extracellular, metabolically inactive
  • Reticulate body (RB): Non-infectious, intracellular, metabolically active, divides by binary fission
• Cannot be cultured on ordinary media; require cell culture

Important Species

Diagnosis

• Direct fluorescent antibody (DFA)
• ELISA
• PCR (most sensitive and specific)
• Cell culture

Treatment

• Azithromycin (single dose for uncomplicated genital infections)
• Doxycycline (7-14 days)
• Erythromycin (in pregnancy)

---

11. MYCOPLASMA

Key Features

• Smallest free-living organisms (no cell wall)
• Pleomorphic (no fixed shape, as no cell wall)
• Resistant to penicillin and all beta-lactam antibiotics (no cell wall target)
• Require cholesterol for membrane function (obtained from host)
• Grow slowly on special enriched media; "fried-egg" colonies

Important Species

Diagnosis

• Serology: Cold agglutinin test (positive in M. pneumoniae)
• PCR (most accurate)
• Culture on Eaton's agar (very slow - 2-3 weeks)

Treatment

• Doxycycline or Azithromycin (macrolides)
• Fluoroquinolones
• NOT penicillin (no cell wall)

---

QUICK COMPARISON TABLE - INTRACELLULAR PATHOGENS

---

IMPORTANT EXAM MNEMONICS

---

NURSING POINTS FOR ALL PATHOGENS

1. Hand hygiene is the single most important measure to prevent pathogen transmission
2. Standard precautions for all patients; droplet precautions for TB (N95 mask)
3. Contact tracing for TB, syphilis, and cholera
4. Notifiable diseases: Cholera, plague, typhoid, TB, leprosy are notifiable under Indian law
5. Vaccination records: BCG at birth, TT for wound management, typhoid and cholera vaccines
6. Monitor patients on anti-TB therapy for hepatotoxicity (liver function), optic neuritis (ethambutol), peripheral neuropathy (INH - give pyridoxine prophylactically)
7. Isolation requirements: TB (airborne precautions), MRSA (contact precautions)
8. ORS preparation: 1 litre water + 6 tsp sugar + 1/2 tsp salt (for cholera/diarrheal management)

---