Notes on Pathogenic Bacteria Classification and Identification

Pathogenic Bacteria Classification

Micrococcus
  • Category: Opportunistic pathogen
  • Gram Stain: Positive (Gram +)
  • Morphology: Nonmotile, tetrad-arranging, pigmented (yellow), aerobic coccus
  • Biochemical Properties:
    • Urease positive
    • Catalase positive
  • Clinical Relevance:
    • Cutaneous skin infections
    • Rarely pathogenic in healthy humans, but opportunistic in hospital settings
    • Colonizes the human mouth and respiratory tract
    • Produces acids that inhibit other pathogens
Streptococcus (Enterococcus)
  • Category: Group includes many species from normal flora; some pathogenic
  • Morphology: Forms chains
  • Gram Stain: Positive (Gram +)
  • Clinical Relevance:
    • Opportunistic pathogens (e.g., Streptococcus faecalis) may cause infections in hospital settings
    • Leading cause of healthcare-associated infections
    • Exhibits intrinsic resistance to multiple antibiotics
Bacillus
  • Category: Endospore forming bacteria
  • Morphology: Rod-shaped
  • Gram Stain: Positive (Gram +)
  • Clinical Relevance:
    • Commonly found in soil and can cause food poisoning
    • Forms biofilms; Bacillus cereus is known for causing nausea, vomiting, and diarrhea due to toxin production
    • 'Emetic' form linked to improperly cooked rice
Pseudomonas
  • Category: Environmental pathogen
  • Morphology: Rod-shaped
  • Gram Stain: Negative (Gram -)
  • Clinical Relevance:
    • Causes infections in burn patients, nails, and eyes (e.g., Pseudomonas aeruginosa)
    • Multidrug resistant; produces blue-green pigment (pyocyanin)
    • Found in medical equipment, particularly on moist surfaces

Media Types for Bacterial Identification

Mannitol Salt Agar (MSA)
  • Type: Differential & selective media
  • Features:
    • High salt concentration (7.5% NaCl) selects for salt-tolerant bacteria (mainly Staphylococcus)
    • Differential agent for fermentation of mannitol using a pH indicator; ferments to turn agar yellow
MacConkey Agar (MAC)
  • Type: Differential & selective media
  • Features:
    • Selectively isolates Gram-negative enteric bacteria
    • Lactose fermenters turn the media red or pink; non-fermenters remain unchanged
    • Contains crystal violet and bile salts to inhibit Gram + growth
Blood Agar Plates (BAP)
  • Type: Differential media
  • Features:
    • Standard nutrient agar augmented with 5% sheep's blood
    • Used to detect hemolysis patterns:
    • Beta hemolysis: Clear zone (complete lysis)
    • Alpha hemolysis: Greenish zone (partial lysis)
    • Gamma (non-hemolytic): No change in agar

Rapid Detection of Pathogenic Bacteria

API 20 E Test
  • Purpose: Rapid identification of enteric pathogenic bacteria
  • Procedure:
    • Hydrate the API 20 E panel with distilled water
    • Inoculate with bacterial samples and overlay appropriate tests
    • Incubate and read results within 24 hours
  • Overview: 20 tests for species/subspecies identification; results provide a numerical code for species identification

Gram-Positive Pathogenic Bacteria

Staphylococcus
  • Key Species: Staphylococcus aureus (pathogenic) and Staphylococcus epidermidis (opportunistic)
  • Clinical Relevance:
    • S. aureus associated with MRSA infections, which can lead to severe conditions such as:
    • Osteomyelitis
    • Cellulitis
    • Scalded skin syndrome
  • Distinguishing Properties:
    • S. aureus produces coagulase; can ferment mannitol
Enterococcus faecalis
  • Characteristics:
    • Commensal in gut; opportunistic in other body parts
    • Causes serious infections (e.g., sepsis, endocarditis)
  • Transmission: Poor hygiene, handwashing after using the bathroom is crucial
Bacillus cereus
  • Significance:
    • Responsible for food poisoning (fried rice syndrome)
    • Two types of toxins: one causing diarrhea and cramps, the other causing nausea and vomiting, linked to starchy foods

Gram-Negative Pathogenic Bacteria

Escherichia coli (E. coli)
  • Characteristics:
    • Diverse; some strains are harmless, while others (e.g., E. coli O157:H7) can cause severe illness
    • Causes UTIs, foodborne illnesses, and respiratory infections
  • Public Health Concern: Fecal contamination can lead to outbreaks
Citrobacter
  • Clinical Importance:
    • Opportunistic pathogen; significant in immunocompromised patients
    • Associated with infections of the respiratory tract and urinary tract
Pseudomonas aeruginosa
  • Infection Risks:
    • Strong association with hospital-acquired infections, especially in patients with invasive devices

Conclusion

  • Understanding the classification, identification methods, and clinical implications of pathogenic bacteria is crucial for accurate diagnosis and treatment in healthcare settings.