Microbiology Quiz on Pathogenic Bacteria

Bacterial Culture Methods and Diagnoses

• N. meningitidis carriers: Specimens are nasopharyngeal swabs (West swab). Culture specimens on Thayer martin medium for 24 hours at 37°C with 5-10% CO2.

Moraxella catarrhalis (commensal neisseria)

• Growth: Grows at room temperature and on nutrient agar at 37°C. Growth occurs on chocolate agar in 24 hours.
• Diseases: Causes otitis media, sinusitis, and pneumonia
• Specimen: Ear discharge or sputum
• Microscopy: Gram negative diplococci
• Culture: On nutrient agar
• Colony Characteristics: Dry, greyish-white, opaque colonies. Colonies "scoot" across the media when pushed with a loop.
• Biochemical Reactions: Identified by biochemical reactions. No acid production from glucose, maltose, lactose, and sucrose (or variable).

Commensal Neisseria

• Location: Present in throat, nasopharynx, nose, mouth, and less frequently external genitalia
• Misidentification: Can be mistaken for Neisseria meningitidis

Corynebacterium diphtheriae

• Morphology: Gram-positive bacilli, non-motile, non-capsulated, non-spore forming. Bacilli arranged in Chinese letter patterns. Beaded appearance due to metachromatic granules (phosphate granules). Methylene Blue stains the bacilli, but not the granules.
• Growth Requirements: Aerobic bacteria that grow best on Loeffler's serum at 37°C. Will also grow on blood agar.
• Virulence: Depends on the production of exotoxin.

Diphtheria

• Transmission: Transmitted by oral droplets or physical contact. Also transmitted by saliva.
• Source: Case or carrier
• Causative Organism: Corynebacterium diphtheriae
• Disease Pathogenesis: Released exotoxin disseminates through the blood, affecting the heart, muscle, peripheral nerves, adrenals, and kidneys.
• Symptoms: Characterized by sore throat, low-grade fever, and pseudomembrane on tonsils. Removal of the membrane causes bleeding and toxin spread. Respiratory obstruction and suffocation may occur.
• Clinical Signs: Low-grade fever, bull neck, sore throat, difficulty swallowing, severe myocarditis

Laboratory Diagnosis of Diphtheria

• Specimen: Throat swabs/membrane exudates
• Direct Smears: Gram stain reveals gram positive bacilli
• Cultures: Incubated on Loeffler's serum
• Colonial Morphology Identification: Beaded gram positive bacilli arranged in Chinese letters
• Subsequent Cultures: Blood agar (excludes Str. Pyogenes)
• Biotyping: Blood tellurite agar differentiate gravis, mitis, and intermedius
• Toxigenicity Test: Elek's test
• ELISA: Used to detect diphtheria toxin in the specimen
• Carrier Diagnosis: Throat or nasal swaps are tested for toxigenicity to diagnose carriers.

Diphtheroids

• Description: Normal flora of the mucous membrane in the throat, skin, respiratory tract
• Morphology/characteristics: Gram-positive bacilli, non-spore-forming, non-motile, arranged in angles or palisades.
• Disease associations: Urinary tract infection, Bacteraemia, Osteomyelitis

Tuberculosis

• Causative Organism: M. tuberculosis
• Morphology/Characteristics: Acid-fast bacilli (thin straight/curved rods)
• Growth Characteristics: Grows slowly, typically 2-8 weeks on egg-selective medium (Lowenstein-Jensen medium). 6-8% CO2 enhances growth.
• Virulence: Ability to survive intracellularly inside macrophages
• Other characteristics: Rapid grow on liquid medium form serpentine form, Acid-fast bacilli in chains due to the cord factor
• Learning Outcomes: The student will be able to know morphology and growth of M. tuberculosis. Recognize clinical forms of tuberculosis, understand cell-mediated immunity in tuberculosis; Analyze tuberculin test results; know and perform Quanti-Feron test; diagnose tuberculosis using laboratory techniques; understand prevention and treatment strategies
• Typical vs. saprophytic M. avium, M. kansasii, M. intracellulare: Rapid grower or slow grower, Pigmented or non-pigmented colonies, acid-fast/alcohol-fast staining characteristics

Mycobacteria

• Description: Acid-fast bacilli, Aerobic, non-spore forming organisms. Cell wall contains mycolic acids, making it impermeable to most stains.
• Medically Important Species: M. tuberculosis, M. bovis, M. leprae, atypical mycobacteria (NTM: M. avium, M. kansasii, M. intracellulare), environmental saprophytic mycobacteria (like M. smegmatis)
• Growth Requirements: Grows slowly (weeks) on egg-selective medium (Lowenstein-Jensen medium) 6-8% CO2 inhance growth.
• Virulence Factors: The ability to survive inside macrophages

Bacillus Group (Bacillus anthracis)

• Morphology: Large aerobic, Gram-positive, spore-forming bacilli
• Pathogenic Species: Bacillus anthracis (anthrax)
• Saprophytic Species: Anthracoids (often found in water, soil, and air). Some strains can be pathogenic (e.g., B. cereus, food poisoning agent)
• Bacillus cereus: Gram-positive bacilli, non-capsulated, motile, spore-forming
• Lab Identification: Gram-stained smears show Gram-positive bacilli with square/bamboo stick ends arranged in chains, and surrounded by an unstained halo capsule. Organisms appear blue while capsule is pink using polychrome methylene blue stain, non-hemolytic colonies. The specimen shows inverted fire-tree appearance on gelatin medium.

Anaerobic Non-Spore Forming Bacteria

• Definition: Do not need oxygen.
• Classification:
  • Gram negative bacilli (Bacteroides)
  • Gram positive bacilli (Lactobacillus, Actinomyces)
• Important Characteristics: These organisms do not survive in the presence of oxygen
• Key Organisms: Bacteroides, Lactobacillus, and Actinomyces
• Gram Negative Bacilli: The specimens are normal inhabitants of bowel , vagina and oral cavities
  • Intraabdominal infection
  • Post operative wound infection following abdominal operation
  • Puerperal sepsis
  • Periodontitis
  • Lung abscess

Anaerobic Spore Forming Bacteria (Clostridium)

• Characteristics: Gram-positive, spore forming, anaerobic bacteria.
• Habitat: Intestinal tract, soil, water
• Important Species: C. tetani, C. perfringens (gas gangrene), C. botulinum, and C. difficile
• C. Perfringens: Gram-positive bacilli, non-motile, non-capsulated, subterminal spores. Grows beta-hemolytic colonies on blood agar, produce thin film and swarming. Can be diagnosed by motility, catalase and indole test. Gelatin liquefaction test, stormy clot formation with litmus milk, and opaque zones on egg yolk medium.
• C. Tetani: Gram-positive bacilli, anaerobic, non-capsulated, drumstick shape, terminal spores in culture
• Lab Identification: Gram stain shows gram positive bacilli with drumstick appearance. Colonies on Robertson cooked meat medium grow. subculture on blood agar and incubated anaerobically. Produce a thin-film or swarming appearance. Are a-hemolytic followed by β-hemolysis , motility. Gelatin liquefaction
• C. difficile: Antibiotic-associated diarrhea

Listeria monocytogenes

• Characteristics: Gram-positive bacilli, arranged in short chains, non-capsulated, motile, non-spore forming, cold tolerant.
• Disease Association: Neonatal listeriosis and adult listeriosis.
• Culture: Grows well on blood agar. Colonies produce a narrow zone of hemolysis similar to Group B Streptococcus.
• Identification: Direct smear of specimen shows gram-positive bacilli, catalase-positive, tumbling motility at 25°C (umbrella type).
• Differentiation: Differentiated from other bacteria based on various tests (e.g., catalase+, motility, esculin hydrolysis).

Haemophilus influenzae

• Description/Characteristics: Gram-negative pleomorphic coccobacilli. Fastidious, requiring specific growth factors (X and V factors) like hemin and NAD.
• Disease Association: Acute bacterial meningitis in young children. Other potential diseases: meningitis, urethritis, associated with eye infections, chancroid.
• Culture: Grows on chocolate agar (both X and V factors are present). Requires 35-37°C, pH 7.6, and 5% CO2. Will not grow on normal blood agar.
• Identification/Diagnosis:
  • Direct stain of CSF/pus/specimen/body fluid will reveal pleomorphic gram-negative coccobacilli
  • Capsular polysaccharide detection by quellung or other methods
  • Culture: Use chocolate or other enriched media, colonies are identified by their morphology

This quiz covers various aspects of pathogenic bacteria including Neisseria meningitidis, Corynebacterium diphtheriae, and Moraxella catarrhalis. It tests knowledge on their diagnostic methods, culture conditions, morphological characteristics, and virulence factors. Perfect for students in microbiology or medical programs.