Bacterial Infections: Gardnerella & Diphtheria

Aerobic Gram Positive Bacilli

• Spore-forming: Bacillus
• Non-spore former: Corynebacterium, Arcanobacterium, Rhodococcus, Listeria, Erysipelothrix, Gardnerella, Rothia
• Branching non-spore former: Actinomycetes, Nocardia
• Many are contaminants or commensals
• Some cause significant diseases

Non-Spore Forming, Non-Branching Catalase Positive Bacilli

• Corynebacterium:

  • More than 60 species, 40 clinically significant
  • Normal microbiota of skin and mucous membranes
  • Cell walls contain m-DAP
  • All are catalase-positive, and non-motile
  • Divided into lipophilic and non-lipophilic
  • Lipophilic are fastidious and grow slowly (at least 48 hours) on standard media
  • Appear slightly curved, club-shaped bacilli on Gram stain
  • Coryneform-like isolates require 16s rRNA sequencing for species identification
  • Species include: C. bovis, C. ulcerans, C. xerosis, C. jeikeium, C. pseudodiphtheriticum, C. pseudotuberculosis
  • Corynebacterium diphtheriae:
    • Toxin produced by strains infected with lysogenic β-phage
    • Non-toxigenic strains can be converted to toxin-positive by appropriate β-phage infection
    • Toxin composed of two fragments (A and B) linked by a disulfide bridge
    • Fragment A is cytotoxic
    • Fragment B binds to eukaryotic cell receptors
    • Potent and lethal to humans (130 ng/kg body weight)
    • Blocks protein synthesis
    • Non-toxic until exposed to trypsin

• Listeria monocytogenes:

  • Six species, only L. monocytogenes and L. ivanovii are pathogenic
  • Found in soil, water, vegetation, and animal products (raw milk, cheese, poultry, processed meats)
  • Isolated from crustaceans, flies, and ticks
  • Causes serious infections in neonates, pregnant women, the elderly, and immunocompromised hosts
  • Virulence factors include listeriolysin O (hemolysin), catalase superoxide dismutase, phospholipase C, and surface protein p60
  • Clinical infections:
    • Pregnant women (most common in the third trimester, causing spontaneous abortion and stillbirth)
    • Flu-like illness (fever, headache, myalgia)
    • Neonates (intrauterine infection or shortly after birth, often resulting in sepsis; and late-onset in several days to weeks after birth, often manifesting as meningitis)
    • Immunocompromised (most commonly CNS infection and endocarditis from eating contaminated food)
  • Lab Diagnosis:
    • Microscopy: Gram-positive coccobacillus, short chains, palisades
    • Culture: Grows on SBA, chocolate agar, nutrient agar, BHIB and Thio broth
    • Colonial appearance: Small, round, smooth, and translucent, surrounded by a narrow zone of β-hemolysis
    • Growth occurs over a wide range (0.5-45°C); optimum at 30-35°C
    • Cold enrichment may help
    • Identification:
      • Catalase-positive
      • Tumbling motility in wet smear; umbrella pattern in motility medium at room temperature (but not at 35°C), Hippurate hydrolysis +, BE +, CAMP + Block type of hemolysis on CAMP Produces acid from glucose VP and MR +

• Erysipelothrix rhusiopathiae:

  • Gram-positive, non-spore-forming, pleomorphic rods (can produce long filaments)
  • Distributed in nature
  • Can cause disease in animals; swine as the main reservoir
  • Humans acquire infection via occupational exposure (cuts, scratches)
  • Clinical infections: Erysipeloid (self-limiting localized infection at site of inoculation, painful swelling usually on hands or fingers; heals within 3-4 weeks), endocarditis (may occur in those with valve replacements), disseminated infections rarely due to exacerbation of erysipeloid lesions)
  • Lab Diagnosis:
    • Microscopy: Pleomorphic, Gram-positive thin rods, long filaments or short rods, arranged singly, in short chains, or in a V shape, easily decolorized (gram variable)
    • Culture: CO2 is required; grows on blood or chocolate agar
    • Colonial appearance: Colonies appear gray or translucent, pinpoint
    • Identification:
      • Catalase-negative
      • Nitrate-negative
      • Urease-negative
      • Non-motile
      • Production of H2S on TSI
      • VP-negative
      • Does not grow on HOH esculin in semisolid motility media
      • 2 colony types: Pinpoint, non-hemolytic, glistening or larger, rough colonies with matte/curled/irregular edges

• Arcanobacterium haemolyticum:

  • Pharyngitis, desquamation of skin on hands and feet, soft tissue infections, sepsis, endocarditis
  • Produces small colonies with a narrow zone of β-hemolysis after 24-48 hours of incubation
  • Lipase+ and Lecithinase+
  • Penicillin-resistant, erythromycin is the drug of choice.

• Gardnerella vaginalis:

  • Member of normal flora of female genital tract
  • Associated with bacterial vaginosis
  • Foul vaginal odor
  • Vaginal pH > 4.5
  • Lab Diagnosis:
    • Wet prep: clue cells (large epithelial cells with various bacterial types on edges)
    • Gram stain: small, thin,gram-variable rods
    • Culture: growth on BAP, CA; no growth on MAC, human blood bilayer tween "V" agar, beta-hemolytic; requires CO2 environment
    • Catalase-negative

• Mycobacterium tuberculosis:

  • Causes tuberculosis, a classic human disease.
  • Pathogenesis: inhaled aerosols ingested by alveolar macrophages -> bacilli replicated -> macrophages die -> infected macrophages migrate to local lymph nodes -> Ghon's focus develops -> cell-mediated immune response halts the cycle of destruction and spread -> viable, non-replicating bacilli remain in macrophages
  • Clinical Presentations: pulmonary (primary complex asymptomatic -> acute pulmonary disease/systemic spread -> latent disease/military tuberculosis), later disease (renal/CNS)
  • Diagnosis: chest X-ray (hilar lymphadenopathy or calcification), PPD (Mantoux or Heaf test), sputum for AFB
  • Treatment: anti-tuberculous drugs (INH, Rifampicin, Ethambutol, Pyrazinamide), DOT (directly observed therapy)
  • Prevention: improved social conditions, case detection and treatment, contact tracing, treatment of infected/diseased contacts, BCG (bacille Calmette-Guérin)

• Other Non-tuberculous Mycobacteria:

  • M. kansasii: Infection of respiratory compromised hosts, presenting like pulmonary tuberculosis, treatment-resistant to anti-TB drugs
  • M. scrofulaceum: Infection of cervical lymph nodes, presenting as cervical lymphadenopathy, treatment-surgery
  • M. avium intracellulare: Immuno-competent host infection of cervical lymph nodes, presenting as cervical lymphadenopathy, treatment-surgery; Immuno-deficient host infection, presenting with severe RTI, severe GI infections,septicaemia
  • M. ulcerans: Causes "Buruli" ulcer, needs prolonged incubation for growth
  • M. fortuitum / M. chelonae: Injection-related abscesses; associated with sternal wound infections following cardiothoracic surgery

Other relevant note information

• Bacillus anthracis:
  • Large bacilli (3-5 μm)
  • Single, or paired
  • Polypeptide capsule and exotoxins
  • Highly resistant central spores
  • Forms medusa head colonies
  • Anthrax toxin: three proteins (PA, LF, EF)
    • PA serves as binding
    • PA+ EF = edema
    • PA+ LF = death
    • Spores of B. anthracis live in soil for >60 years
  • Zoonotic disease of herbivorous livestock
  • Method of infection: contact with infected animal products (hides, wool, hair), ingestion of contaminated food products, inhalation of contaminated dust (Woolsorter's disease)
  • Clinical findings: cutaneous, gastrointestinal, pulmonary, and injectional anthrax
    • Cutaneous findings: malignant pustule, incubation 2-3 days, erythematous papule, increasingly necrotic →later ruptures→ painless black eschar
    • Gastrointestinal findings: contaminated meat (Asia, Africa), cardiovascular collapse within hours, death
    • Pulmonary findings: toxemia, capillary thrombosis, cardiovascular shock
    • Injectional findings: skin popping, injection drug use, tissue infection, necrotizing fasciitis, organ failure, shock, coma, meningitis; absence of black eschar formation
  • Laboratory Diagnosis: specimen (aspirate or swab from cutaneous lesion), blood culture, sputum, laboratory investigation (Gram stain, Culture, Identification of isolate)
  • Treatment: penicillin, tetracycline/chloramphenicol, erythromycin, clindamycin; prevention: vaccination of animal herds, proper disposal of carcasses
  • Bacillus cereus:
    • Large, motile, saprophytic bacillus
    • Heat-resistant spores
    • Airborne and dust-borne contaminants
    • Preformed heat and acid stable toxin (emetic syndrome)
    • Heat labile enterotoxin (diarrhoeal disease)
    • Multiplies readily in cooked foods (rice, potato, meat)
    • Lab diagnosis: demonstration of large number of bacilli in food
    • Clinical presentations: emetic form (incubation period <6 hours, vomiting, lasts 8-10 hours); diarrhoeal form (incubation period >6 hours, diarrhoea, lasts 20-36 hours)

Test your knowledge on the morphology, diagnosis, and treatment of bacterial infections, specifically Gardnerella Vaginitis and Diphtheria. This quiz covers questions related to culture methods, clinical signs, and identification techniques used in the microbiology field. Perfect for students and professionals in the health sciences!