7. Bacillus spp. Clostridium spp.

Laboratory Diagnosis of Infectious Diseases

  • Focus on aerobic spore-forming bacteria (Bacillus spp.) and anaerobic spore-forming bacteria (Clostridium spp.)

  • Instructor: Dr. Denitsa Tsaneva-Damyanova

Family Bacillaceae

  • Diverse collection of bacteria: Both aerobic and anaerobic.

  • Morphology:

    • Shapes: Cocci and rods.

    • Staining: Some are gram-positive, others gram-negative.

  • Common feature: Ability to form endospores.

  • Clinically important genera:

    1. Bacillus

    2. Clostridium

Genus Bacillus

  • Contains about 250 species.

  • Characteristics:

    • Gram-positive rods.

    • Oval, centrally located spores.

    • Nonmotile.

    • Mostly obligate aerobes, with some facultative anaerobes.

  • Important species:

    • Bacillus anthracis (causes anthrax).

    • Bacillus cereus (causes gastroenteritis, eye infections, pneumonia).

Bacillus anthracis

  • Characteristics:

    • Obligate pathogen; large size (1×3 to 8μm), usually arranged as single or paired rods.

    • Forms short chains; resembles bamboo sticks (R-forms).

    • Nonmotile, nonhemolytic gram-positive rods.

    • Spores: Observed in re-cultured samples but not seen in clinical specimens.

    • Virulence: Produces a polypeptide capsule of poly-D-glutamic acid and is associated with clinical samples.

Physiology of B. anthracis

  • Growth: Obligate aerobe; grows well on meat-peptone agar, meat-peptone broth, blood agar.

  • Temperature range: 12-45 °C (optimal 35–37 °C).

  • Incubation: Examine after 18-24 hours.

Staining Methods

  • Appearance in blood: Can be seen with Gram stain as long, thin, gram-positive rods.

  • Preliminary identification based on microscopic and colonial morphology.

Culture Properties

  • Blood agar colony appearance:

    1. Flat to slightly convex, white to grey colonies (24 hrs).

    2. Edges may have irregular or wavy protrusions ("Medusa head").

    3. Sticky and adherent, resembling beaten egg whites.

  • Nonhemolytic (weak hemolysis may occur in older cultures).

  • Growth in broth: Forms cotton-like sediment with a translucent supernatant, becomes dull when turning into S-form, which is avirulent.

Spores

  • Definition: Dehydrated cell with a thick wall, can remain inactive for years until favorable conditions arise.

  • Location and characteristics:

    • Observed mainly in special culture conditions.

    • Can be round, oval, or cylindrical, highly resistant to heat and many disinfectants.

Capsules

  • Capsule production: Produced in vivo, detectable with contrasting stains (e.g., Indian ink).

  • Observation methods:

    1. Gins method utilizes Indian ink and methylene blue staining to visualize the capsule.

    2. Klett method for staining capsules shows red capsules around blue cells.

Determinants of Pathogenicity

  • Major antigens in B. anthracis:

    1. Group somatic antigen (thermostable polysaccharide).

    2. Capsule composed of D-glutamic acid.

    3. Exotoxins consisting of three proteins that synergistically induce cell death and edema.

Epidemiology

  • B. anthracis as a soil microorganism; infects herbivores and can transmit to humans.

  • Routes of infection:

    1. Inoculation (95% of cases).

    2. Ingestion.

    3. Inhalation.

Clinical Disease

  • Four clinical forms of anthrax:

    1. Cutaneous anthrax (most common).

    2. Pulmonary anthrax (wool sorter's disease).

    3. Gastrointestinal anthrax (contaminated food).

    4. Bacteremia with secondary infections, particularly meningitis.

Diagnosis

  • Confirmation of anthrax infection involves analyzing: blood, sputum, wounds, environmental samples.

  • Microscopy and culture methods; presence of gram-positive, nonmotile rods crucial for diagnosis.

Identification

  • Based on biochemical properties, susceptibility to antibiotics, and serology tests (Ascoli test for anthrax).

Treatment and Prevention

  • Treatment: Penicillin is the drug of choice; ciprofloxacin or doxycycline in combination with other antibiotics is recommended.

  • Prevention through animal vaccination and control of human disease.

Bacillus cereus and Bacillus subtilis

  • Opportunistic pathogens; B. cereus most notable for causing gastroenteritis and infections in immunocompromised patients.

Genus Clostridium

  • Contains over 200 species characterized by:

    1. Endospore formation.

    2. Strict anaerobic metabolism.

    3. Gram-positive structure.

  • Common pathogens: C. perfringens, C. difficile, C. botulinum, and C. tetani.

Clostridium tetani

  • Characteristics: Obligate anaerobe, gram-positive, motile with terminal spores resembling drums.

Culture Properties

  • Growth conditions: Fastidious, prefers specific pH levels and temperatures, sensitive to oxygen toxicity.

Epidemiology

  • Ubiquitous presence in soil; greater risks in developing countries with inadequate vaccination.

Determinants of Pathogenicity

  • Produces tetanolysin and tetanospasmin which cause spastic paralysis. Toxins bind irreversibly.

Pathogenesis

  • Acquired primarily through penetrating trauma; toxins interfere with nervous system function.

Clinical Disease

  • Forms of tetanus:

    1. Local.

    2. Cephalic.

    3. Generalized (most common).

    4. Neonatal (high mortality).

Laboratory Diagnosis

  • Based on clinical presentation, microscopy, culture techniques, but culture results may be low.

Treatment

  • Immediate based on clinical diagnosis; includes debridement and antibiotics.

Control and Prevention

  • Prevention through immunization with tetanus toxoid; regular booster shots recommended.

Clostridium perfringens

  • Causes soft tissue infections and food poisoning; characterized by rapid growth and specific toxin production.

Clinical Disease

  • Includes necrotizing enteritis and gas gangrene; treatment requires immediate medical attention.

Laboratory Diagnosis

  • Essential for treatment initiation; considers microscopy and culture methodologies.

Treatment

  • Immediate treatment for soft tissue infections includes antibiotics and debridement.

Clostridium botulinum

  • Causative agent of botulism; characterized by potency of toxins causing paralysis.

Clinical Disease

  • Presents as foodborne botulism; various botulism forms, including infant and wound types.

Laboratory Diagnosis

  • Diagnosis relies on toxin detection methods from various samples.

Treatment

  • Supportive measures crucial; antitoxin administered for unbound circulating toxins.

Clostridium difficile

  • Known for antibiotic-associated gastrointestinal diseases.

Laboratory Diagnosis

  • Confirmed through assays for toxins in feces; culture methods used for definitive diagnosis.