Microbiology: Gram Positive Rods and Bacillus

Gram Positive Rods

• Medically important GPR: Bacillus, Clostridium, Listeria, Corynebacterium, Actinobacteria, Streptomyces, Nocardia, and Mycobacterium

Spore Forming Bacteria

• Bacillus and Clostridium are spore-forming bacteria
• Spores are highly resistant structures formed in response to adverse conditions
• Spore formation occurs when nutrients such as carbon and nitrogen sources are depleted
• Spores contain bacterial DNA, a small amount of cytoplasm, cell membrane, peptidoglycan, and a thick keratin-like coat
• The keratin-like coat is responsible for the spore's remarkable resistance to heat, dehydration, radiation, and chemicals
• Dipicolinic acid, a calcium ion chelator, may mediate the spore's resistance
• Spores have no metabolic activity and can remain dormant for many years
• Upon exposure to water and nutrients, specific enzymes degrade the coat, water and nutrients enter, and germination into a potentially pathogenic bacterial cell occurs

Bacillus

• Characteristics: large gram-positive rod with square ends, capsule, central endospore, and non-motile
• Reservoir: hoof stock (sheep, cattle, goats), textile workers, and soil contaminated with spores
• Virulence factors: capsule, spore, and exotoxin (anthrax toxin)
• Exotoxin causes necrosis and edema

Bacillus anthracis

• Can cause gastrointestinal, cutaneous, and inhalation anthrax
• Cutaneous anthrax: painless ulcer with a black eschar, local edema, and a malignant pustule
• Gastrointestinal anthrax: vomiting, abdominal pain, and bloody diarrhea
• Inhalation anthrax: nonspecific respiratory symptoms, dry cough, and substernal pressure, progressing to hemorrhagic mediastinitis, bloody pleural effusions, septic shock, and death
• Laboratory diagnosis: gram-positive rod with square ends, nonhemolytic colonies on blood agar, and PCR
• Treatment: ciprofloxacin or doxycycline
• Prevention: immunization with cell-free vaccine and incinerating animals that die of anthrax to prevent soil contamination

Bacillus cereus

• Characteristics: aerobic, non-encapsulated, motile, and spore-forming
• Virulence factors: spores, enterotoxins, hemolysins, and lecithinase (pLC)
• Causes food poisoning through contaminated rice
• Pathogenesis: produces two enterotoxins, one with a mode of action similar to cholera toxin and the other a superantigen
• Clinical findings: two syndromes, one characterized by abdominal cramps, diarrhea, and vomiting, and the other by a more severe syndrome resembling staphylococcal food poisoning