Notes on Bacillus, Listeria, Erysipelothrix, and Corynebacterium

Bacillus, Listeria, Erysipelothrix, Corynebacterium - Gram Positive Rods

Bacillus

  • General Characteristics:

  • Large, gram-positive, spore-forming rods

  • Found in soil and water

  • Only Bacillus anthracis is an obligate pathogen; others are opportunistic

Bacillus anthracis

  • Non-Motile, Aerobic Characteristics:

  • Encapsulated, non-hemolytic, spore-forming rod

  • Spores can survive in the environment for decades; require 120 °C for 15 minutes to kill

  • Etiological agent of anthrax, primarily affects cattle and sheep

  • Human infections typically occur through contact with animal hides or hair

  • Classified as a biowarfare agent

  • Virulence Factors:

  • Capsule: Antiphagocytic property

  • Toxins (plasmid-encoded):

    • Protective antigen (PA)

    • Edema factor (EF)

    • Lethal factor (LF)

  • Mechanism:

    • PA binds to receptors on host cells (e.g., brain, heart, intestine)

    • EF and/or LF bind to PA; stimulates endocytosis

    • LF stimulates cytokine release, resulting in cell death

    • EF acts as an adenylate cyclase, increasing intracellular cAMP levels and resulting in edema

Epidemiology

  • Natural Infection:

  • Anthrax is primarily a disease of herbivores; human infections arise from exposure to contaminated animals or animal products

  • Rare in the U.S. due to livestock vaccination but a serious problem in other countries, particularly in African wildlife

Clinical Disease

  • Transmission Routes:

  • Cutaneous anthrax: Most common in humans

    • Spores enter through cuts in the skin, causing a painless papule that progresses to an ulcer and necrotic eschar; 20% mortality if untreated

  • Gastrointestinal anthrax: Rare in humans

    • Results from ingestion of undercooked contaminated meat; high mortality (50-100%) if untreated

  • Inhalation anthrax (Wool-sorters disease):

    • Associated with processing goat hair; initial symptoms are non-specific (fever, cough, etc.) and can progress rapidly to severe conditions with high mortality (100% if untreated)

Laboratory Diagnosis

  • Methods:

  • Culture and Gram stain show long, thin gram-positive rods arranged in chains (spores not often observed)

  • Non-motile, non-hemolytic

  • Direct Fluorescent Antibody (DFA) test against the capsule

  • PCR tests available for confirmation

Treatment, Prevention, and Control

  • Treatment Options:

  • Penicillin, doxycycline, ciprofloxacin (resistance noted in some strains)

  • Prevention Measures:

  • Vaccination of animals in endemic areas, appropriate disposal of carcasses, and development of vaccines for humans

Bioterrorism

  • Recognized as an ideal bioweapon due to ease of dispersal and high mortality rates; notable incidents include:

  • Sverdlovsk (1979) accidental leak, Aum Shinrikyo cult release in Japan, and the 2001 U.S. postal attacks

Listeria monocytogenes

  • Characteristics:

  • Gram-positive, non-spore-forming coccobacilli that can be mistaken for streptococci

  • Grows at a broad temperature range (1-45 °C); motile at room temperature

  • Weak beta-hemolytic and catalase positive

Virulence Factors

  • Intracellular Growth:

  • Survives and replicates in macrophages and epithelial cells

  • Produces Listeriolysin O (disrupts membranes to escape phagosome) and phospholipases

Epidemiology

  • Sources:

  • Found in soil, water, animals, and contaminated food (e.g., soft cheese, raw vegetables); can spread transplacentally

  • Higher risk for young, elderly, pregnant women, and immunocompromised individuals

Clinical Disease

  • Digestive Tract Infections:

  • Typically mild or asymptomatic in healthy adults but severe in those with cellular immunity defects

  • Neonatal disease may involve high mortality, early onset presenting with disseminated abscesses, late onset presenting with meningitis

Laboratory Diagnosis

  • Commonly diagnosed through CSF culture and Gram stain (showing no organisms), as well as culture on blood agar

Treatment/Prevention

  • Treatments:

  • Primarily penicillin, but resistance is developing

  • Prevention:

  • Safe food handling practices (cooking, washing)

Erysipelothrix rhusiopathiae

  • Characteristics:

  • Gram-positive, non-spore forming, pleomorphic rods

  • Microaerophilic, prefers low oxygen and enhanced carbon dioxide environment

Epidemiology

  • Transmission:

  • Zoonotic disease primarily among occupationally exposed individuals (butchers, veterinarians)

Clinical Disease

  • Erysipeloid:

  • Characterized by localized skin infection that develops after a 2-7 day incubation; presents as a violaceous lesion with raised edges and no suppuration

Laboratory Diagnosis

  • Cultured from lesion margins using conventional media under specific carbon dioxide conditions

Treatment/Prevention

  • Responds well to penicillin; prevention involves use of protective equipment for those in high-risk occupations

Corynebacterium diphtheriae

  • Characteristics:

  • Gram-positive, non-spore forming, forms palisade arrangements; produces metachromatic granules

Clinical Disease

  • Diphtheria:

  • Spread through droplets; can cause mild to severe respiratory illness with a characteristic thick pseudomembrane in the throat leading to potential airway obstruction and death

Laboratory Diagnosis

  • Diagnosed via culture on blood agar and selective media; toxin testing is crucial for confirmation

Treatment/Prevention

  • Antitoxin is essential; vaccines (DTP) provide immunity for future infections.