Spore-Forming Gram-Positive Bacilli

Gram-positive bacteria form spores
Diverse collection of bacteria
Grow aerobically or anaerobically
Form endospores
Two clinically important genera: Bacillus and Clostridium

Bacillus

Almost 300 species in this genus
Bacillus anthracis: Agent of anthrax
Bacillus cereus: An opportunist, causes food poisoning

Bacillus anthracis (B. anthracis)

Large (1 x 3 to 8 µm) rods
Arranged as single or paired rods, or as long serpentine chains
Spores (in 2- to 3-day-old cultures, not in clinical specimens)
Virulence factor: Capsule
Toxins

Virulence Factor: Capsule

Polypeptide capsule
In clinical specimens
Consisting of poly-D-glutamic acid
Plasmid pX02 transfers gene
Inhibits phagocytosis

Virulence Factor: Toxins

Exotoxin: Protective antigen (PA), Lethal factor (LF), Edema factor (EF)
Non-toxic individually
Plasmid pXO1 carries genes for three toxin protein components
Edema toxin: PA + EF
Lethal toxin: PA + LF

Lethal Factor (LF)

Zinc-dependent protease
Cell death - tissue necrosis

Edema Factor (EF)

Adenylate cyclase
Converts adenosine triphosphate (ATP) to cyclic AMP (cAMP)
Increases intracellular cyclic AMP, results in edema
Fluid accumulation observed in anthrax

Epidemiology (B. anthracis)

Primarily infects herbivores
Humans infected through exposure to contaminated animals or animal products
Human anthrax disease is acquired by:
- Inoculation (95%)
- Ingestion (herbivores)
- Inhalation

Clinical Diseases (B. anthracis)

1- Cutaneous: Inoculation of spores through exposed skin from contaminated soil or infected animal products. Painless papule, rapidly progresses to ulcer surrounded by vesicles, necrotic eschar (ulcer with black center)
- Systemic signs: painful lymphadenopathy, massive edema
2- Gastrointestinal: Is very rare in humans, contaminated raw meat, upper intestinal tract to cecum/ileum causing ulcers, edema, sepsis, nausea, vomiting, malaise, rapidly progresses to systemic disease.
3- Inhalation (Wool-sorters' disease): Inhalation of spores (animal hair or hides), Incubation period prolonged (2 months or more), spores remain latent in nasal passages or reach lower airways.
Injection anthrax: related to this

Inhalation Anthrax

Initial symptoms: nonspecific (fever, myalgias, nonproductive cough, malaise)
Second stage: fever, edema, massive enlargement of mediastinal lymph nodes, respiratory failure, sepsis, pneumonia (rarely develops), meningeal symptoms (in half of patients)

Laboratory Diagnosis

Microscopic morphology: Long, thin gram-positive rods, arranged singly or in long chains, organisms present in wounds, involved lymph nodes, blood, spores not observed in clinical specimens, Capsule of B. anthracis is produced in vivo, special spore stain.
Colonial morphology: Nonhemolytic, grow rapidly, firmly adherent to the agar, "medusa head".
Microscopy of blood, wound samples and/or lymph node samples is critical

Treatment & Control of Anthrax

Treatment: Ciprofloxacin or doxycycline combined with other antibiotics (rifampin, vancomycin, penicillin, etc.)
Control: Control of animal disease with vaccination of herds in endemic regions, burning of animals that die of anthrax. Vaccination is also available for animals, people in endemic areas, those who work with animal products, and military personnel.

Bacillus cereus

Spore-forming, motile gram-positive rods
Opportunistic pathogens
Gastroenteritis, ocular infections, intravenous catheter, severe pneumonia are some issues.

Bacillus cereus Gastroenteritis

Emetic form implicates rice, incubation period (<6 hrs, mean 2), vomiting, nausea, abdominal cramps (8-10 hrs average). Heat-stable enterotoxin
Diarrheal form affects meat & vegetables, incubation period is >6 hrs, mean 9, diarrhea, nausea, abdominal cramps (20-36 hrs average), Heat-labile enterotoxin toxin

Bacillus cereus Ocular Infections

After traumatic, penetrating injuries with a soil contaminated object.
Rapid destruction from a combination of toxins (necrotic toxin, Cereolysin, Phospholipase C), rapid aggressive disease causing complete eye loss in 48 hrs.

Other common Bacillus cereus infections

Intravenous catheter, central nervous system shunts
Endocarditis (most common in drug abusers)

One rare disease of B. cereus (Immunosuppressed patients)

Severe pneumonia
Strains contained B. anthracis pXO1 toxin genes
Transferring B. anthracis virulence genes into the ubiquitous B. cereus

Laboratory Diagnosis

B. cereus gastroenteritis: diagnosed by epidemiologic criteria, impacted food is cultured for verification of foodborne disease.
Treatment: Symptomatic for gastroenteritis, other Bacillus infections- Penicillins/Cephalosporins are ineffective; Vancomycin, clindamycin, ciprofloxacin and gentamicin can be used. Eye infections must be rapidly treated.

Anaerobic Gram-Negative Bacteria

Most important Gram-negative anaerobes colonize human upper respiratory, gastrointestinal, and genitourinary tracts.
Include: Bacteroides, Fusobacterium, Parabacteroides, Porphyromonas, Prevotella, and Veillonella (cocci)

Physiology & Structure of Anaerobic Bacteria

Very small or elongated bacteria.
Stain weakly using Gram stain.
B. fragilis has a gram-negative cell wall surrounded by a polysaccharide capsule.
LPS has little to no endotoxin activity.

Pathogenesis of Anaerobic Bacteria

Capsule, fimbriae adhere to epithelial cells & extracellular molecules.
Succinic acid inhibits phagocytosis.
IgA, IgM, IgG proteases, tolerate exposure to oxygen, catalase, & toxins. Note: Enterotoxigenic toxin B. fragilis is a heat-labile zinc metalloprotease toxin.

Clinical Diseases: Anaerobic Bacteria

Respiratory: half of chronic infections of sinuses/ears/periodontal involve mixtures of gram-negative anaerobes ( Prevotella, Porphyromonas, Fusobacterium, Non-fragilis Bacteroides).
Brain abscesses typically involve chronic sinusitis/otitis (Prevotella, Porphyromonas, Fusobacterium, Peptostreptococcus) and multiple abscesses.
Intraabdominal Infections: B. fragilis is the most prevalent organism, often involving strains of enterotoxin-producing B. fragilis, resulting in self-limited watery diarrhea in children younger than 5 years
Gynecologic infections: Infections of the female genital tract (e.g., pelvic inflammatory disease, abscesses, endometritis); mixtures of anaerobes, Prevotella, fragilis also responsible for abscess formation.
Skin & soft tissue Infections; bacteremia, less than 5% of all cases, B. fragilis being commonly isolated in blood cultures, not part of normal skin flora, bite or wound exposure can be root causes.

Epidemiology: Anaerobic Bacteria

Colonize the human body in large numbers, they can prevent colonization by pathogenic organisms from external sources, digestion of food may stimulate host immunity, these normal organisms create disease when relocating to normally sterile sites (endogenous). Endogenous infections are polymicrobial.

Laboratory Diagnosis: Anaerobic Bacteria

Bacteria may stain faintly and irregularly pleomorphic rods in microscopy.
Most endogenous infections specimens are not contaminated, drying can cause losses of bacteria. Specimens should be kept in a moist environment for analysis. Requires long incubation (only Bacteroides grow rapidly.
Many polymicrobial infections involve different species, a broad spectrum of species are present.

Treatment: Anaerobic Bacteria

Antibiotic therapy combined with surgical intervention needed, many species of anaerobic bacteria produce beta-lactamases, and are resistant to penicillin and cephalosporins.
Metronidazole, Carbapenems (imipenem, meropenem), Beta-lactam-beta-lactamase inhibitors (piperacillin tazobactam), are used for prophylactic treatment.