Notes
Bacillus
Sections
Bacillus anthracis
"Anthrax" means "charcoal," which describes appearance lesions it produces on the skin.
Is the causative agent of anthrax, the infection.
Resides in the soil, where its spores can persist for years; spores are also found on animals, especially on hides and wool.
Non-motile
In clinical specimens, spores are rarely seen; colonies are non-hemolytic.
Treatment: ciprofloxacin or doxycycline with antitoxins; amoxicillin is often used to treat cutaneous anthrax.
Prevention: In areas where B. anthracis is endemic, vaccination of humans and animals can help control disease.
Infected animals should be incinerated, as spores can remain in the soil for many years.
Virulence factors:
Capsule
Unique polypeptide capsule comprising D-glutamic acid, which enables host immune evasion (recall most bacterial capsules comprise polysaccharides).
Exotoxins: Edema toxin and Lethal toxin.
Toxins comprise A and B subunits:
B subunit = Protective antigen (PA); this is the region of the toxin that binds with host cells.
A subunits = Factors that combine with Protective antigen to form active toxins:
Edema factor (EF) is an adenylate cyclase that increases intracellular cyclic adenosine monophosphate (cAMP), resulting in edema.
Lethal factor (LF) is a protease that inactivates mitogen-activated protein kinase (MAPK) pathways, resulting in cell death.
Be aware that these factors are nontoxic on their own; they must combine with protective antigen to enter host cells and cause damage.
Infections:
B. anthracis primarily infects non-human animals.
Interaction with contaminated animal products can lead to three types of human infections:
- Cutaneous, Inhalation, Gastrointestinal.
Cutaneous anthrax
Most common form.
Skin lesions with central necrotic eschar surrounded by edema. The pustule is initially painless, but infection can progress to produce systemic signs of edema, and bacteremia can be fatal.
Typically, cutaneous anthrax is due to exposure to contaminated soil or animal hides, hair, or wool; however, outbreaks have also been reported among injection drug users.
Inhalation anthrax
Initially presents with nonspecific symptoms, including fever, non-productive cough, and myalgias.
However, as the spores travel from the lungs to the nearby lymph nodes, edema and mediastinal lymph node enlargement occurs; in chest X-rays, mediastinal widening is an important diagnostic cue. Respiratory failure can ensue, and, in about half the cases, meningeal symptoms occur.
Historically, inhalation anthrax in humans was associated with spore inhalation while working with animal products. However, weaponized anthrax has been used in bioterrorism; person-to-person transmission does not occur, because bacterial replication occurs in the lymph nodes.
Gastrointestinal anthrax
Aka, ingestion anthrax, occurs upon consumption of contaminated meat.
Manifests in the upper and lower gastrointestinal tract:
In the oral cavity, pharynx, and esophagus, anthrax produces lymphadenopathy, edema, sore throat, and can lead to sepsis. In some patients, pseudeomembranes form; these are grayish coverings that comprise fibrin, leukocytes, and
other exudates.
In the lower gastrointestinal tract, particularly the ileum and cecum, infection causes ulcerative lesions and edema, with nausea, vomiting, and bloody diarrhea.
Bacillus cereus
"Cereus" means "wax-like"; these bacteria produce flat, grayish white colonies on blood agar plates.
Environmentally ubiquitous, motile, and beta-hemolytic.
Causes two forms gastroenteritis, aka, food poisoning, upon consumption; symptoms depend on the type of toxin ingested.
Symptomatic treatment is given, infection resolves itself.
Emetic food poisoning
Intoxication caused by pre-formed cereulide, which is heat stable.
Spores survive initial cooking, and germinate if the food is not refrigerated; importantly, reheating does NOT kill the enterotoxin.
Commonly associated with rice and other starchy foods left at room-temperature for long periods of time.
Toxin produces nausea, abdominal cramps, and vomiting; in rare complications, liver failure occurs when large quantities impair mitochondrial fatty acid metabolism.
Quick: the incubation period < 6 hours after consumption, and illness duration < 24 hours.
Diarrheal food poisoning
Infection by vegetative cells that produce heat labile enterotoxin in the intestine.
Within the intestinal epithelial cells, the toxin increase the concentration of cyclic AMP.
The bacteria tend to reside on meats and vegetables.
They multiply in the gastrointestinal tract, where heat labile enterotoxin produces nausea, abdominal cramps, and watery diarrhea.
Slow: Incubation period is > 6 hours; duration > 24 hours.
Ocular infections
Associated with trauma, surgery, or bacteremia.
At least three toxins are associated with ocular infection: necrotic toxin, cereolysin, and phospholipase C. Interactions of these toxins, and perhaps factors, leads to rapid infection progression and eye loss.
Treatment: Clindamycin or vancomycin is urgent.
Severe pneumonia
Mimicks inhalation anthrax.
Intravenous catheter and CNS shunt infections, endocarditis, bacteremia, and meningitis.
Especially in immunocompromised patients.
References
Murray, P. R., Rosenthal, K. S., & Pfaller, M. A. Medical microbiology. Philadelphia: Elsevier/Saunders. (2013).
Levinson, W. E. Review of Medical Microbiology and Immunology. 14th Ed. Lange (2016)
Arnesen, L.P.S., Fagerlund, A., & Granum, P.E. (2008). From soil to gut: Baccilus cereus and its food poisoning toxins. FEMS Microbiol Rev. 32:579-606.
Liu, S., Moayeri, M., Leppla, S.H. (2014). Anthrax lethal and edema toxins in anthrax pathogenesis. Trends Microbiol. 22(6): 317-325.
doi:10.1016/j.tim.2014.02.012.
Images
Bacillus anthracis (Wikipedia; Author BruceBlaus).
Bacillus cereus (CDC/ Courtesy of Larry Stauffer, Oregon State Public Health Laboratory).
Cutaneous anthrax (CDC/ James H. Steele).
Inhalation anthrax (Centers for Disease Control).