Bacillus: Medical Importance and Human Health Implications

Introduction Bacillus

Large gram positive (young/fresh culture) bacilli with square end. Some are gram negative in old culture. Bacillus species can be obligate aerobes or facultative anaerobes. Test positive for the enzyme catalase. Some species are natural flora in the human intestines. Most species of Bacillus are harmless saprophytes but two species are considered medically significant B. anthracis and B. cereus. Natural habitat: Saprophytes, widely distributed in natural environment (mainly in soil and water) Contaminants of operating rooms and surgical dressings, pharmaceutical products and dried foods. Include thermophilic, psychrophilic, acidophilic, alkalophilic, halotolerant, halophilic.

Medically importance species

1.       B. anthracis:

Causes anthrax in cows, sheep, and sometimes humans Anthrax is transmitted to humans via direct contact with animal products or inhalation of endospores. Bacillus anthracis is a gram-positive and rod-shaped bacterium that causes anthrax, a deadly disease to livestock and, occasionally, to humans. Its infection is a type of zoonosis. Untreated B. anthracis infection is usually deadly. Infection is indicated by inflammatory, black, necrotic lesions. Different animal and human vaccines are now available. The infection can be treated with common antibiotics such as penicillins, quinolones, and tetracylines. B. anthracis produces a single antigenic type of capsule and several exotoxins. B. anthracis is responsible for the disease anthrax.

Diseases Caused by Bacillus anthracis

1. CUTANEOUS ANTHRAX:

Usually acquired via injured skin, spores from the soil or a contaminated animal or carcass inoculated. Death results from respiratory failure and anoxia caused by actions of the toxin on the central nervous system.  The spores germinate, vegetative cells multiply characteristic gelatinous edema develops at the site.

2. PULMONARY ANTHRAX:

Results from inhalation of spore-containing dust where animal hair or hides are being handled.  Spore germinate in the lung and spreads leads to a fatal septicemia or meningitis. The most serious form of disease progresses rapidly to a systemic hemorrhagic pathology and is often fatal.

3. GASTROINTESTINAL ANTHRAX:

Analogous to cutaneous anthrax but occurs on the intestinal mucosa organisms invade the mucosa through a preexisting lesion, results from the ingestion of poorly cooked meat from infected animals. Has an extremely high mortality rate.

Clinical findings

• Skin infection (malignant pustule)

• Meningitis

• Hemorrhagic pulmonary

Haw to diagnosis B.anthracis

Specimen: fluid or pus from a local lesion, blood, and sputum. Microscopic: Stained smears from the local lesion or of blood from dead animals often show chains of large gram-positive rods. Loeffler’s polychrome (McFadyean) methylene blue stained smear- Square ended blue-black bacilli surrounded by a pink/purple capsule. Anthrax can be identified in dried smears by immunofluorescence staining techniques.

B. ANTHRACIS TREATMENT

Antibiotics should be given to unvaccinated individuals exposed to inhalation anthrax.  Penicillin, tetracyclines and fluoroquinolones are effective if administered before the onset of lymphatic spread or septicemia, estimated to be about 24 hours. Antibiotic treatment is also known to lessen the severity of disease in individuals who acquire anthrax through the skin.

Prevention and control of B. anthracis treatment

• Disposal of animal carcasses by burning or by deep burial in lime pits

• Decontamination of animal products.

• Wearing of protective clothes & gloves for handling infected materials- Eg. Animal’s hair, hide bone.

• Immunization of domestic animals with live attenuated vaccines.

• Immunization of individuals with high occupational risk- E.g. Leather factory.

2. B. cereus:

 Bacillus cereus is a bacterium linked to food poisoning. It is commonly found in the environment, especially in soil and on plants. This bacterium is gram-positive and rod-shaped, adaptable to various conditions. Bacillus cereus can form heat-resistant endospores that survive cooking and drying. There are two main types of foodborne illnesses caused by bacillus cereus. The emetic form leads to nausea, vomiting, and abdominal cramps within 1 to 6 hours after ingestion. The emetic form is caused by a pre-formed toxin called cereulide. The diarrheal form results in diarrhea and stomach cramps, starting 6 to 15 hours after eating contaminated food. This form is due to enterotoxins produced in the intestine. Recognized as an agent of food poisoning since 1955 Habitat: air, soil, water & dust, most likely to cause opportunistic infection has no capsule, motile, and produces an enterotoxin. Food poisoning caused by B. cereus has two distinct forms. Bacillus cereus thrives in  improperly stored starchy foods, particularly rice.

Clinical manifestation

Food poisoning caused by B. cereus is manifested by nausea, vomiting, abdominal cramps, and occasionally diarrhea.

Haw to diagnosis B. cereus

• Suspected food (eg, rice, meat, vegitable) and vomitus of patients are cultured on ordinary media.

• Smear from colonies show large gram positive sporing bacilli

• B. cereus, unlike B. anthracis, is motile, non-capsulate, and produces haemolytic colonies on blood agar.

• The organism is non-lactose fermenting, producing pale colonies on MacConkey agar.

• On egg-yolk agar, B. cereus gives a strong lecithinase reaction. It rapidly liquefies gelatin stabs.

Prevention and control

Rice should not be kept warm for a long period of time.

Conclusion

Bacillus species are important bacteria found widely in nature, some being beneficial while others cause diseases.