Bacillus

Lab Twenty:- ______________ Medical Microbiology
Prepared by: Luma J. Witwit


Bacillus

Bacillus represents a genus of Gram-positive bacteria which are ubiquitous in nature (soil, water, and airborne dust ). Some species are natural flora in the human intestines.

When grown on blood agar, Bacillus produces Large spreading, gray-white colonies with irregular margins. A unique characteristic of this bacterium its ability to produce endospores when environmental conditions are stressful. The only other known spore-producing bacterium is Clostridium. Although most species of Bacillus are harmless saprophytes, two species are considered medical significant: B. anthracis and B. cereus.

B. anthracis

B. anthracis is the bacterium which cause anthrax in cows, sheep , and sometimes humans. Authrax is transmitted to humans via direct contact with animal products or inhalation of endospores.

Under the microscope, B. anthracis cells appear to have square ends and seem to be attached by a joint to other cells. The spores are best observed when the bacterium is cultured on artificial media. Sources of infection are usually industrial or agricultural and the infection is classified as one of three types:

- Cutaneous infection (95% of human cases )
- Inhalation anthrax (rare)
- Gastrointestinal anthrax (very rare ) .

Specimens

Malignant pustule : films are made from exudates and stained by grams method. The finding of bacilli morphologically like B. anthracis is suggestive, hair, tissue, bone.

Laboratory indications:-
Non hemolytic (Sheep blood agar ).
Non – motile.
Gel hydrolysis
Catalase.

B. cereus

Unlike B. anthracis, B. cereus is a motile bacterium which can cause toxin-mediated food poisoning It is known to inhabit any kinds of food including stew, cereal, and milk. Most recently, however, it has been found in fried rice. The two toxins released by the bacterium lead to vomiting and diarrhea, symptoms similar to those of Staphylococcus food poisoning. Because toxin production usually takes place after the infected foods are cooked, proper cold story of food is recommended immediately after preparation.

The presence of B.cereus in a patient’s stool is not sufficient to make a diagnosis of its disease, since the bacteria may be presenting normal stool specimens, a concentration of 105 bacteria or more per gram of food is considered diagnostic.

Disease:

Food poisoning(emetic and diarrheal form). B.cereus is important cause of eye infections, localized and systemic infections including endocarditis, meningitis and pneumonia.

Laboratory indications:-
Hemolytic (sheep blood agar).
Motile.
Gel hydrolysis (+).
Glucose, Maltose, and Salicin fermentative.