GRAM-POSITIVE BACILLI
GENUS BACILLUS
This genus contains nearly 50 species, most of which are soil organisms.
There are two species of major medical importance:
- B. anthracis and
- B. cereus.
Bacillus anthracis
Characteristics:
Large (4-10 µm) Gram-positive spore-forming encapsulated rods. Spores are formed only after the organism is shed from the body. Respires aerobically.
Transmission:
Soil organisms; B. anthracis can survive for many years. The carcasses of animals dying with anthrax are buried six feet deep to prevent organisms being carried to the surface. Humans are accidental hosts and infection is usually acquired when spores enter abrasions on the skin or are inhaled.
Diseases:
Anthrax is exotoxin mediated significant disease in both domestic and wild animals. It is a zoonosis and humans are usually infected by contact with infected hides or bones. Woolsorter’s disease (respiratory anthrax) is now rare. Intestinal anthrax is rare in humans, but remains a possibility that attracts interest as an aspect of biological warfare.
Laboratory Identification:
In smears of body fluids the capsule can be stained with polychrome methylene blue (McFadyean reaction). This is diagnostic for B. anthracis. The species is non-fastidious; grows well on simple media. Characteristic colonies (Medusa head) are probably related to chaining of the long rods. Non-hemolytic on horse blood agar. Growth in CO2 encourages the formation of the capsule and smooth colonies. Biochemical reactions are unhelpful.
Bacillus cereus
Characteristics:
Large Gram-positive spore-forming rod. This and many other Bacillus species are similar to B. anthracis in many respects except most are motile and non-capsulate. Respires aerobically.
Transmission:
B. cereus spores are found on many foods, especially rice, pulses, and vegetables. Infection is acquired by ingestion of organisms or toxins.
Diseases:
B. cereus causes toxin mediated food poisoning, the commonest association being with reheated cooked rice and pulses. Two different syndromes are recognized, due to different toxins. The organism is also a rare cause of bacteremia in immunocompromised hosts.
Laboratory Identification:
Non-fastidious. Produces hemolysis on horse and sheep blood agar. Lecithinase production and inability to utilize mannitol used as distinguishing features on a specially- designed selective medium.
GENUS LISTERIA
These organisms are Gram-positive bacilli. L. monocytogenes is the major species of medical importance.
Listeria monocytogenes
Characteristics:
Short Gram-positive rods, often coccobacillary in clinical material; frequently Gram variable. Motile at 250C with a characteristic tumbling movement; non-motile at 370C.
Transmission:
Widely distributed in nature, survives well in cold. Reaches food chain via spillage as well as more directly via for example vegetables. Excreted in large numbers in cow’s milk. Humans may carry Listeria in gut as normal flora. Infection may be acquired by ingestion or transplacentally to the baby in utero. Serotyping has been used to investigate outbreaks. The serotype 4b appears to be associated with outbreaks.
Diseases:
Meningitis and sepsis in neonates. Infection in the immunocompromised and in pregnant women.
Laboratory Identification:
Hemolytic on sheep or horse blood agar. Selective medium aids recovery of these organisms, especially from food samples. Cold enrichment at +4°C for several weeks is also an effective selective technique. On translucent, non-blood containing agar colonies appear green-blue in oblique light. Catalase-positive, nitrite reduction negative; coupled with motility at room temperature these results are useful identifying features.
