Bacillus cereus, a volatile human pathogen

Abstract

Bacillus cereus is a Gram-positive aerobic or facultatively anaerobic, motile, spore-forming, rod-shaped bacterium that is widely distributed environmentally. While B. cereus is associated mainly with food poisoning, it is being increasingly reported to be a cause of serious and potentially fatal non-gastrointestinal-tract infections. The pathogenicity of B. cereus, whether intestinal or nonintestinal, is intimately associated with the production of tissue-destructive exoenzymes. Among these secreted toxins are four hemolysins, three distinct phospholipases, an emesis-inducing toxin, and proteases. The major hurdle in evaluating B. cereus when isolated from a clinical specimen is overcoming its stigma as an insignificant contaminant. Outside its notoriety in association with food poisoning and severe eye infections, this bacterium has been incriminated in a multitude of other clinical conditions such as anthrax-like progressive pneumonia, fulminant sepsis, and devastating central nervous system infections, particularly in immunosuppressed individuals, intravenous drug abusers, and neonates. Its role in nosocomial acquired bacteremia and wound infections in postsurgical patients has also been well defined, especially when intravascular devices such as catheters are inserted. Primary cutaneous infections mimicking clostridial gas gangrene induced subsequent to trauma have also been well documented. B. cereus produces a potent beta-lactamase conferring marked resistance to beta-lactam antibiotics. Antimicrobials noted to be effective in the empirical management of a B. cereus infection while awaiting antimicrobial susceptibility results for the isolate include ciprofloxacin and vancomycin.

FIG. 1.

Gram stain of blood culture showing Gram-positive slender bacilli with rounded ends singly, in pairs, and in short chains.

FIG. 2.

Gram stain of hemorrhagic brain biopsy specimen with histological sections showing clusters of elongated bacillary forms.

FIG. 3.

Filamentation phenomenon consisting of intertwined beaded bacilli as shown in Gram stains of agar cultures.

FIG. 4.

Gray, opaque, granular, spreading colonies with irregular perimeters growing on 5% sheep blood agar. Note the smaller smooth colonies admixed among spreading growth.

FIG. 5.

Smooth colonies on 5% sheep blood agar surrounded by a uniform zone of beta-hemolysis.

FIG. 6.

Numerous Gram-positive bacilli in a smear of an anterior-chamber aspiration sample from a patient with post-cataract surgery endophthalmitis.

FIG. 7.

Hemorrhagic necrosis of brain due to B. cereus invasion in a patient with lymphocytic leukemia.

FIG. 8.

Gram-positive uniform bacilli in a smear of a myonecrosis lesion that grew B. cereus and C. perfringens.

FIG. 9.

India ink preparation of exudate from a gangrenous lesion showing encapsulated bacilli, which allowed the differentiation of C. perfringens from B. cereus. The smear was prepared by emulsifying fragments of necrotic tissue in India ink and smearing contents with another glass slide across the prepared slide. After drying, crystal violet was applied to the slide, and the slide was rinsed gently, air dried, and examined under oil immersion.

FIG. 10.

Rapidly spreading erythematous cellulitis in a 17-year-old patient following a puncture wound to sole of the foot while walking barefoot in a garden. Aspiration of spreading erythema grew B. cereus. Note the site of puncture on the heel of the foot.