Bacillus pumilus in the induction of clindamycin-associated enterocolitis in guinea pigs

Antibiotic-associated enterocolitis was induced in guinea pigs by the intraperitoneal injection of clindamycin. The colonic and cecal mucosa and feces of acutely ill animals were cultured under aerobic and anaerobic conditions on 5% sheep blood agar plates and on a selective and differential medium for Clostridium difficile. All morphologically distinct colony types were isolated in pure culture and identified. A sterile cell-free filtrate of each isolate was tested for ability to induce morphological changes in cultured monolayers of mouse adrenal cells. The filtrate of a predominant isolate, Bacillus pumilus, induced an alteration of cellular morphology; the sterile filtrate of other isolates were unreactive. Toxin contained in cell-free filtrates of B. pumilus caused a syndrome identical to clindamycin-associated enterocolitis when injected intracecally into guinea pigs. The toxin had a molecular weight of 6,500 daltons as determined by molecular sieve chromatography and was inactivated with pronase, lipase, and trypsin. The minimal inhibitory concentrations of clindamycin and vancomycin for B. pumilus were 50 micrograms/ml and less than or equal to 0.4 micrograms/ml, respectively.