Pus was obtained from patients with polymicrobial intraabdominal abscesses or polymicrobial empyema. Physical and chemical characteristics of 12 specimens were examined, and bacterial isolates were enumerated. Pus supernatant of six specimens rapidly inactivated penicillin, cephalothin, and cefazolin. Carbenicillin and ticarcillin were similarly degraded by supernatant of certain pus specimens. Cefoxitin, chloramphenicol, and clindamycin were not appreciably inactivated by pus supernatant. Degradation of penicillin and cephalosporin congeners in pus was due to the presence of beta-lactamase, as shown by chemical interaction with nitrocefin, chromatography, and inhibition by the beta-lactamase inhibitor clavulanic acid. Pus supernatant containing beta-lactamase activity reduced the bactericidal activity of carbenicillin against Bacteroides fragilis in whole pus in an abscess model in vitro. Bactericidal activity of clindamycin or cefoxitin was not impaired in pus containing beta-lactamase.

To define factors contributing to the adverse prognosis of patients with gram-negative bacillemia and abscess formation, we studied the interaction between polymyxin B, colistin sulfate, gentamicin, or carbenicillin with purulent material. Carbenicillin activity was not significantly altered by incubation with pus. Equal volumes of antibiotic and purulent sediment decreased the effective concentration of polymyxin B, colistin sulfate, or gentamicin from 100 yg/ml to 3 to 6 ug/ml. One milliliter of purulent sediment bound more than 700 ,ug of gentamicin and 1,500 ,ug of polymyxin B or colistin sulfate. This effect occurred rapidly, proceeded at 4 and 37 C, was stable for 24 to 48 h, and was altered, but not abolished, by varying the pH of the solution. Antibiotic activity could be removed from pus by high concentrations of protamine sulfate, heparin, sodium chloride, or potassium chloride, suggesting binding rather than inactivation.