No abstract

(8,9,14). There is a need for alternative agents in cases where the use of penicillin drugs is indicated and also in cases of therapeutic failures.Over the last 5 years, we have accumulated 12 patients with serious infections caused by L. monocytogenes. This study looks at the antibiotic susceptibility and synergy of the organisms isolated from these patients.Twelve strains of L. monocytogenes isolated from blood or cerebrospinal fluid were available for study. Organisms were maintained on blood agar plates at 37°C and were identified by Gram stain, hemolysis on blood agar plates, and motility at 25°C. Standard antibiotic disk susceptibility tests used the Kirby-Bauer method. Antibiotic susceptibilities and drug synergy were studied with the use of the microtiter system (6).All dilutions for assays performed in this study were done in tryptic soy broth (BBL Microbiology Systems). For assays with trimethoprim-sulfamethoxazole (TMP-SMX), 0.1 IU of thymidine phosphorylase per ml was added to tryptic soy broth to convert the thymidine pool to thymine. Overnight broth cultures were adjusted to 5 x 106 organisms/ml for a standard inoculum. The 1 ratio of SMZ to TMP; Burroughs-Wellcome); and ticarcillin (Beecham Laboratories). Stock sterile solutions were stored at -20°C and were diluted with tryptic soy broth immediately before use. Rifampin and TMP at concentrations ranging from 0.0045 to 2.5 ,ug/ml, and SMZ at 0.065 to 47.5 p.g/ ml, were tested against each strain. All other drugs were assayed at concentrations of 0.039 to 200 ,ug/ml.Minimal inhibitory concentrations (MICs) and minimal bactericidal concentrations (MBCs) were determined for each drug separately, and checkerboard studies were used to determine synergistic activity of rifampin plus penicillin, ampicillin, gentamicin, erythromycin, or cefamandole. The MIC was defined as the lowest concentration of antibiotic which allowed no visible growth of the organism in the microtiter plate after 18 to 24 h of incubation at 370C. MBCs were determined by subculturing 10 ,ul of solution from each well of the microtiter plate onto blood agar plates and reincubating for 18 to 24 h at 37°C. The MBC was defined as the lowest concentration of drug which permitted the growth of 10 or fewer colonies per blood agar plate.Combinations of antimicrobial agents were considered fully synergistic if inhibition of growth occurred at s25% of the MIC of each drug. Antagonism was defined as the MIC of the 525 on May 9, 2018 by guest http://aac.asm.org/ Downloaded from

Street heroin and injection paraphernalia have been implicated as sources of bacteria causing infections in drug abusers [1]. Staphylococcus aureus, Pseudomonas aeruginosa, and Bacillus cereus are common etiologic agents. In a previous study of the microbiology of street heroin and injection paraphernalia, Bacillus species was the predominant isolate [2]. We did not find S. aureus, but one study reported isolates of identical phage type from heroin powder and from an infected patient [3]. To reconcile the results of our recent investigation among drug abusers with panophthalmitis, we theorized that the drug mixture might have an antibacterial effect. Of the samples of street heroin tested, all except one were bactericidal against S. aureus. The single sample with no heroin content was not bactericidal against one isolate of S. aureus and was the only sample that exhibited some degree of inhibitory and bactericidal activity against P. aeruginosa. All samples were bactericidal against the two isolates of B. cereus tested. Quinine exhibited bactericidal activity against S. aureus and B. cereus but was ineffective against P. aeruginosa. Our findings indicate that most samples of street heroin have antibacterial effects against S. aureus and B. cereus but no activity against P. aeruginosa. Such activity may be due to the quinine content of the mixture. The apparent lack of recovery of S. aureus from street heroin may be partially explained by this phenomenon.