The stability of the aminoglycosides gentamicin, tobramycin, and amikacin stored in combination with carbenicillin, piperacillin, cefotaxime, and moxalactam was evaluated at four temperatures (25, 4, -8, and -70 degrees C) over a 3-week period. Amikacin was the most stable of the aminoglycosides and demonstrated no loss of activity when stored with either carbenicillin or piperacillin. Gentamicin and tobramycin were inactivated by carbenicillin and piperacillin at 25 and 4 degrees C, with aminoglycoside activity declining substantially after 8 to 48 h of storage; virtually no loss of gentamicin or tobramycin activity occurred with storage at -8 or -70 degrees C. Cefotaxime and moxalactam produced no degradation of any of the three aminoglycosides.

Time-kill curves were used to assess the relative in vitro efficacy of the early interaction of three semisynthetic penicillins with two aminoglycosides against 48 Enterobacteriaceae strains. The most efficacious combinations were piperacillin plus amikacin, which demonstrated synergism (>2 logs of increased kill after 7 h of incubation) against 43 of 48 (901%) strains, and piperacillin plus gentamicin, which exhibited synergism against 25 of 48 (52%) strains. With the combinations of carbenicillin or ticarcdillin plus amikacin or gentamicin, early synergistic killing was demonstrated against only 12 to 29% of the strains.Semisynthetic penicillins such as ampicillin, carbenicillin, and ticarcillin have been used for several years in the therapy of severe gramnegative baciliary infections. Piperacillin is a new semisynthetic penicillin which has demonstrated greater in vitro activity than carbenicillin or ticarcillin against Pseudomonas aeruginosa and members of the Enterobacteriaceae (1,5,13,15,16,19,20 Antibiotic synergism. The interaction between each of the three semisynthetic penicillins and each of the two aminoglycosides was evaluated by time-kill curves in Mueller-Hinton broth as described previously (6). A starting inoculum of 105 organisms per ml was prepared by appropriate dilution of an overnight broth culture. The concentrations of antibiotics in the flasks were selected to be at or below the MIC for each drug and within a clinically achievable range for each drug::125 pg/ml for each of the penicillins, s4 pg/ml for gentamicin, and <8 .g/ml for amikacin. The culture flasks (final volume, 20 ml) were incubated without agitation at 35°C, and 0.5-mi samples were removed at 0, 4, and 7 h for determination of colony counts with 10-fold serial dilutions. Early synergy is herein defined as a decrease of 100-fold or more in the number of 902 on May 11, 2018 by guest http://aac.asm.org/ Downloaded from

Volume 26, no. 3, p. 380: Table 2 should appear as shown below. TABLE 2. Effectiveness of ,B-lactam antibiotics plus amikacin against Enterobacteriaceae strains resistant to one or both agentsa No. of strains showing early synergism/total no. with indicated pattern of susceptibility Therapeutic combination

An in vitro comparison of the early synergistic interaction between amikacin and each of six IT-lactam antibiotics was made by using time-kill curves against 48 multiply resistant members of the family Enterobacteriaceae. Overall, these six combinations demonstrated early synergism (x2 logs of increased kill after 7 h of incubation) against the 48 strains on 74% (range, 67 to 85%) of occasions; cefotaxime-amikacin and piperacilin-amikacin were the most efficacious combinations. Antagonism was not observed with any of the combinations against any of the 48 Enterobacteriaceae strains tested.Combination therapy with an aminoglycoside plus a semisynthetic penicillin or cephalosporin commonly is used in the treatment of life-threatening infections caused by gramnegative bacilli, especially in the immunocompromised patient (1, 16). These antibiotic combinations, shown to be synergistic both in vivo (1, 11) and in vitro (7-9), are used in place of single-drug therapy with the aim of obtaining increased therapeutic efficacy as well as reducing the likelihood of emergence of resistance among infecting gramnegative bacilli (5,18).In a prior investigation with timne-kill curves to evaluate the early synergistic interaction between gentamicin or amikacin and three ,-lactams against 48 members of the family Enterobacteriaceae, we demonstrated that amikacin ih combination with carbenicillin, ticarcillin, or piperacillin produced early synergism against 46% of the strains, whereas gentamicin in combination with these same P-lactams exhibited early synergism against only 28% of the strains (4). Of the P-lactams studied previously, piperacillin in combination with either amikacin or gentamicin demonstrated early synergism against 71% of the strains, whereas carbenicillin plus amikacin or gentamicin and ticarcillin plus amikacin or gentamicin produced early synergism against 16 (14).Susceptibility testing. Antimicrobial susceptibility testing was performed by the agar dilution method (19). Serial twofold dilutions of the respective antibiotics were incorporated into Mueller-Hinton agar, and an inoculum of 1 organisms per ml was delivered onto each plate with a Steers replicator (Craft Machine, Inc., Chester, Pa.). The MIC was defined as the lowest concentration of antibiotic producing no visible growth after 18 h of incubation at 37°C.Ahtibiotic synergism. The synergistic interaction between amikacin and each of the six P-lactam drugs was evaluated by time-kill curves during 7 h of incubation, as described previously (4). The following concentrations of antibiotics in the flasks were selected to be at or below the MIC for each drug and within a clinically achievable range (10, 17) for each drug: <125 ,ug/ml for piperacillin, -62 ,ug/ml for each of the cephalosporins, and s8 ,ug/ml for amikacin. A starting inoculum of 105 organisms per ml was prepared by appropriate dilutiod of an overnight broth culture and added to each flask (final volume, 20 ml of Mueller-Hinton broth). The culture flasks were incubated without agitation at 37...