Antimicrobial susceptibility studies Activity of piperacillin/tazobactam in combination with amikacin, ciprofloxacin, and trovafloxacin against Pseudomonas aeruginosa by time-kill David S. Burgess a,b, *, Rhonda W. Hastings a,b a College of Pharmacy, The University of Texas at Austin, Austin, Texas 78712, USA b Department of Pharmacology, The University of Texas Health Science Center at San Antonio, San Antonio, Texas 78284, USA Received 16 February 2000; received in revised form 9 June 2000; accepted 9 June 2000 Abstract Pseudomonal infections have a high rate of morbidity and mortality, thus combination therapy is often recommended. We compared the activity of piperacillin/tazobactam in combination with amikacin, ciprofloxacin, or trovafloxacin at different concentrations against P. aeruginosa using time-kill methodology. MICs were determined for 4 clinical isolates of P. aeruginosa. Time-kill studies were conducted over 24 h. Each drug was tested alone and in combination using the following concentrations: 2 and 1/4, 1/4 and 2, and 1/4 and 1/4MIC of piperacillin/tazobactam and amikacin, ciprofloxacin, or trovafloxacin. Combinations were classified as synergistic, indifferent, or antagonistic. Synergy was defined as  2-log 10 decrease in CFU/mL at 24 h with the combination when compared to the most active single agent and the number of surviving organisms for the antimicrobial combination was 2-log 10 less than the initial inoculum. The MICs for piperacillin/tazobactam, amikacin, ciprofloxacin, and trovafloxacin, ranged from 4/4 –512/4, 0.5– 4, 0.125– 4, and 0.5– 8g/mL, respectively. Fifty eight percent of the combinations using concentrations of 1/4MIC of piperacillin/tazobactam and 2MIC of amikacin, ciprofloxacin, and trovafloxacin or 2MIC of piperacillin/tazobactam and 1/4MIC of amikacin, ciprofloxacin, and trovafloxacin were synergistic. Although no differences existed in synergistic activity between the two combinations, the 1/4 and 2MIC maintained colony counts below the limit of quantification for 24 h for a significantly greater percentage of isolates than the 2 and 1/4MIC combinations (75 and 25%, respectively; p = 0.04). Overall, synergy was most frequently (42%) noted with the piperacillin/tazobactam and amikacin combinations followed by 33 and 8% of the piperacillin/tazobactam and trovafloxacin and ciprofloxacin combinations. No combination demonstrated antagonism. Further more extensive studies are necessary to determine clinical significance. © 2000 Elsevier Science Inc. All rights reserved. 1. Introduction Pseudomonas aeruginosa is a difficult to treat patho- genic bacterium that typically infects immunocompromised, neutropenic, and critically ill patients. Such infections have a high rate of morbidity and mortality, and development of resistance to monotherapy can be problematic. Conse- quently, dual antimicrobial coverage is often employed (Hilf et al. 1989; Korvick et al. 1991; Stratton 1990). Traditionally, combinations are comprised of an anti- pseudomonal penicillin and an aminoglycoside such as pip- eracillin and gentamicin. Today, selected fluoroquinolone antibiotics (e.g., ciprofloxacin and trovafloxacin) offer a reasonable alternative for treating pseudomonal infections. Advantages of using a fluoroquinolone include less neph- rotoxicity as opposed to the aminoglycosides and oral ad- ministration. The activity of antimicrobial combinations can be as- sessed in vitro using checkerboard and time-kill methodol- ogies. While most data available has been generated from checkerboard methods, some investigators have used time- kill studies in order to provide a more dynamic description of antimicrobial activity and interaction over time. Synergy against P. aeruginosa has been investigated using combi- nations of extended spectrum -lactams, aminoglycosides, and fluoroquinolones. In this experiment, we compared the activity of piperacillin/tazobactam in combination with ami- * Corresponding author. Tel.: +1-210-567-8329; fax: +1-210-567- 8328. E-mail address: burgessd@uthscsa.edu (D.S.Burgess). Presented at the Interscience Conference on Antimicrobial Agents and Chemotherapy, San Francisco, California, September 26 –29, 1999. www.elsevier.com/locate/diagmicrobio Diagnostic Microbiology and Infectious Disease 38 (2000) 37– 41 0732-8893/00/$ – see front matter © 2000 Elsevier Science Inc. All rights reserved. PII: S0732-8893(00)00162-0