Results of a local combination therapy antibiogram for Pseudomonas aeruginosa isolates: is double worth the trouble?

Purpose: To determine the frequency at which fluoroquinolones and aminoglycosides demonstrate in vitro activity against non-urinary, non-skin/skin structure Pseudomonas aeruginosa isolates exhibiting decreased susceptibilities to one or more beta-lactam agents. Methods: beta-lactam-non-susceptible P. aeruginosa isolates recovered from blood, bone, lower respiratory tract, pleural fluid, cerebrospinal fluid, or peritoneal fluid cultures between October 2010 and October 2014 were reviewed from four community hospitals within a single health-system. Only the first isolate per patient was included for analysis. The likelihood that each isolate was susceptible to a non-beta-lactam antimicrobial was then determined and summarized within a combination antibiogram. Results: In total, 179 P. aeruginosa isolates with decreased susceptibilities to one or more beta-lactam agents were assessed. Because no appreciable differences in antimicrobial susceptibility profile were observed between hospitals, the isolates were evaluated in aggregate. Susceptibility rates for beta-lactam monotherapy ranged from 34% to 75%. Aminoglycosides possessed increased antibacterial activity compared to fluoroquinolones. Tobramycin was the non--lactam most likely to expand antimicrobial coverage against beta-lactam-non-susceptible P. aeruginosa with activity against 64%, 66%, and 65% of cefepime-, piperacillin-tazobactam-, and meropenem-non-susceptible isolates, respectively (p<0.001 for all). Conclusions: The results of this study support the use of aminoglycosides over fluoroquinolones for achieving optimal, empiric antimicrobial combination therapy for P. aeruginosa when dual antimicrobial therapy is clinically necessary. Future efforts aimed at optimizing combination therapy for P. aeruginosa should focus on systemic interventions that limit the selection of fluoroquinolones in combination with beta-lactams to expand coverage based on local susceptibility rates.