In vitro bactericidal effect of a beta-lactam plus aminoglycoside combination against multiresistant Pseudomonas aeruginosa and Acinetobacter baumannii

Pseudomonas aeruginosa and Acinetobacter baumannii are frequently isolated in hospital outbreaks of nosocomial infections. In our hospital, among 1018 strains isolated one year in an intensive care unit, 84 strains (8.3%) of P. aeruginosa and 155 strains (15.2%) of A. baumannii were considered responsible for infections. The major problem related to these bacteria is their multiresistant characteristic which confers great difficulty in treating infections. We carried out a 24 h time-kill study to assess the bactericidal effect of three beta-lactams [imipenem (IPM), ticarcillin + clavulanic acid (TCC), piperacillin + tazobactam (PTB)] in combination with each other and with sulbactam (SUL) and amikacin (AKN) against 8 P. aeruginosa strains and 8 A. baumannii strains. The initial inoculum was 10(6) cfu/ml. Antibiotics were tested at clinically achievable concentrations: TCC (112 mg/l), PTB (100 mg/l), IPM (25 mg/l) and AKN (15 mg/l). The results showed: IMP+TCC+AKN = PTB+SUL+AKN = PTB+TCC+AKN much greater than IMP+SUL+AKN against P. aeruginosa; and PTB+SUL+AKN = PTB+TCC+AKN > IMP+SUL+AKN or IMP+TCC+AKN against A. baumannii. When infection due to these multiresistant strains was suspected, PTB+AKN combined with either TCC or SUL was bactericidal against both strains. These combinations appeared to be an alternative therapy in the treatment of undocumented nosocomial infections in intensive care units. These in vitro results are being evaluated in patients and seem to give good results for the moment.