Argentinean collaborative multicenter study on the in vitro comparative activity of piperacillin-tazobactam against selected bacterial isolates recovered from hospitalized patients.

The in vitro activity of piperacillin-tazobactam and several antibacterial drugs commonly used in Argentinean hospitals for the treatment of severe infections was determined against selected but consecutively isolated strains from clinical specimens recovered from hospitalized patients at 17 different hospitals from 9 Argentinean cities from different geographic areas during the period November 2001-March 2002. Out of 418 Enterobacteriaceae included in the Study 84% were susceptible to piperacillin-tazobactam. ESBLs putative producers were isolated at an extremely high rate since among those isolates obtained from patients with hospital acquired infections 56% of Klebsiella pneumoniae, 32% of Proteus mirabilis and 25% Escherichia coli were phenotypically considered as ESBLs producers Notably P.mirabilis is not considered by for screening for ESBL producers. ESBLs producers were 100% susceptible to imipenem and 70% were susceptible to piperacillin-tazobactam whereas more than 50% were resistant to levofloxacin. The isolates considered as amp C beta lactamase putative producers showed 99% susceptibility to carbapenems while 26.7% were resistant to piperacillin-tazobactam and 38.4% to levofloxacin. Noteworthy only 4% of the Enterobacteriaceae isolates were resistant to amikacin. Piperacillin-tazobactam was the most active agent against Pseudomonas aeruginosa isolates (MIC(90): 128 microg/ml; 78% susceptibility) but showed poor activity against Acinetobacter spp (MIC(90):>256 microg/ml; 21.7% susceptibility). Only 41.7% Acinetobacter spp isolates were susceptible to ampicillin-sulbactam. Piperacillin-tazobactam inhibited 100% of Haemophilus influenzae isolates (MIC(90) < 0.25 microg/ml) but only 16.6% of them were ampicillin resistant. The activity of piperacillin-tazobactam against oxacillin susceptible Staphylococcus aureus or coagulase negative staphylococci was excellent (MIC(90) 2 microg/ml; 100% susceptibility). Out of 150 enterococci 12 isolates (8%) were identified as E.faecium and only three isolates (2%), 2 E.faecium and 1 E.faecalis were vancomycin resistant. All the enterococci isolates were susceptible to linezolid. Piperacillin-tazobactam showed excellent activity (MIC(90) 2 microg/ml; 92% susceptibility). Regarding pneumococci all the isolates showed MICs of 16 microg/ml for piperacillin-tazobactam. Among 34 viridans group streptococci only 67% were penicillin susceptible and 85.2% ceftriaxone susceptible whereas piperacillin-tazobactam was very active (MIC(90) 4 microg/ml).Piperacillin-tazobactam is therefore a very interesting antibacterial drug to be used, preferably in combination (IE: amikacin-vancomycin) for the empiric treatment of severe infections occurring in hospitalized patients in Argentina. Caution must be taken for infections due to ESBL producers considering that the inoculum effect MICs can affect MIC values.