A multicentre in vitro evaluation of piperacillin/tazobactam in Germany

In a multicentre study in Germany 10584 fresh clinical aerobic isolates were tested for susceptibility to piperacillin (30 micrograms or 100 micrograms discs) and piperacillin/tazobactam (30 micrograms/10 micrograms or 100 micrograms/10 micrograms discs). Preliminary breakpoints for piperacillin/tazobactam according to the German standard methods (Deutsches Institute für Normung) were: Resistant: inhibition zone < or = 14 mm; MIC > or = 64 mg/l. Susceptible: inhibition zone > or = 22 mm; MIC D or = 4 mg/l. For comparison, the National Committee for Clinical Laboratory Standards (USA) breakpoints for enterobacteria (100/10 micrograms/disc) are: Resistant: inhibition zone < or = 17 mm; MIC > or = 128/4 mg/l. Susceptible: inhibition zone > or = 21 mm; MIC < or = 6/4 mg/l. Using the DIN criteria, 73.2% of Enterobacteriaceae tested were susceptible to piperacillin and 91.2% to piperacillin/tazobactam. Of Staphylococcus aureus strains tested (including methicillin resistant Staphylococcus aureus, MRSA), 20.8% were susceptible to piperacillin and 78.7% to piperacillin/tazobactam. The MIC of piperacillin/tazobactam for most staphylococci and Enterobacteriaceae that had been classified on the first test as having intermediate susceptibility or resistance to piperacillin/tazobactam by DIN criteria were re-evaluated to solve methodological problems and inconsistencies among the participating laboratories. This second study resulted in a reduction of the zone diameter for susceptibility of staphylococci to > 14 mm and found that an additional 55.7% of resistant/intermediate Enterobacteriaceae and 91.0% of resistant/intermediate S aureus were susceptible to piperacillin/tazobactam, giving totals for susceptibility of 96.1% for Enterobacteriaceae and 98.1% for S aureus.