In a multicentre study in Germany 10584 fresh clinical aerobic isolates were tested for susceptibility to piperacillin (30 mu g or 100 mu g discs) and piperacillin/tazobactam (30 mu g/10 mu g or 100 mu g/10 mu g discs). Preliminary breakpoints for piperacillin/tazobactam according to the German standard methods (Deutsches Institute fur Normung) were: Resistant: inhibition zone less than or equal to 14 mm; MIC greater than or equal to 64 mg/l. Susceptible: inhibition zone greater than or equal to 22 mm; MIC less than or equal to 4 mg/l. For comparison, the National Committee for Clinical Laboratory Standards (USA) breakpoints for enterobacteria (100/10 mu g/disc) are: Resistant: inhibition zone less than or equal to 17 mm; MIC greater than or equal to 128/4 mg/l. Susceptible: inhibition zone greater than or equal to 21 mm; MIC less than or equal to 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.
