High isolation rate of Staphylococcus aureus from surgical site infections in an Indian hospital

sampling. He was empirically treated with 5.2 g of intravenous piperacillin/tazobactam (Tazocin, Wyeth Pharmaceuticals) every 8 h. Due to a good clinical response, the treatment was not changed after the notification of the heteroresistance to piperacillin/tazobactam. No growth was noticed in repeated urine cultures 1 week after the end of treatment with piperacillin/tazobactam and 1 month after his hospital discharge. We have previously described heteroresistance to carbapenems in P. aeruginosa and A. baumannii. In the present study, we analyse the heterogeneous mode of growth of P. aeruginosa in piperacillin/tazobactam. As this heteroresistant phenotype has not been reported previously in pseudomonads, it is not known what percentage of P. aeruginosa isolates are heteroresistant to the drug and what clinical impact this heteroresistance may have. Also, whether the use of piperacillin/tazobactam may gradually lead to the selection of mutant subpopulations that might subsequently compromise treatment still remains unknown. In P. aeruginosa, a heterogeneous mode of resistance to blactams has been shown to emerge readily in hypermutable strains. The heteroresistant phenotype of our isolate resembles persister cells rather than mutants, due to the fact that the heteroresistant subpopulations return to the native phenotype when re-tested and that might partially explain the successful treatment with piperacillin/tazobactam. The fact that the drug achieves high urine concentrations may also have contributed to the favourable outcome. A similar strategy of bacteria to develop persister cells under antibiotic pressure has also been observed in A. baumannii. Should antimicrobials against which bacteria produce persisters still remain effective, as shown for piperacillin/tazobactam in P. aeruginosa, it is important when it comes to the treatment of multidrug-resistant pseudomonal infections. Larger studies are necessary to determine the frequency of infections due to piperacillin/tazobactamheterore istant isolates and their therapeutic consequences.