In vitro interaction of micafungin and ﬂuconazole against Candida

fingerprint patterns. The antibiotic discs used (Table 1) were from HiMedia Lab. Ltd, India, except piperacillin, piperacillin/tazobactam, ceftriaxone/sulbactam (kindly provided by Ranbaxy Lab. Ltd, India) and imipenem, which were obtained from Oxoid, Basingstoke, UK. E. coli ATCC 25922, E. coli ATCC 35218 and Klebsiella pneumoniae ATCC 700603 were used as control strains. The results were interpreted as per the CLSI (formerly NCCLS) criteria, except for ceftriaxone/sulbactam. Because there are no published breakpoints for the latter combination, the susceptibility criterion used for this combination was . 8 mm increase in zone diameter of ceftriaxone/sulbactam in comparison with that of ceftriaxone alone. Phenotypic ESBL detection by disc synergy tests was performed on all 54 isolates using co-amoxiclav and piperacillin/tazobactam discs as a source of inhibitors. Briefly, the test inoculum (equivalent in turbidity to that of a 0.5 McFarland standard) was streaked on Mueller– Hinton agar. Discs of co-amoxiclav (20/10 mg) or piperacillin/ tazobactam were placed 20 and 30 mm, centre to centre, from discs containing cefotaxime (30 mg), ceftazidime (30 mg) and cefpirome (30 mg) and plates were incubated at 378C overnight. Enhancements of zones of inhibition of cephalosporins towards piperacillin/tazobactam or co-amoxiclav were considered as positive ESBL results. All tests were performed in duplicate. Comparative susceptibilities of the isolates against individual b-lactam antibiotics and b-lactam/b-lactamase inhibitor combinations are shown in Table 1. A total of 96.3% of isolates were susceptible to ceftriaxone/sulbactam in comparison with only 12.9% susceptible to piperacillin/tazobactam. This contrasts with previous international and national studies, which reported that combinations of b-lactam with tazobactam showed greater activity than b-lactam/sulbactam combinations against E. coli isolates. This marked difference may reflect different mechanism(s) in the different bacterial collections. In the present study, we specifically looked at CTX-M group-1 and CTX-M-15 producers and found ceftriaxone/sulbactam to be a highly effective combination (activity being equal to that of imipenem) in contrast to piperacillin/tazobactam and ticarcillin/clavulanate. Two isolates that were resistant to ceftriaxone/sulbactam were also resistant to imipenem (Table 1); they carried blaCTX-M-15 and blaampC alleles (data not shown). In disc synergy tests, none of the 54 ESBL producers was detected when co-amoxiclav discs were placed 30 mm from cephalosporin discs, and only 4 isolates (7.4%) were detected when this was reduced to 20 mm. It is known that optimal disc placement is an important issue in such tests. However, when piperacillin/tazobactam discs were used and the discs were placed 20 mm apart, we readily detected most ESBL-producing isolates; synergy with ceftriaxone, cefotaxime and ceftazidime allowed detection of 92.6% (50/54), 90.7% (49/54) and 74.1% (40/54) of ESBL producers, respectively. When the discs were placed at 30 mm, ESBL could still be detected in all the isolates; most (88.9% with ceftriaxone and 83.3% with cefotaxime) of the isolates still giving excellent synergistic patterns. On the basis of these findings, we feel that piperacillin/ tazobactam (or tazobactam alone) could be a better indicator when combined with ceftriaxone and cefotaxime for the phenotypic detection of ESBLs, especially in CTX-M producers. This needs to be evaluated further and also against other classes of b-lactamases. To conclude, ceftriaxone/sulbactam proved to be effective in vitro against 54 diverse CTX-M-15-producing E. coli strains. This novel combination merits further investigation, as does the use of piperacillin/tazobactam discs for the phenotypic detection of ESBLs.