INFLUENCE OF ZINC ON PSEUDOMONAS-AERUGINOSA SUSCEPTIBILITIES TO IMIPENEM

Serial dilution susceptibility testing of imipenem against 59 clinical isolates of Pseudomonas aeruginosa, conducted simultaneously on singl e lots of Difco and BBL Mueller-Hinton agar (MHA), resulted in MICs fo r 90% of strains tested of 8 and 16 mug/ml, respectively. MICs for Esc herichia coli, Klebsiella pneumoniae, and Pseudomonas spp. were also h igher on BBL MHA. Quantification of the cation content of the two MHAs by atomic absorption spectroscopy demonstrated that the zinc concentr ation in BBL MHA was 15 times greater than that measured in Difco MHA (2.61 and 0.17 mug/mi, respectively). Concentrations of calcium, magne sium, iron, manganese, and copper in the two agars were similar. Addit ion of zinc to Difco MHA resulted in increases in MICs of imipenem for P. aeruginosa but not in the MICs of ceftazidime or cefpirome for P. aeruginosa (P < 0.01). A lesser zinc effect was seen on the activity o f imipenem against E. coli, K. pneumoniae, and Pseudomonas spp. The ac tivities of ceftazidime and cefpirome were similar on both MHAs when t ested against all gram-negative organisms in this study. Thus, the eff ect of zinc in MHA was clearly demonstrated by a significant increase in the MICs of imipenem for P. aeruginosa, and, to a lesser extent, fo r other gram-negative bacilli.