On occasion we encounter isolates of Enterobacter cloacae that are susceptible to ampicillin and cefazolin by our commercial MIC test system. Should we report as susceptible? Repeat the test? Report as resistant?

Answer:

Repeat the antimicrobial susceptibility test or the identification test or both of these. It would be very uncommon to have an E. cloacae truly susceptible to ampicillin and cefazolin. There are no data to support use of these agents in treating infections caused by E. cloacae. Even if test performance steps are followed as the manufacturer of your system states and your QC testing is satisfactory, the patient test results may be erroneous. Some labs may choose to report ampicillin and cefazolin as resistant rather than expend resources to repeat the test. If there is any doubt that the test was improperly performed (e.g. use of inadequate inoculum), repeat testing is definitely warranted. There is concern that drugs other than ß-lactan agents in addition to ampicillin and cefazolin may be falsely susceptible.

References:

Courvalin, P. 1992. Interpretive reading of antimicrobial susceptibility tests; molecular analysis and therapeutic interpretation of in vitro tests to improve antibiotic therapy. ASM News 58:368-375.

Ferraro, MJ and JH Jorgensen. 1999. Susceptibility testing instrumentation and computerized expert systems for data analysis and interpretation. pp. 1593-1600. In Murray, PR, EJ Baron, MA Pfaller, FC Tenover, and RH Yolken, (eds). Manual of Clinical Microbiology, seventh ed. ASM Press, Washington, DC.