Direct susceptibility testing of blood culture isolates with the AutoMicrobic System (AMS)

To decrease the time needed to obtain preliminary antimicrobial susceptibility results with blood culture isolates, we inoculated a suspension of centrifuged organisms from blood culture broth directly into the AutoMicrobic System Gram-Positive (GPS) and Gram-Negative (GSC+) susceptibility cards (AMS, Vitek Systems Inc., Hazelwood, MO). Interpretive category results (susceptible, moderately susceptible, resistant) obtained by this direct method (DAMS) were then compared with results obtained by conventional inoculation (i.e., using 18-hr subcultures) of both AMS cards (CAMS method) and broth microdilution panels (MIC method, Micro-Media Systems Inc., Potomac, MD). Ninety-six Gram-positive cocci (951 antimicrobial agent--organism combinations) and 112 Gram-negative bacilli (1006 antimicrobial agent-organism combinations) were tested. When only very major (false susceptible DAMS results) and major (false resistant DAMS results) discrepancies were considered, 95% of the DAMS results for Gram-positive cocci agreed with CAMS results and 93% agreed with MIC results. Most discrepancies were observed when staphylococci were tested against oxacillin and when enterococci were tested against several antimicrobial agents. For Gram-negative bacilli, 94% of DAMS results agreed with CAMS results and 93% agreed with MIC results. Most discrepancies occurred when Enterobacter spp. and Serratia marcescens were tested against ampicillin and cefamandole. The DAMS method provides accurate and rapid preliminary susceptibility test results, usually within 6 to 7 hr of the time a positive blood culture is first detected.