[Influence of the mixed inoculum on the species-identification and antimicrobial susceptibility test results by the automated microbial systems].

The influence of the mixed inoculum on the species-identification and antimicrobial susceptibility test results was evaluated for the three automated microbial systems, WalkAway-40 (Dade MicroScan, West Sacramento, CA, USA), VITEK 2 Compact (bioMérieux, Marcy l'Etoile, France) and RAISUS (Nissui Pharmaceutical, Tokyo). In the evaluation, two different species or two different strains were mixed in serial ratios and adjusted to the inoculum cell suspension for the respective systems, and then tested for the species-identification and antimicrobial susceptibility. For the species-identification, all the three automated systems experienced incorrect identifications others from the species inoculated with higher likelihoods (> 90%), e.g. Enterobacter cloacae plus Klebsiella pneumoniae resulted in K. ornithinolytica or E. aerogenes with 93% to 97% likelihoods at the mixing ratio 9 to 1. Whereas, the mixings extended-spectrum beta-lactamase (ESBL)-producing and non-producing Escherichia coli, methicillin-resistant (MRSA) and methicillin-susceptible Staphylococcus aureus, and vancomycin-resistant (VR) and vancomycin-susceptible (VS) Enterococcus faecalis, always resulted in correct detection of a small portion of resistant cells. However, minimum ratios of resistant cells for the correct detection varied by the systems, that is, RAISUS required 70% of MRSA, VITEK 2 Compact was 8%, and the WalkAway-40 was 1%. Also, when the cell suspension of VS E. faecalis spiked with Proteus mirabilis was tested, the WalkAway-40 reported as being very rare biotype, but both VITEK 2 Compact and RAISUS reported as the test inoculum being VR E. faecalis. With these results, it can be concluded that: First, incorrect species-identifications others from the inoculated species easily occur when the inoculum contains different species even at the ratio visibly indiscernible on the primary isolation agar plate. Secondly, the automated microbial systems always intend to detect antimicrobial resistant cells in the inoculum rather than to detect major susceptible cells to prevent us from reporting very major error interpretation.