[Species-identification and antimicrobial susceptibility tests by the fully automated RAISUS using an early-harvested...

We evaluated the usefulness of an early-harvested bacterial cell suspension to the fully automated RAISUS (Nissui Pharmaceuticals Co., Ltd., Tokyo) to provide the results of species-identification and antimicrobial susceptibility testings within a day after overnight-incubation of the primary cultures. A single, well-separated colony appeared on the primary culture plate was transferred onto a blood agar or chocolate agar plates, then incubated for 3 to 6 hours. The cell suspension to the RAISUS was properly prepared to the McFarland 0.5 turbidity from the early-harvested bacterial cells. When the five ATCC reference strains, consisting of Staphylococcus aureus ATCC 29213, Enterococcus faecalis ATCC 29212, Streptococcus pneumoniae ATCC 49619, Escherichia coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853, were repeatedly tested for the species-identification, all the identification results were acceptable. Antimicrobial susceptibility tests were evaluated with the above five strains and Haemophilus influenzae ATCC 49247. The results obtained indicated that the most susceptibility test results were comparable to those MICs obtained by the standard test procedure, but some strains, in particular, H. influenzae and P. aeruginosa gave significantly discrepant MICs for certain antimicrobial agents. The significant discrepancy in MIC determinations regarded the difference of viable cell concentrations in the cell suspension prepared respectively. Through the analysis of laboratory workflow, it became to apparent that 18S to 20S of the tests were completed by 5:00 p.m., and it required to wait until 3:00 a.m. to complete 90S of the tests. With these results, the early-harvested bacterial cell suspension is applicable to species-identification by RAISUS, but it is necessary to adjust viable cell concentrations to antimicrobial susceptibility test. Also, it is urgent to reconstitute a daily workflow to improve the rapidity of RAISUS test function.