Higa Miyako, Kisanuki Kyoko, Yamane Nobuhisa, Nakasone Isamu
Clinical Laboratories, University Hospital of the Ryukyus, Nakagami-gun, Okinawa-pref. 903-0215.
Rinsho Byori. 2008 Apr;56(4):283-9.
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.
针对三种自动化微生物检测系统，即沃克威40（美国加利福尼亚州西萨克拉门托市达德微生物扫描公司）、VITEK 2 Compact（法国马西伊图瓦勒市生物梅里埃公司）和RAISUS（日本东京日水制药公司），评估了混合接种物对菌种鉴定和抗菌药敏试验结果的影响。在评估过程中，将两种不同的菌种或两种不同的菌株按系列比例混合，并针对各自的系统调整为接种菌悬液，然后进行菌种鉴定和抗菌药敏试验。对于菌种鉴定，所有这三种自动化系统均出现了高于90%可能性的与接种菌种不同的错误鉴定，例如阴沟肠杆菌加肺炎克雷伯菌在9比1的混合比例下，有93%至97%的可能性被鉴定为解鸟氨酸克雷伯菌或产气肠杆菌。然而，将产超广谱β-内酰胺酶（ESBL）和不产ESBL的大肠埃希菌、耐甲氧西林金黄色葡萄球菌（MRSA）和甲氧西林敏感金黄色葡萄球菌、耐万古霉素粪肠球菌（VR）和万古霉素敏感粪肠球菌（VS）混合时，总能正确检测到一小部分耐药菌。然而，正确检测所需的耐药菌最小比例因系统而异，即RAISUS需要70%的MRSA，VITEK 2 Compact为8%，沃克威40为1%。此外，当检测添加了奇异变形杆菌的VS粪肠球菌菌悬液时，沃克威40报告为非常罕见的生物型，但VITEK 2 Compact和RAISUS均报告检测接种物为VR粪肠球菌。基于这些结果，可以得出以下结论：第一，即使接种物中不同菌种的比例在初次分离琼脂平板上肉眼难以区分，也很容易出现与接种菌种不同的错误菌种鉴定。第二，自动化微生物检测系统总是倾向于检测接种物中的抗菌耐药菌，而不是检测主要的敏感菌，以防止我们报告非常严重的错误解读。
