Evaluation of the Four-Hour Micro-ID Technique for Direct Identification of Oxidase-Negative, Gram-Negative Rods from Blood Cultures

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Abstract
A 4 h Micro-ID technique for direct identification of oxidase-negative gram-negative rods from positive blood cultures was compared to subculture and species identification of single colonies by API 20E and Micro-ID, using standardized inocula. A total of 127 patients (220 positive cultures) were studied. Isolates included 96 Escherichia coli, 46 Klebsiella pneumoniae, 7 K. oxytoca, 8 Enterobacter aerogenes, 17 E. cloacae, 19 Serratia marcescens, 2 S. liquefaciens, 8 Proteus mirabilis, 1 Salmonella spp., 1 Morganella morganii, 6 Haemophilus influenzae, 2 H. parainfluenzae, 3 Bacteroides fragilis, 3 Acinetobacter calcoaceticus biotype anitratus and 1 Pseudomonas maltophilia. In 90% of the cultures, identification by Micro-ID was identical to that obtained after subculture; if the 15 non-enterobacterial isolates were excluded, the corresponding figure was 96.6%. Enterobacteria identified incorrectly by direct Micro-ID were 3 S. marcescens (2 identified as S. liquefaciens, 1 as Hafnia alvei), 2 S. liquefaciens (both identified as E. cloacae), and 2 K. pneumoniae (1 identified as K. ozaenae, the other as S. rubidaea). None of the 15 non-enterobacterial cultures were correctly identified by Micro-ID (non-identifiable, or classified as Providencia/Yersinia/Klebsiella spp.). Although biochemical discrepancies between direct and final Micro-ID tests occurred in 41% of the enterobacterial cultures, this did not seriously interfere with identification. Direct species identification of Enterobacteriaceae from blood cultures by direct Micro-ID is accurate and easily performed and identified organisms within 4 h compared to at least 24 h by most other methods; the direct Micro-ID technique would be rendered even more valuable by the additional capability of identifying non-enterobacterial gram-negative isolates.