Modified inoculum for the enteric Minitek system from positive blood cultures

Identification of Enterobacteriaceae with the Minitek system

A total of 417 strains (361 Enterobacteriaceae, 56 Vibrionaceae) was examined in all the available Minitek system tests. The results were processed through four successive identification schemes devised by the manufacturer and the proportion of strains correctly identified, not identified or incorrectly identified determined for each scheme. From the results, a probability matrix was constructed incorporating all 35 Minitek tests. Test results for each strain were then processed through this matrix to determine its success in identification. From the matrix the order of separating value of the tests was determined. Forty-three of the strains were each tested three times to assess the level of test reproducibility; the corrected error rate was 0.85%.

Comparison of four rapid methods for identification of Enterobacteriaceae from blood cultures

Positive blood cultures containing gram-negative bacilli were utilized for direct identification by two automated systems, the AutoMicrobic system (Vitek Systems, Inc., Hazelwood, Mo.) and the MS-2 (Abbott Diagnostics, Dallas, Tex.), and two commercial kits, the Micro-ID system (General Diagnostics, Warner-Lambert Co., Morris Plains, N.J.) and the same-day API 20E (Analytab Products, Plainview, N.Y.). Samples of 10 to 15 ml were aseptically removed from radiometrically positive BACTEC bottles (Johnston Laboratories, Cockeysville, Md.) and divided among four sterile tubes. The tubes were centrifuged at 107 X g for 10 min. The supernatants were centrifuged at 1,510 X g for 10 min, and pellets were tested for cytochrome oxidase by means of Pathotec strips. Oxidase-negative pellets were suspended in appropriate media as suggested by the manufacturers. All systems were inoculated, incubated, and interpreted according to the instructions of the manufacturers. The Micro-ID system was read after 4 h of incubation; the three remaining systems were read after 5 h. Results were compared with those of the 18-h API 20E inoculated from pure subcultures of the organisms. Correlation of 90% or more with the API 20E was achieved by the AutoMicrobic and Micro-ID systems. The same-day API 20E and the MS-2 demonstrated 60 and 44% correlation, respectively, with the 18-h API 20E.

Rapid identification and antimicrobial susceptibility testing of gram-negative bacilli from blood cultures by the AutoMicrobic system

A procedure was developed which allows direct identification and antimicrobial susceptibility testing of fermentative and nonfermentative gram-negative bacilli from positive blood cultures. A 10-ml sample was removed from turbid blood culture bottles, and the bacteria were washed and concentrated by centrifugation. The bacterial pellet was used to inoculate an Enterobacteriaceae Plus Identification Card and a Gram-Negative General Susceptibility Card of the AutoMicrobic system. Results with these cards were compared with results obtained with standard technique for 196 blood cultures seeded with recent clinical isolates. Identification of most cultures was available in 8 h, whereas the antimicrobial susceptibility results were available in an average of 4.7 h for all organisms. Direct identification was correct for 95% of the cultures, whereas the antimicrobial susceptibility data had an average agreement of 87% with 3.8% very major and 1.4% major errors. In using this procedure it was possible to provide accurate preliminary identification and results of antimicrobial susceptibility tests for gram-negative bacilli on the same day that a blood culture was determined to be positive.

Evaluation of the four-hour Micro-ID technique for direct identification of oxidase-negative, Gram-negative rods from blood cultures

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 Klebsiella oxytoca, 8 Enterobacter aerogenes, 17 Enterobacter cloacae, 19 Serratia marcescens, 2 Serratia liquefaciens, 8 Proteus mirabilis, 1 Salmonella species, 1 Morganella morganii, 6 Haemophilus influenzae, 2 Haemophilus 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 three S. marcescens (two identified as S. liquefaciens, one as Hafnia alvei), two S. liquefaciens (both identified as E. cloacae), and two K. pneumoniae (one identified as Klebsiella ozaenae, the other as Serratia rubidaea). None of the 15 non-enterobacterial cultures were correctly identified by Micro-ID (non-identifiable, or classified as Providencia/Yersinia/Klebsiella species). 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.