Rapid identification of Enterobacteriaceae with the micro-ID system versus API 20E and conventional media.

The Micro-ID system for rapid (4 h) identification of Enterobacteriaceae was evaluated by testing 433 enteric bacilli and 9 other gram-negative bacilli. Each isolate was identified with conventional tubed media and was also tested in the Micro-ID and API 20E systems. The overall accuracy of both systems was 97%. Micro-ID tests for the Voges-Proskauer reaction, indole and H2S production, and ornithine and lysine decarboxylase all demonstrated a 97 to 99% correlation with conventional methods. Only 86% of the Micro-ID urease tests agreed with Christenson urea agar. Two inoculum densities were tested in Micro-ID panels, with 157 stock cultures. Over 90% of the tests were unaffected by changes in inoculum density. Tests with four control strains suggested that the Micro-ID system was more reproducible when a light inoculum was used. The Micro-ID system was found to be a very convenient method for rapid, accurate, and precise identification of the Enterobacteriaceae.     

Comparison of Micro-ID, API 20E, and Conventional Media Systems in Identification of Enterobacteriaceae

The Micro-ID, a new identification kit for Enterobacteriaceae, consists of 15 biochemical tests, with substrates and reagents impregnated in filter paper disks. A 0.2-ml amount of an organism suspension equal to a 0.5 McFarland standard is pipetted into each of the compartments. After 4 h of incubation and addition of potassium-hydroxide (KOH) to the Voges-Proskauer test, the color reactions are read according to the recommendations of the manufacturer. A five-digit octal code number is derived from each set of reactions from which an identification is derived by using a code book. In a single-blind, comparative study of the Micro-ID system with the API 20E system (Analytab Products Inc.) and conventional biochemical tube media, we found that the Micro-ID and the API 20E systems gave a 90% identification correlation when each was compared with the conventional tube media. A comparison of all three systems gave an 82% overall identification correlation. When the common tests of Micro-ID and API 20E were compared with conventional tube media, we found that the tests in the Micro-ID performed as well as or better than those of the API 20E. Certain groups of organisms, i.e., Citrobacter, Enterobacter, Proteus, Salmonella, and Serratia genera, were found to give low correlation on certain common tests. When using primary isolation MacConkey plates from the clinical laboratory, only 74% of the plates with Enterobacteriaceae had sufficient numbers of colonies of each enteric organism to produce the 0.5 McFarland inoculum density required. Problems concerning the misidentification of some organisms are discussed.     

Comparison of the automicrobic system with API, enterotube, micro-ID, micro-media systems, and conventional methods for identification of Enterobacteriaceae.

Identification of Enterobacteriaceae by the AutoMicrobic System Enterobacteriaceae Biochemical Card was evaluated. Recent clinical isolates of enteric gram-negative bacilli (192) and glucose nonfermenters (3) were identified by the AutoMicrobic System, Micro-Media Systems, Micro-ID, API, and Enterotube II in comparison with conventional methods. The AutoMicrobic System and Micro-Media Systems correctly identified 97% of the organisms tested. Micro-ID, API, and Enterotube II correctly identified 94, 92, and 84% of the organisms, respectively. In addition to a high degree of identification accuracy, the AutoMicrobic System was convenient to operate and produced identification results in 8 h. Operation of the AutoMicrobic System also required minimal personnel time because it automatically monitored and interpreted the biochemical reactions and reported organism identifications. The AutoMicrobic System appears to be an efficient and accurate system for the identification of Enterobacteriaceae.     

Evaluation of the modified Micro-ID system for identification of Enterobacteriaceae.

Micro-ID is a system designed to identify the Enterobacteriaceae by utilizing reagent-impregnated disks for 15 biochemical tests. Since its initial evaluations, the system has undergone modification in formulation and in its computer data base. In a dual-center evaluation, 306 isolates of Enterobacteriaceae were tested: 145 common and typical isolates at the Mayo Clinic and 161 unusual or atypical isolates at the Center for Disease Control. Each laboratory also exchanged 50 cultures to test the system's reproducibility. Micro-ID correctly identified 142 (98%) of the common clinical isolates and 123 (76%) of the unusual or atypical organisms. However, in this latter group, three species tested were not in the system's data base. When these organisms were deleted from the analysis, 138 of 146 (95%) of the unusual or atypical isolates were correctly identified. Analysis of the 100 isolates identified in duplicate revealed 93% reproducibility of genus and species identification and 62% reproducibility of octal code numbers. Of the 31 strains with the same identification but different code numbers, 74% differed in only one biochemical test.     

Evaluation of the Micro-ID system for the identification of Yersinia pestis.

One hundred isolates of Yersinia pestis identified by conventional means were tested by the Micro-ID system to assess its reliability for distinguishing Y. pestis from other members of the family Enterobacteriaceae. The Micro-ID system gave Y. pestis as a choice for the identification of 89 of these cultures, although not always as the first choice. Most nitrate-negative strains of Y. pestis keyed out with Yersinia pseudotuberculosis as first choice and Y. pestis as second or fourth choice.     

Comparison of micro-ID and API 20E in rapid identification of Enterobacteriaceae.

The effectiveness of Micro-ID and API 20E as same-day identification systems for Enterobacteriaceae was evaluated in comparison with conventional identification by using 315 clinical isolates and 90 stock strains. The API 20E system was heavily inoculated according to manufacturer's recommendations for same-day identification. We found that 83 and 81% of isolates provided adequate inocula for Micro-ID and API 20E, respectively, and purity of the heavy inocula was not a problem with either system. Overall agreement with conventional identification at genus and species levels was 93.5% with Micro-ID and 90.2% with API 20E. However, when Klebsiella pneumoniae and K. oxytoca were considered as a single species and Proteus morganii was equated with Morganella morganii, agreement was 95.8 and 90.5%, respectively. Only 83.% of isolates were identified on the day of inoculation by API 20E, in contrast to 94.3% with Micro-ID. The remaining isolates required supplementary overnight testing. Provisional (low selectivity) determinations were constant with conventional identification with 49.3% of isolates with API 20E and 82.6% with Micro-ID. Telephone consultations with the manufacturers to resolve unprinted octal codes required a maximum of 15 min with Micro-ID and from 2 to greater than 48 h with API 20E.     

Four hour identification of Enterobacteriaceae with the API Rapid 20E and Micro-ID systems.

One hundred strains of Enterobacteriaceae were examined in parallel with the API Rapid 20E and Micro-ID commercial four hour identification systems. With the API Rapid 20E system 78% of the strains were correctly identified, 15% were not identified, and 7% were misidentified. The respective figures with the Micro-ID system were 74%, 11%, and 15%.     

Comparison of Micro-ID and API 20E systems for identification of Enterobacteriaceae.

The Micro-ID 4-h identification system for Enterobacteriaceae was compared to the API 20E overnight method, using 230 fresh clinical isolates and 74 stock cultures. Agreement was 97.8% for the clinical isolates and 93.2% for the stock cultures. Eighty-seven percent of primary culture plates containing gram-negative rods yielded sufficient growth to perform the 4-h Micro-ID identification on the same day the organisms were isolated.     

Update and evaluation of the AutoMicrobic yeast identification system.

The AutoMicrobic system (AMS) Yeast Biochemical Card (Vitek Systems Inc., Hazelwood, Mo.) is a system which has been designed for rapid and automated reporting of yeast identification in the clinical laboratory. Recent improvements have been implemented in the AMS data base to expand and enhance its yeast identification capabilities. These improvements include the addition of seven biotypes, changes in data analysis scheme, and construction of the taxonomic keys. The updated system was compared with the API 20C (Analytab Products, Plainview, N.Y.) yeast identification system and a rapid conventional method, using 1,106 clinical and stock yeast isolates. With these improvements, the AMS Yeast Biochemical Card had a correlation of 98.8% with the API 20C system and 93.4% with the rapid conventional method and significantly increased its capability of identifying Cryptococcus neoformans (98%). The most difficult organisms for the system to identify in 22 to 24 h were Cryptococcus terreus (58%) and Cryptococcus uniguttulatus (73%). The updated AMS not only provided more rapid results which were comparable to the other two systems but gave a substantial savings in set-up and reporting time as well.     

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.     

Evaluation of a New Assay for Vi Antibody in Chronic Carriers of Salmonella typhi

Biotyping of Haemophilus influenzae into five type and H. parainfluenzae into three types based on indole production, ornithine decarboxylase, and urease has been reported (M. Kilian, Acta Pathol. Microbiol. Scand. Sect. B 82:835--842, 1976). A commercially available test system designed for the 4-h identification of Enterobacteriaceae. Micro-ID, proved efficacious for the rapid biotyping of these two Haemophilus species. The nitrate reductase, indole production, ornithine decarboxylase, urease, and o-nitrophenyl-beta-D-galactopyranoside hydrolysis tests in Micro-ID correlated over 99% with conventional methodology. By utilizing the indole and o-nitrophenyl-beta-D-galactopyranoside tests it was possible, with 261 of 272 (96.1%) isolates, to distinguish H. influenzae from H. parainfluenzae. Cerebrospinal fluid isolates were over 90% H. influenzae biotype I, and conjunctival isolates were approximately 70% biotype II. Type b H. influenzae were predominantly biotypes I and II; these type b isolates were also overwhelmingly indole producers. Although over 90% of biotypes I and II have been reported to produce beta-lactamase, this was not confirmed by the small number of beta-lactamase producers encountered here. The 4-h Micro-ID should prove a useful mechanism, amenable to the routine clinical laboratory, for the further exploration of the association of Haemophilus with the site of isolation, antigenicity, and antibiotic resistance.     

Time-motion and cost comparison study of micro-ID, API 20E, and conventional biochemical testing in identification of Enterobacteriaceae.

A total of 730 Enterobacteriaceae strains isolated from 567 cultures were evaluated by a rapid kit method (Micro-ID; General Diagnostics, Morris Plains, N.J.; 4 h), an overnight incubation kit method (API 20E; Analytab Products, Plainview, N.Y.), and conventional biochemical test methodology (mostly overnight incubation and some rapid methods) to compare the amount of laboratory effort required, timing, and cost parameters. We assessed the amount of technologist time expended, the time sequence of culture reporting to physicians, the number of isolates requiring repeat testing or additional biochemical testing, the number of cultures held due to the need for identification of other organisms, the cost of total work-up, etc. Cultures evaluated included urines, respiratory cultures, wound cultures, body fluids, genital cultures, and cultures from miscellaneous categories. A total of 64% of the Enterobacteriaceae strains processed by the Micro-ID method could be identified within 24 h of receipt of the specimens in the clinical laboratories, in contrast to the need for an additional day required by the API or conventional biochemical methods. The Micro-ID method also required less technologist time (4.5 min) for set-up and interpretation than did either the API method (6 min) or conventional methods (7 min). Total direct costs (June 1981) per organism identified were: Micro-ID, $4.30; API 20E, $4.96; conventional biochemicals with commercially prepared media, $5.66. An estimate of 80% technologist time efficiency was made in all procedures.     

Evaluation of the automicrobic system for detection of resistance of Staphylococcus aureus to methicillin

The AutoMicrobic system (AMS) (Vitek System, Inc., Hazelwood, Mo.) was tested for its ability to determine oxacillin and gentamicin susceptibility of 98 known oxacillin-susceptible and 103 known oxacillin-resistant Staphylococcus aureus isolates. AMS and reference oxacillin susceptibility results were in agreement for all 95 (100%) oxacillin-susceptible isolates. In contrast, only 23 (22.3%) of the 103 known oxacillin-resistant isolates were correctly reported. For the known oxacillin-resistant isolates, 65 received AMS reports at 3 to 4 h, with only 9% being correct, whereas 38 were reported at 5 to 6 h, with 47% being correct. The reliability of AMS gentamicin susceptibility results was evaluated by testing the 198 S. aureus isolates in parallel with MIC-2000 broth dilution tests. AMS gentamicin susceptibility results were found to be reliable and essentially identical to MIC-2000 results. The possibility of improving AMS oxacillin resistance detection by using gentamicin resistance as a linked screening marker for oxacillin resistance was evaluated with data from the parallel AMS and MIC-2000 gentamicin susceptibility tests and from data accrued on recent clinical laboratory isolates. By these two approaches, respective sensitivities of 97 and 99.8%, and specificity of 72%, were found for detection of oxacillin-resistant isolates by using gentamicin resistance as a marker.     

Comparison of the autoSCAN-W/A rapid bacterial identification system and the Vitek AutoMicrobic system for identification of gram-negative bacilli

The autoSCAN-W/A (W/A; Baxter MicroScan, West Sacramento, Calif.) with the new fluorometric Rapid Neg Combo 1 (RNC) panel is a fully automated fluorometric system for identification of both enteric and nonenteric gram-negative bacilli within 2 h. We compared the W/A with the Vitek AutoMicrobic System (Vitek AMS; Vitek Systems, Inc., Hazelwood, Mo.) for identification of 383 clinical isolates of gram-negative bacilli. The API 20E (Analytab Products, Plainview, N.Y.) and conventional biochemical testing were used as the reference systems. The W/A correctly identified 336 isolates (87.7%) to the species level and classified an additional 29 isolates (7.6%) as correct with low probability (overall identification = 95.3%); the Vitek AMS correctly identified 355 isolates (92.7%) to the species level and classified an additional 8 isolates (2.1%) as correct with low probability (overall identification = 94.8%). A common set of 134 isolates of gram-negative bacilli was tested in both participating laboratories as a means of assessing interlaboratory agreement with both the W/A and the Vitek AMS. The overall agreements between the two laboratories were 86% with the W/A and 92% with the Vitek AMS. The W/A performed comparably to the Vitek AMS for identification of most gram-negative bacilli, actually exceeding the Vitek AMS for identification of nonenteric bacilli. Rapid time to identification and a high level of automation make the W/A an attractive system for clinical microbiology laboratories.     

Rapid species identification and biotyping of respiratory isolates of Haemophilus spp

Three commercially available systems, the 4-h Minitek Enterobacteriaceae III, the Haemophilus Trio-Tube, and the Micro-ID, were evaluated for their capacities to identify and biotype 308 respiratory isolates of Haemophilus spp. When compared with aminolevulinic acid test results, the definitive identification method used in this study, these systems demonstrated no significant differences in their capacities to differentiate Haemophilus influenzae from Haemophilus parainfluenzae. They were in agreement with the standard method of species identification approximately 50% of the time. When sucrose was added to the Minitek and Trio-Tube configurations, the efficiency rate of species identification increased to more than 95%. The Micro-ID could not be modified to incorporate this additional biochemical parameter. The performance of the sucrose-supplemented Minitek and Trio-Tube systems, compared to the combined results of Micro-ID and aminolevulinic acid, produced correlations of 94 and 90%, respectively. Rapid and accurate methodologies are available for combined species identification and biotyping of Haemophilus spp.     

The Automicrobic System for urines.

An evaluation of the Automicrobic System (AMS) for Urines (Vitek Systems, Inc.) was carried out under the auspices of the Product Evaluation Committee of the College of American Pathologists from the period June 1977 through October 1978. Data generated during this evaluation indicated that, when comparing the AMS methodology to our clinical microbiology laboratory methodology, a 37% time saving could be realized by utilizing the AMS. Quantitation with the AMS showed a 99% correlation with the clinical microbiology laboratory method except for yeast which correlated only 50% of the time. The average overall identification accuracy was 95%. Negative response accuracy was 99%. Other members of the Enterobacteriaceae which the instrument is not designed to identify may produce erroneous results if they occur in urine specimens. Specimens containing two organisms were identified with a 94% correlation when compared to our conventional methodology. The time when a well becomes positive may be used as a fairly reliable indicator of significance (count greater than 70,000 colony-forming units per ml) for Escherichia coli, Klebsiella-Enterobacter group, and group D Enterococcus, but not for Proteus sp., Pseudomonas aeruginosa, and yeast. Specimen collection must be performed properly since specimens considered as contaminated by conventional plating-out techniques may be reported out by the AMS as only one or two organisms and thus lead to an erroneous assumption as to significance. Cost per specimen was $1.83 more by utilizing the AMS method as compared to our conventional method. This is offset by a saving of 1.74 h daily of personnel time and a final report in 13 h. At least 30 urine specimens would be needed daily to pay for the instrument and specimen costs in 1 year. The AMS can provide significant aid to a clinical microbiology laboratory when all factors are considered.     

Clinical laboratory evaluation of the Auto-Microbic system for rapid identification of Enterobacteriaceae.

The capability of the Auto-Microbic system (Vitek Systems, Inc., Hazelwood, Mo.) has been expanded to identify members of the family Enterobacteriaceae with the use of a sealed, disposable accessory card (the Enterobacteriaceae Biochemical Card) containing 26 biochemical tests. To judge the accuracy of the AutoMicrobic system's identification in a hospital laboratory, 933 Enterobacteriaceae isolates were studied. The AutoMicrobic system provided the correct identification for 905 of the isolates (97%). For Enterobacteriaceae excluding Escherichia coli (533 isolates not E. coli), 506 isolates were correctly identified (94.9%). The AutoMicrobic system integrates well into the clinical microbiology laboratory, providing a simple, accurate, and efficient automated system for the rapid identification of Enterobacteriaceae.     

Rapid automated identification of gram-negative bacilli from blood cultures with the AutoMicrobic system.

Automated identification of gram-negative bacilli directly from blood culture bottles by using the AutoMicrobic System (AMS) was evaluated with a modified procedure for the AMS Enterobacteriaceae-plus nonfermenter identification card. A total of 150 strains were tested (44 clinical and 106 seeded) and compared with a conventional identification procedure. These strains included 107 Enterobacteriaceae and 43 oxidase-positive or glucose-nonfermenting, or both, organisms. AMS identifications on one of these strains were not interpretable owing to equal probability AMS identification values. Of the remaining 149 strains, 138 (92.6%) were correctly identified within 8 to 13 h of the first reading. Of 69 identifications analyzed after 6 h of incubation, 91% were correct. This procedure was found to be rapid, convenient, and nonlabor intensive and is recommended for presumptive identification of gram-negative bacilli in blood cultures.     

Evaluation of the AutoMicrobic system for identification and susceptibility testing of gram-negative bacilli

The AutoMicrobic system (AMS) (Vitek Systems, Inc., Hazelwood, Mo.) was compared with the API-20E system for the identification of gram-negative bacilli by using 380 stock clinical isolates and 377 immediately encountered fresh clinical isolates. For the stock isolates, with Enterobacteriaceae-Plus Biochemical Cards and automated interpretation, 364 (95.8%) were in agreement to the species level. For the fresh clinical isolates, agreement at the genus and species levels was 89.7 and 85.9%, respectively, when Enterobacteriaceae-Plus Cards were interpreted by the AMS. Manual interpretation of Enterobacteriaceae-Plus Biochemical Cards improved species level agreement to 91.0%. Subsequent retesting of all discrepant isolates with the Gram-Negative Identification Card resulted in significant improvement of results, and for the stock and fresh clinical isolates, species level agreement was 98.7 and 97.3%, respectively. AMS susceptibility testing was evaluated by comparing ampicillin and cephalothin MICs determined in parallel by AMS and a reference broth microdilution test for stock isolates, and by comparison of AMS and standardized disk agar diffusion test results for fresh clinical isolates. For the stock isolates, AMS mean integer MICs approximated microdilution mean integer MICs with AMS, providing excellent MIC replicability. For ampicillin and cephalothin, 50 and 46.8%, respectively, of AMS integer MICs were within +/- 1 microgram/ml of the reference values, and 89.3 and 63.1% of AMS integer MICs were within +/- 2 micrograms/ml of the reference values. For the fresh clinical isolates, AMS and reference results were in disagreement for 4.5% of the antimicrobial agents tested, with 2.3% as a combination of "major" and "very major" errors.