Clinical Evaluation of the MICRO-ID, API 20E, and Conventional Media Systems for Identification of Enterobacteriaceae

MICRO-ID (General Diagnostics, Morris Plains, N.J.) is a new kit system designed for the identification of Enterobacteriaceae in 4 h. It consists of 15 biochemical tests of paper disks. Each test is in its own compartment in a molded plastic tray. Only one reagent need be added to the system (2 drops of 20% KOH, which is added to the Voges-Proskauer test). Based on the pattern of positive and negative biochemical test results, a five-digit octal code number is calculated. An identification is derived from a computer-generated identification manual. A study was conducted to compare three systems-the MICRO-ID 4-h and the API 20E (Analytab Products Inc., Plainview, N.Y.) 18- to 24-h systems and a conventional media system-to measure the ability of each to identify members of the family Enterobacteriaceae. Comparison tables, rather than simple percentage agreement tables, were generated to define the particular strengths and weaknesses of each system and allow the laboratory to best use the data. The MICRO-ID compared quite favorably with conventional media. MICRO-ID yielded incorrect identifications with 1.5% of the isolates tested (API 20E, 4.7% misidentification rate). Half the MICRO-ID misidentifications occurred when the system identified a Citrobacter diversus as a lysine-negative Escherichia coli; all gave one octal number. A direct comparison of the MICRO-ID and API 20E was of limited value because percentage agreements were merely the sums of the errors of each. The ease of inoculation, the requirement for the addition of only one reagent, and the 4-h capability make the MICRO-ID system an extremely attractive development in the field of bacterial identification.     

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.     

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 with API 20E for identification ofSalmonella,Shigella andYersinia species

Micro-ID is a new kit system for the identification ofEnterobacteriaceae in 4 h. It consists of 15 biochemical tests with paper discs. A study was conducted to compare the accuracy of the Micro-ID system with that of the API 20E system for identification ofSalmonella, Shigella andYersinia species. Both systems correctly identified 57 out of 59 (97%)Shigella strains. Micro-ID correctly identified twoSalmonella typhi strains, whereas API 20E incorrectly identified one. Of 138Salmonella strains of other serotypes, 135 (98%) were correctly identified by Micro-ID and 130 (94 %) by API 20E. The respective figures for theYersinia strains were 40 out of 45 (89 %) and 30 out of 45 (66 %). It is concluded that Micro-ID is at least equal to API 20E in its ability to identifySalmonella, Shigella andYersinia species.     

Identification of Enterobacteriaceae by the API 20E system.

Since the introduction of the API 20E kit a number of identification schemes have been developed by the manufacturer for use with the kit. We evaluated the success of these various schemes in identifying 206 strains belonging to 34 taxa of the family Enterobacteriaceae. Many of the strains were atypical and only 94% could be identified by our own system of 50 conventional tests and a computer program. The most advanced identification scheme so far developed for the API 20E kit (the Analytical Profile Index and complementary Computer Service) allowed 88% of the 206 strains to be correctly identified, although 2% were incorrectly identified. The tests in the API 20E kit and 52 conventional tests were separately evaluated for their ability to discriminate between the 34 taxa considered in this study. Our results suggest that replacing some of the tests in the present API 20E kit might further improve its diagnostic performance.     

Evaluation of the Micro-ID,the API 20E and the Rapid 20E for same-day identification ofEnterobacteriaceae

European Journal of Clinical Microbiology & Infectious Diseases - In a comparative study of three methods for same-day identification, the Rapid 20E identified 91.8 % of 328 clinically...     

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.     

Comparison of identification of Enterobacteriaceae by API 20E and Sensititre Autoidentification System.

Of 251 isolates of the Enterobacteriaceae identified to species level by API 20E, 208 (83%) were similarily identified by the Sensititre Autoidentification System. Both systems shared a common problem in that discrimination between species of the genera Klebsiella, Enterobacter, and Serratia was poor. The eight digit biocode generated by the Sensititre system for individual isolates is not reproducible and therefore not of epidemiological value.     

Comparison of Enteric-Tek with API 20E and conventional methods for identification of Enterobacteriaceae.

Enteric-Tek (Flow Laboratories, Inc., Roslyn, N.Y.) is a new system designed for the identification of Enterobacteriaceae and consists of a round, multicompartmented plastic plate with 11 peripheral wells and 1 center well incorporating 14 biochemical reactions. Only one reagent (Kovacs reagent) is required to complete the biochemical test results for generation of a five-digit number for computer code identification. The identification accuracy of the Enteric-Tek system was compared with those obtained by conventional methods and API 20E. The Enteric-Tek system was found to be not only a convenient and simple method for rapid identification of Enterobacteriaceae, but also to be highly reliable, giving excellent identification performance as compared to API 20E and conventional methods.     

Rapid Identification of Enterobacteriaceae by Using Noncommercial Micro-Tests in Conjunction with API 20E Profile Data

A total of 371 (97%) of 382 strains of Enterobacteriaceae representing 26 species were identified cheaply, accurately, and rapidly by using a test system based upon standard 96-well microdilution plates and the API 20E profile data.     

Correlation studies of entero-set 20, API 20E and conventional media systems for Enterobacteriaceae identification.

The Entero-Set kit (Fisher Diagnostics) is a 20-biochemical-test system used in the identification of members of the Enterobacteriaceae. This kit was compared with the API 20E (Analytab Products) and conventional media systems, using 505 (303 stock and 202 clinical) strains of Enterobacteriaceae. When the Entero-Set and API 20E results were compared with those of the conventional media system, the Entero-Set performed as well as the API 20E in overall identification. Comparison of common biochemical tests among the various systems showed that citrate, arabinose, adonitol, inositol, and malonate gave correlations below 90%. The majority of the discrepancies were found among stock cultures. In addition, most discrepancies occurred with species of Enterobacter, Salmonella, Proteus, Klebsiella, and Serratia. Reproducibility studies showed the Entero-Set system to perform with a high degree of accuracy and reproducibility.     

Identification of the Enterobacteriaceae: a comparison of the Enterotube II with the API 20E.

Two commercial methods for the identification of the Enterobacteriaceae, API 20E and Enterotube II, were compared using the results obtained with 235 cultures of fresh clinical isolates. Enterotube II was an improvement on the original Enterotube, but the availability of two differing indices, one using the Voges-Proskauer (VP) test result and one without, is criticised.     

Use of the API 20E system to identify veterinary Enterobacteriaceae.

A total of 503 veterinary enteric bacterial pathogens obtained from state veterinary diagnostic laboratories were tested on API 20E strips to determine whether this rapid microidentification system could be utilized for veterinary clinical microbiology. The API 20E strip accurately identified 96% of the veterinary isolates and misidentified 3%. Identifications by the API system and the diagnostic laboratories were in agreement in 85% of the isolates, disagreement on 16% of the isolates, and 1% were not identified by the API strip. Differences in identification occurred primarily in distinguishing between Klebsiella and Enterobacter and between Enterobacter and Escherichia coli. These disagreements were most often due to incorrect identifications by the diagnostic laboratory rather than by the API system. Biotype differences between human and veterinary isolates were compared. Significant differences were noted in several biochemical reactions. The main differences observed for E. coli isolates were in ornithine decarboxylase production and melibiose fermentation. The largest differences for Salmonella occurred in arginine dihydrolase production, citrate utilization, and inositol fermentation, whereas for Klebsiella pneumoniae the main differences were noted in urease production and nitrate reduction. These biotype differences, however, did not affect the accurate identification of organisms on the API strip.     

Comparison of API 20E, API rapid E, and API rapid NFT for identification of members of the family Vibrionaceae.

Sixty isolates, from nine species of the family Vibrionaceae, were tested by the API 20E, API Rapid E, and API Rapid NFT systems. Results were compared with those obtained with standard biochemicals. Included were 7 Aeromonas caviae isolates, 27 Aeromonas hydrophilia isolates, 10 Aeromonas sobria isolates, 3 Plesiomonas shigelloides isolates, 3 Vibrio alginolyticus isolates, 3 Vibrio cholerae isolates, 1 Vibrio fluvialis isolate, 5 Vibrio parahaemolyticus isolates, and 1 Vibrio vulnificus isolate. The API 20E correctly identified all the isolates within 24 h. The API Rapid E correctly identified only 77%, misidentified 8%, and failed to identify 2% of the isolates in 4 h. The remaining 13% of the isolates gave a low selectivity identification, with one of the choices being correct. The API Rapid NFT correctly identified 87%, misidentified 5%, gave a low selectivity identification for 8% of the isolates, and in some instances, required up to 48 h of incubation. The API 20E is a valid system for use in the identification of the more commonly occurring members of the family Vibrionaceae and the most accurate and efficient of the three systems tested.     

Comparison of the API rapid E four-hour system with the API 20E overnight system for the identification of routine clinical isolates of the family Enterobacteriaceae.

Four hundred forty-one clinical isolates of the family Enterobacteriaceae were identified in parallel by using the API Rapid E 4-h and the API 20E overnight procedures (Analytab Products, Plainview, N.Y.). The results obtained by using the API Rapid E were compared with those obtained by using the API 20E. Discrepancies were resolved by using standard biochemicals. The API 20E identified 98.9% (436 of 441) of the isolates without the use of additional biochemicals and was found to be correct in each case of a discrepancy among the 436 isolates. The API Rapid E gave the same identification as the API 20E for 94.0% (410 of 436) of the isolates, misidentified 3.0% (13 of 436), and gave a correct but low-selectivity answer for the remaining 3.0% (13 of 436). The API Rapid E is a suitable alternative for the rapid identification of the Enterobacteriaceae.     

Identification of Yersinia species by the API 20E.

A prospective study was performed to assess the effectiveness of the API 20E in the identification of 183 Yersinia isolates incubated at 28 degrees C for 18 to 24 h. The results showed an overall correct-identification rate of 90%, with positive predictive values for Yersinia enterocolitica and Yersinia frederiksenii of 94 and 92%, respectively. Yersinia intermedia results were unacceptable.     

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.     

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.     

Identification of Yersinia spp. with the API 20E system.

The ability of the API 20E system to identify 105 clinical isolates of Yersinia spp. was compared with those of conventional biochemical tests at 28 and 37 degrees C. Elimination of the Voges-Proskauer test (recorded as a negative result) increased the percentage of correct identifications for Yersinia spp. from 66 to 93% when the API 20E strips were incubated at 28 degrees C.