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

Enzymatic substrates in microbiology

Enzymatic substrates are powerful tools in biochemistry. They are widely used in microbiology to study metabolic pathways, to monitor metabolism and to detect, enumerate and identify microorganisms. Synthetic enzymatic substrates have been customized for various microbial assays, to detect an expanding range of both new enzymatic activities and target microorganisms. Recent developments in synthetic enzymatic substrates with new spectral, chemical and biochemical properties allow improved detection, enumeration and identification of food-borne microorganisms, clinical pathogens and multi-resistant bacteria in various sample types. In the past 20 years, the range of synthetic enzymatic substrates used in microbiology has been markedly extended supporting the development of new multi-test systems (e.g., Microscan, Vitek 2, Phoenix) and chromogenic culture media. The use of such substrates enables an improvement in time to detection and specificity over conventional tests that employ natural substrates. In the era of intense developments in molecular biology, phenotypic tests involving enzymatic substrates remain useful to analyse both simple and complex samples. Such tests are applicable to diagnostic and research laboratories all over the world.

An evaluation of manual and mechanical methods to identify Candida spp. from human and animal sources

To compare two yeast identification methods, i. e, the manual and the VITEK mechanical methods, 62 clinical samples from hemocultures and animal sources were analyzed. After identification as Candida yeasts by the VITEK method, the strains were recharacterized using manual assimilation methods and sugar fermentation tests. Our findings reveal 58% concurrent identification between the two methods for animal strains, and 51% for human hemoculture strains.

Evaluation of the BD Phoenix SMIC/ID, a new streptococci identification and antimicrobial susceptibility panel, for potential routine use in a university-based clinical microbiology laboratory

We evaluated the new BD Phoenix automated microbiology system (Becton Dickinson Diagnostic Systems, Sparks, MD) SMIC/ID-4 panel for routine identification (ID) and antimicrobial susceptibility testing (AST) of streptococci in a university-based laboratory. Clinical isolates of Streptococcus pneumoniae (n = 92), Streptococcus pyogenes (n = 24), and Streptococcus agalactiae (n = 10) were collected, and comparisons were made with the routine manual methods used in our microbiology laboratory for ID and susceptibility testing. ID concordance with manual methods was 85.9%, 95.8%, and 90.0% for S. pneumoniae, S. pyogenes, and S. agalactiae, respectively. With respect to AST concordance for S. pneumoniae and beta-hemolytic streptococci (S. pyogenes and S. agalactiae) using Phoenix and standard broth microdilution panels, overall essential agreement was 93.0% and 97.5%, respectively, whereas overall category agreement was 92.4% and 98.9%, respectively. Major and minor error rates for S. pneumoniae and beta-hemolytic streptococci were 0.5% and 0.3%, and 7.1% and 0.8%, respectively. Very major errors were not observed in this study. Mean time for ID and AST test completion was 13.6 +/- 1.6, 10.7 +/- 2.4, and 11.2 +/- 2.3 h for S. pneumoniae, S. pyogenes, and S. agalactiae, respectively. We have demonstrated that Phoenix ID results show high agreement with manual ID and that AST performance was equivalent to standard broth microdilution in less time.

Evaluation of the automated phoenix system for potential routine use in the clinical microbiology laboratory

A comparative study was designed to evaluate the identification (ID) and antimicrobial susceptibility testing (AST) performances of the BD Phoenix Automated Microbiology System (Becton Dickinson Diagnostic Systems [BD], Pont de Claix, France). A total of 305 single clinical isolates were collected, and comparisons were made with routine manual methods in use in our microbiology laboratories. The percentages of correct IDs were 93.3, 89.4, 91.8, and 85.7% for enterobacteria, nonfermenting gram-negative bacilli, staphylococci, and streptococci-enterococci, respectively. The median ID time was 3 h, and the median time for AST was 10 h 30 min. AST results showed variable percentages of errors for the different antibiotics. None of the enterobacteria and 0.3% of Pseudomonas aeruginosa isolates showed a very major error (VME). Only one strain of Staphylococcus aureus showed a VME with oxacillin. We demonstrate here the efficiency of the Phoenix system, which can be used for the majority of strains encountered in a university-based laboratory, for ID and AST.

Particular biochemical profiles for enterohemorrhagic Escherichia coli O157:H7 isolates on the ID 32E system

The ability of the ID 32E system to identify and discriminate 74 Escherichia coli O157 isolates among 106 E. coli non-O157 isolates was evaluated. The results showed atypical biochemical reactions but accurate identification at the species level and no unique biochemical profile numbers for E. coli O157, although these numbers were distinct from those of other serotypes.

Evaluation of autoSCAN-W/A and the Vitek GNI+ AutoMicrobic system for identification of non-glucose-fermenting gram-negative bacilli

The autoSCAN-W/A (W/A; Dade Behring Microscan Inc., West Sacramento, Calif.) and Vitek AutoMicrobic System (Vitek AMS; bioMérieux Vitek Systems, Inc., Hazelwood, Mo.) are both fully automated microbiology systems. We evaluated the accuracy of these two systems in identifying nonglucose-fermenting gram-negative bacilli. We used the W/A with conventional-panel Neg Combo type 12 and Vitek GNI+ identification systems. A total of 301 isolates from 25 different species were tested. Of these, 299 isolates were identified in the databases of both systems. The conventional biochemical methods were used for reference. The W/A correctly identified 215 isolates (71. 4%) to the species level at initial testing with a high probability of >/=85%. The Vitek GNI+ correctly identified 216 isolates (71.8%) to the species level at initial testing with a high probability of >/=90%. After additional testing that was recommended by the manufacturer's protocol, the correct identifications of the W/A and Vitek GNI+ improved to 96.0 and 92.3%, respectively. The major misidentified species were Sphingomonas paucimobilis and Agrobacterium radiobacter in the W/A system and Acinetobacter lwoffii, Chryseobacterium indologenes, and Comamonas acidovorans in the Vitek GNI+ system. The error rates were 4.0 and 7.6%, respectively. The overall accuracy for both systems was above 90% if the supplemental tests were applied. There was no significant difference in accuracy (P > 0.05) between the two systems.

Evaluation of accuracy and repeatability of identification of food-borne pathogens by automated bacterial identification systems

The performances of five automated microbial identification systems, relative to that of a reference identification system, for their ability to accurately and repeatedly identify six common food-borne pathogens were assessed. The systems assessed were the MicroLog system (Biolog Inc., Hayward, Calif.), the Microbial Identification System (MIS; MIDI Inc., Newark, Del.), the VITEK system (bioMérieux Vitek, Hazelwood, Mo.), the MicroScan WalkAway 40 system (Dade-MicroScan International, West Sacramento, Calif.), and the Replianalyzer system (Oxoid Inc., Nepean, Ontario, Canada). The sensitivities and specificities of these systems for the identification of food-borne isolates of Bacillus cereus, Campylobacter jejuni, Listeria monocytogenes, Staphylococcus aureus, Salmonella spp., and verotoxigenic Escherichia coli were determined with 40 reference positive isolates and 40 reference negative isolates for each pathogen. The sensitivities of these systems for the identification of these pathogens ranged from 42.5 to 100%, and the specificities of these systems for the identification of these pathogens ranged from 32.5 to 100%. Some of the systems had difficulty correctly identifying the reference isolates when the results were compared to those from the reference identification tests. The sensitivity of MIS for the identification of S. aureus, B. cereus, E. coli, and C. jejuni, for example, ranged from 47.5 to 72. 5%. The sensitivity of the Microlog system for the identification of E. coli was 72.5%, and the sensitivity of the VITEK system for the identification of B. cereus was 42.5%. The specificities of four of the five systems for the identification of all of the species tested with the available databases were greater than or equal to 97.5%; the exception was MIS for the identification of C. jejuni, which displayed a specificity of 32.5% when it was tested with reference negative isolates including Campylobacter coli and other Campylobacter species. All systems had >80% sensitivities for the identification of Salmonella species and Listeria species at the genus level. The repeatability of these systems for the identification of test isolates ranged from 30 to 100%. Not all systems included all six pathogens in their databases; thus, some species could not be tested with all systems. The choice of automated microbial identification system for the identification of a food-borne pathogen would depend on the availability of identification libraries within the systems and the performance of the systems for the identification of the pathogen.

Direct bacterial identification from positive BacT/Alert blood cultures using MicroScan overnight and rapid panels

Studies were conducted on a method of direct inoculation of MicroScan overnight and rapid panels from positive BacT/Alert blood culture bottles containing standard aerobic media to determine the correlation with inoculation of the corresponding panels with a standardized bacterial suspension obtained following subculture to agar. For Gram-negative organisms, 122 of 127 (96%) overnight panels and 85 of 118 (72%) rapid panels showed complete agreement with the standard method for species identification. Highest concordance (99%) occurred with Enterobacteriaceae inoculated directly into overnight panels. For Gram-positive organisms, 70 of 85 (82%) overnight panels and 45 of 86 (52%) rapid panels showed complete agreement. These findings suggest that direct inoculation of Gram-negative overnight MicroScan panels yields results most comparable to standard methods when Enterobacteriaceae are detected and allows reporting of results 18 to 24 h sooner. Direct inoculation of Gram-positive overnight or rapid panels and Gram-negative rapid panels from this blood culture medium did not yield acceptable identification results and is not recommended.

Evaluation of the VITEK 2 system for rapid identification of medically relevant gram-negative rods

The new VITEK 2 system (bioMérieux) was evaluated at two independent sites with the identification card for gram-negative bacilli (ID-GNB card). Of the 845 strains tested, which represented 70 different taxa belonging to either the family Enterobacteriaceae or the nonenteric bacilli, 716 (84.7%) were correctly identified at the species level. Thirty-two (3.8%) additional strains were identified to the species level after the performance of simple, rapid manual tests (oxidase, hemolysis, indole reaction, motility, and pigmentation). For 80 (9.5%) strains, these additional tests did not lead to an identification at the species level but the correct species identification was given among the organisms listed. Only 7 (0.8%) strains were misidentified, and 10 (1.2%) were not identified. Mistakes were randomly distributed over different taxa. Due to the new, more sensitive fluorescence-based technology of the VITEK 2 system, final results were available after 3 h. Since our evaluation was mainly a stress test, it is predicted that the VITEK 2 system in conjunction with the ID-GNB card would perform well under conditions of a routine clinical laboratory in identifying members of the family Enterobacteriaceae and selected species of nonenteric bacteria. This system is a promising, highly automated new tool for the rapid identification of gram-negative bacilli from human clinical specimens.

Microbiological and clinical aspects of infection associated with Stenotrophomonas maltophilia

The gram-negative bacterium Stenotrophomonas maltophilia is increasingly recognized as an important cause of nosocomial infection. Infection occurs principally, but not exclusively, in debilitated and immunosuppressed individuals. Management of S. maltophilia-associated infection is problematic because many strains of the bacterium manifest resistance to multiple antibiotics. These difficulties are compounded by methodological problems in in vitro susceptibility testing for which there are, as yet, no formal guidelines. Despite its acknowledged importance as a nosocomial pathogen, little is known of the epidemiology of S. maltophilia, and although it is considered an environmental bacterium, its sources and reservoirs are often not readily apparent. Molecular typing systems may contribute to our knowledge of the epidemiology of S. maltophilia infection, thus allowing the development of strategies to interrupt the transmission of the bacterium in the hospital setting. Even less is known of pathogenic mechanisms and putative virulence factors involved in the natural history of S. maltophilia infection and this, coupled with difficulties in distinguishing colonization from true infection, has fostered the view that the bacterium is essentially nonpathogenic. This article aims to review the current taxonomic status of S. maltophilia, and it discusses the laboratory identification of the bacterium. The epidemiology of the organism is considered with particular reference to nosocomial outbreaks, several of which have been investigated by molecular typing techniques. Risk factors for acquisition of the bacterium are also reviewed, and the ever-expanding spectrum of clinical syndromes associated with S. maltophilia is surveyed. Antimicrobial resistance mechanisms, pitfalls in in vitro susceptibility testing, and therapy of S. maltophilia infections are also discussed.

Multicenter evaluation of the MicroScan Rapid Gram-Negative Identification Type 3 Panel

The accuracy and performance of the revised MicroScan Rapid Gram-Negative Identification Type 3 Panel (Dade MicroScan Inc., West Sacramento, Calif.) were examined in a multicenter evaluation. The revised panel database includes data for 119 taxa covering a total of 150 species, with data for 12 new species added. Testing was performed in three phases: the efficacy, challenge, and reproducibility testing phases. A total of 405 fresh and stock gram-negative isolates comprising 54 species were tested in the efficacy phase; 96.8% of these species were identified correctly in comparison to the identification obtained either with the API 20E system (bioMérieux Vitek, Hazelwood, Mo.) or by the conventional tube method. The number of correctly identified isolates in the challenge phase, including new species added to the database, was 221 of 247, or 89.5%, in comparison to the number correctly identified by the conventional tube method. A total of 465 isolates were examined for intra- and interlaboratory identification reproducibility and gave an agreement of 464 of 465, or 99.8%. The overall reproducibility of each individual identification test or substrate was 14,373 of 14,384, or 99.9%. The new Rapid Gram-Negative Identification Type 3 Panel gave accurate and highly reproducible results in this multiple-laboratory evaluation.

Accuracy of four commercial systems for identification of Burkholderia cepacia and other gram-negative nonfermenting bacilli recovered from patients with cystic fibrosis

Burkholderia cepacia has recently been recognized as an important pathogen in chronic lung disease in patients with cystic fibrosis (CF). Because of the social, psychological, and medical implications of the isolation of B. cepacia from CF patients, accurate identification of this organism is essential. We compared the accuracies of four commercial systems developed for the identification of nonfermenting, gram-negative bacilli with that of conventional biochemical testing for 150 nonfermenters including 58 isolates of B. cepacia recovered from respiratory secretions from CF patients. The accuracies of the four systems for identifying all nonfermenters ranged from 57 to 80%, with the RapID NF Plus system being most accurate. The accuracies of these systems for identifying B. cepacia ranged from 43 to 86%, with the Remel system being most accurate. Depending on the commercial system, from two to seven isolates were misidentified as B. cepacia. The relatively poor performance of the commercial systems requires that identification of certain nonfermenters be confirmed by conventional biochemical testing. These organisms include B. cepacia, Burkholderia sp. other than B. cepacia, and infrequently encountered environmental species (Pseudomonas and Flavobacterium species). In addition, conventional biochemical testing should be done if a commercial system fails to assign an identification to an organism. Confirmatory testing should preferably be performed by a reference laboratory with experience in working organisms isolated from CF patients.

A comparative study of the BBL crystal enteric/nonfermenter identification system and the biomerieux API20E and API20NE identification systems after overnight incubation

The BBL Crystal Enteric/Nonfermenter (Crystal, Becton Dickinson Microbiology Systems) is a new multi-test identification system for Gram negative rods requiring no oil overlay or addition of reagents. One hundred and three selected Gram negative rod isolates from routine clinical specimens were tested in parallel using the appropriate API20E or API20NE (BioMerieux) reading after overnight incubation. The isolates included in the study, and the number tested, were as follows: Acinetobacter sp, 8; Aeromonas hydrophila, 1; Citrobacter diversus, 1; Citrobacter freundii, 1; Escherichia coli, 13; Enterobacter aerogenes, 10; Enterobacter agglomerans, 2; Enterobacter cloacae, 6; Klebsiella oxytoca, 6; Klebsiella pneumoniae, 12; Morganella morganii, 2; Proteus mirabilis, 6; Pseudomonas aeruginosa, 21; Salmonella sp, 2; Salmonella typhi, 1; Serratia marcescens, 1; Shigella sonnei, 2; Shigella sp, 2; Vibrio metschnikovii, 1; Xanthomonas maltophilia, 3; and, Yersinia enterocolitica, 2. The API20E and API20NE systems combined identified 74.8% (77/103) and the Crystal 97.1% (100/103) of isolates. Twenty six isolates required repeat and/or additional tests for correct identification with the API systems compared to 3 with the Crystal. Crystal is more convenient than API for routine clinical use because it requires fewer repeat and/or extra tests, and is easier and less time consuming to use.

Comparison of Crystal Enteric/Nonfermenter system, API 20E system, and Vitek AutoMicrobic system for identification of gram-negative bacilli

A comparative evaluation of the Crystal Enteric/Nonfermenter system (Crystal; Becton Dickinson, Cockeysville, Md.), API 20E (API; bioMérieux Vitek, Inc., Hazelwood, Mo.), and the Vitek GNI card (Vitek; bioMérieux Vitek) was performed with 512 clinical isolates of gram-negative bacilli, including 381 members of the family Enterobacteriaceae and 131 nonenteric bacilli. With supplemental testing, API, Crystal, and Vitek correctly identified to the genus and species level 505 (98.6%), 489 (95.5%), and 494 (96.5%) of the 512 isolates, respectively. Supplemental testing, as specified by the manufacturer, was required to identify 119 (23.2%), 18 (3.5%), and 5 (1.0%) of the isolates with the three systems, respectively. Of the 381 isolates from the family Enterobacteriaceae, API and Crystal correctly identified 90.3 and 91.6% by 18 to 24 h without supplemental testing, respectively, and Vitek identified 92.4 and 96.1% following 10 and 18 h of incubation, respectively. Of the 131 nonenteric organisms, API and Crystal correctly identified 28.2 and 93.9% by 18 to 24 h without supplemental testing, respectively, and Vitek identified 84.0% by 10 h and 93.9% by 18 h. Errors in identification with each system were infrequent and appeared to be randomly distributed among the genera evaluated. The three systems were comparable in accuracy when either a weighted clinical laboratory profile of organisms or a group of selected isolates in a stress test sample was evaluated (P > 0.05). There were no significant differences between the three systems in their ability to identify either the isolates in the weighted group or those in the stress test (P > 0.05). Crystal compared favorably with API and Vitek, which have established track records in clinical laboratories, and is acceptable for the identification of members of the Enterobacteriaceae and nonenteric bacilli in a clinical microbiology laboratory.

Evaluation of RapID onE system for identification of 379 strains in the family Enterobacteriaceae and oxidase-negative, gram-negative nonfermenters

The ability of the RapID onE system (Innovative Diagnostic Systems, Inc., Norcross, Ga.) to identify 364 strains in the family Enterobacteriaceae and 15 oxidase-negative, gram-negative, nonfermentative rods was evaluated. Kits were inoculated with no. 2 McFarland standard suspensions, and reactions were interpreted after 4 h of incubation at 35 degrees C. Overall, the method correctly identified (to the species level or to the genus level for salmonellas and non-Shigella sonnei Shigella species) 363 strains (95.8%) without additional tests. For four strains (1.0%), additional tests were required to delineate the correct identification from a range of two or more possibilities; these included one Serratia liquefaciens (Serratia marcescens or Serratia liquefaciens), one Serratia rubidaea (Serratia rubidaea or Serratia odorifera), one Salmonella typhi (Leminorella richardii or Salmonella sp.) and one Yersinia enterocolitica (Yersinia frederiksenii, Yersinia intermedia, or Yersinia enterocolitica). Twelve strains (3.2%) were misidentified or yielded codes with no identification; these comprised one Citrobacter amalonaticus (no identification), three Enterobacter hormaechei (not in the RapID onE database; two Enterobacter amnigenus, one Enterobacter sp.), one Serratia liquefaciens (Enterobacter cloacae), one Serratia rubidaea (no identification), four Serratia fonticola (not in RapID onE database; two Enterobacter aerogenes, one Serratia marcescens, one not identified), one Proteus mirabilis (Proteus penneri), and one Proteus vulgaris (Providencia rustigianii). If the seven strains not included in the database had been excluded, correct identification rates would have risen to 97.6% without additional tests and 98.7% with additional tests, with misidentification rates dropping to 1.3%. The RapID onE system is easy to set up and the results are easy to read, and the system provides an accurate, nonautomated commercially available method for the same-day identification of members of the family Enterobacteriaceae and oxidase-negative, gram-negative nonfermenters.

Comparison of Vitek and Cobas Micro systems with a semiautomated conventional microsystem for identification and susceptibility testing of gram negative bacilli

AIMS

To compare the sensitivity and specificity of two semiautomated systems against a conventional (MIC 2000) test system for the identification and antibiotic susceptibility of Gram negative bacteria.

METHODS

Clinical isolates of Gram negative bacilli (188 urinary and 229 non-urinary strains) were identified and tested for antibiotic susceptibility in the Cobas Micro and MIC 2000 systems. Of these, 359 strains were then tested in the Vitek and MIC 2000 systems. Two hundred and forty three strains were tested in all three systems immediately after isolation. Forty three were also tested only in the Vitek and MIC 2000 systems immediately after isolation. The remaining 174 strains were tested after storage at -20 degrees C for several months.

RESULTS

The Cobas Micro and MIC 2000 systems agreed on the identification of 310 of the 417 (74.3%) strains; the Vitek and MIC 2000 systems agreed on 338 of the 359 (94.2%) strains. The Cobas Micro system correctly identified 86.8% of strains tested after storage and 65.4% of those immediately after isolation. Organism-antibiotic combinations (non-urinary isolates) were tested in the Cobas Micro and MIC 2000 systems (n = 2335), in the Vitek and MIC 2000 systems (n = 999). Essential correlation (complete agreement plus minor errors) was observed in 98% (with 90% complete agreement) in the former and in 97% (with 86% complete agreement) in the latter. For the urinary isolates, 1949 organism-antibiotic combinations were analysed in the Cobas Micro and MIC 2000 systems where complete agreement was observed in 92% (with 3% very major discrepancies), for 1382 urinary organism-antibiotic combinations tested in the Vitek and MIC 2000 systems, the figures were 95% and 2%, respectively.

CONCLUSIONS

The Vitek system is highly accurate in the identification and antibiotic susceptibility testing of Gram negative bacteria. The Cobas Micro system has many shortcomings in its identification of Gram negative rods, especially freshly isolated strains, but it is comparable with the Vitek system in antibiotic susceptibility testing.

Parallel comparison of accuracy of API 20E, Vitek GNI, MicroScan Walk/Away Rapid ID, and Becton Dickinson Cobas Micro ID-E/NF for identification of members of the family Enterobacteriaceae and common gram-negative, non-glucose-fermenting bacilli

We compared the API 20E (21 h) (API; bioMérieux Vitek, Hazelwood, Mo.), the Vitek GNI card (4 to 18 h) (Vitek; bioMérieux Vitek), the identification portion of the MicroScan Walk/Away Rapid Neg Combo 3 panel (2 h) (W/A; Baxter Diagnostics, Inc., West Sacramento, Calif.), and the Becton Dickinson Cobas Micro ID-E/NF rotor (21 h) (Cobas; Becton Dickinson Diagnostic Instrument Systems, Sparks, Md.), versus conventional biochemicals for their abilities to identify accurately 252 strains of biochemically typical and atypical members of the family Enterobacteriaceae and common non-glucose-fermenting gram-negative bacilli. All strains used were included in the data base of each product. At the end of the initial incubation, 194 (77.0%), 213 (84.5%), 198 (78.6%), and 192 (76.2%) strains were correct to the genus and species levels with the API, Vitek, W/A, and Cobas systems, respectively. After additional biochemical tests were performed, as directed by each manufacturer's protocol, the numbers of strains correctly identified to the genus and species levels were 241 (95.6%), 234 (92.8%), 243 (96.4%), and 230 (91.3%) with the four systems, respectively. The errors were random in all systems, with the exception of two atypical Salmonella enteritidis strains, each of which was misidentified by three systems. After the initial recommended incubation period, both API and Cobas were significantly less accurate than Vitek (Yates' corrected P < 0.05). No significant differences were noted between the results of Vitek and W/A or between the results of API and W/A. After additional tests were completed, Cobas was significantly less accurate than W/A (P < 0.05) but was equal in accuracy to Vitek and API. API, Vitek, and W/A were equal in accuracy after these same additional tests. All four systems were significantly more accurate after additional biochemical testing than after the initial reporting period (194 of 252 versus 241 of 252 for API, 213 of 252 versus 234 of 252 for Vitek, 198 of 252 versus 243 or 252 for W/A, and 192 of 252 versus 230 of 252 for Cobas [P<0.05]).

Collaborative evaluation of the Radiometer Sensititre AP80 for identification of gram-negative bacilli

A multicenter trial of the Sensititre AP80 panel read on the Sensititre AutoReader (Radiometer America, Westlake, Ohio) for the automated identification of gram-negative bacilli was conducted with 1,023 clinical isolates (879 members of the family Enterobacteriaceae plus 144 nonenteric organisms). Assignment of taxa was based on the computer-assisted interpretation of the results of a series of reactions with fluorogenic enzyme substrates after 5 h of incubation, with an incubation interval of approximately 18 h used when indicated. Accuracy was determined initially by comparison with the results obtained with the API 20E or Rapid NFT system (Analytab Products, Plainview, N.Y.). Isolates showing discrepancies were identified by using conventional biochemical profiles. Identifications were available after 5 h of incubation for 918 isolates (90%). Agreements with reference results for members of the family Enterobacteriaceae were 95.3 and 92.5% at the genus and species levels, respectively, and for the nonmembers of the family Enterobacteriaceae, the agreements with reference results were 95.1 and 84.7%, respectively. The Sensititre AP80 panel was found to be simple and convenient to use, allowed for the testing of three isolates per panel, required minimal supplementary testing for completion of identification, performed in a reproducible fashion, and demonstrated an accuracy of same-day identification comparable to that reported for other automated systems. The AP80 panel appears well suited for routine use in the clinical microbiology laboratory as an automated means of identifying both members of the family Enterobacteriaceae and nonenteric gram-negative bacilli.

Comparison of the autoSCAN-W/A and Vitek Automicrobic systems for identification and susceptibility testing of bacteria

The identification and susceptibility testing of bacteria by the autoSCAN-W/A rapid panels and Vitek Automicrobic systems were compared. A total of 291 clinical isolates, including 207 members of the Enterobacteriaceae, 41 nonfermentative gram-negative bacilli, 31 Staphylococcus aureus strains and 12 Enterococcus faecalis strains, were tested. autoSCAN-W/A and Vitek correctly identified 93% and 94% respectively of the Enterobacteriaceae, the identification in an additional 2% and 5% of strains respectively being reported as correct with a low probability. autoSCAN-W/A reported all identification results within 2 h after inoculation, whereas Vitek reported the identification results for the majority of gram-negative bacilli between 4 h and 6 h after inoculation. autoSCAN-W/A and Vitek reported 81% and 74% of MIC results respectively after 3-6 h incubation. The rates of very major susceptibility errors were 0.4% and 0.4% for autoSCAN-W/A and Vitek respectively, the rates of major errors 0.4% and 1% respectively, and the rates of minor errors 4% and 3% respectively. Six of 30 Pseudomonas aeruginosa strains failed to grow sufficiently for susceptibility testing in the autoSCAN-W/A system, and four Xanthomonas maltophilia strains had insufficient growth for susceptibility testing in both systems. The majority of results of susceptibility testing of nonfermentative gram-negative bacilli were reported more than 6 h after inoculation. Both systems were reliable and easy to operate, and gave accurate results for common clinical isolates.

Evaluation of the autoSCAN-W/A rapid system for identification and susceptibility testing of gram-negative fermentative bacilli

The autoSCAN-Walk-Away (W/A) system for identification and susceptibility testing was evaluated for 400 gram-negative fermentative bacteria by using the API 20E (366 isolates) and/or tube biochemical tests as the reference identification system and a frozen microdilution MIC tray system for susceptibility testing. The W/A system performed well for identification of this group of organisms representing 14 genera and 30 species, showing a sensitivity of 96% and results available in 2 h. Of the 16 misidentifications, 6 were with Serratia liquefaciens. A total of 63 isolates (17%) required further tests to complete the identification, compared with 106 (29%) of the isolates which required additional tests for the API 20E identification. Approximately half (32) of the additional tests with the W/A system were required in order to separate Citrobacter diversus from C. amalonaticus. For susceptibility determinations, the W/A system demonstrated an overall agreement of 93% (4,102 determinations) with 40 major errors (0.98%). However, of the 906 resistant organism-drug combinations in the study, there were 115 very major errors, for a false-susceptibility rate of 12.7% of the resistance determinations. Among these very major errors, 80% occurred with piperacillin and the cephalosporins. The W/A system completed the MIC determinations in 7 h; however, the difficulty in detecting resistance with some antimicrobial agents limited the advantages of the rapid susceptibility testing.

Automated systems for identification of microorganisms

Automated instruments for the identification of microorganisms were introduced into clinical microbiology laboratories in the 1970s. During the past two decades, the capabilities and performance characteristics of automated identification systems have steadily progressed and improved. This article explores the development of the various automated identification systems available in the United States and reviews their performance for identification of microorganisms. Observations regarding deficiencies and suggested improvements for these systems are provided.

Evaluation of the autoSCAN-W/A system for rapid (2-hour) identification of members of the family Enterobacteriaceae

We evaluated the ability of the Baxter autoSCAN-W/A System (MicroScan Division, Baxter Diagnostics, Inc., West Sacramento, Calif.) to use the rapid (2-h) gram-negative identification panel for accurate identification of members of the family Enterobacteriaceae. At 2 h, 353 of 467 (75.6%) strains in a challenge set of biochemically typical and atypical stock cultures were correctly identified to genus and species. Another 76 (16.3%) strains were correctly identified to genus and species after the performance of recommended additional biochemical testing. Thus, at 24 h, 91.9% of the 467 strains were correctly identified. Twenty-two strains (4.7%) were identified to the correct genus but the incorrect species, and 16 strains (3.4%) were misidentified. Of these 16 strains, 9 were incorrect at 2 h, and 7 were incorrect after the additional testing. Because the system is based on fluorogenic substrates, no conventional tests were readily available with which to compare aberrant reactions. These results suggest that the autoSCAN-W/A with its rapid gram-negative panels is acceptable for the identification of the Enterobacteriaceae in a clinical microbiology laboratory.

Evaluation of the Autoscan Walkaway system for rapid identification and susceptibility testing of gram-negative bacilli

We evaluated the performance of the Autoscan Walkaway (W/A) system (MicroScan, Sacramento, Calif.) in conjunction with the fluorometric Neg Combo panels for rapid identification and susceptibility testing of gram-negative bacilli. Fermentative and nonfermentative gram-negative bacilli were tested in parallel with the W/A system and the Cathra Repliscan replicator (C/R) system (Cathra, St. Paul, Minn.). Conventional biochemical testing and agar dilution testing were used to resolve the identification and susceptibility testing discrepancies. Of 495 clinical isolates tested, 445 (90%) were correctly identified by the W/A system and 483 (98%) were correctly identified by the C/R system. Repeat testing by using updated versions of the W/A system's computer identification software failed to demonstrate improved identification accuracy. For susceptibility testing, the W/A system demonstrated 5.6% total interpretative category errors, including only 0.9% major and very major errors. The comparative C/R system produced only 1% errors overall, including 0.2% major and very major errors. Although the W/A system is highly automated and is capable of producing results rapidly, our findings suggest that additional identification and susceptibility testing refinements are needed before the system will be suitable for routine use.

Reevaluation of the API 20E identification system versus conventional biochemicals for identification of members of the family Enterobacteriaceae: a new look at an old product

The API 20E bacterial identification system has been used for 19 years, often as the standard with which other identification systems are compared. Because the accuracy of this system compared with conventional biochemical tests has not been determined in many years, we evaluated the API 20E linear strip by using 291 typical and atypical strains of the family Enterobacteriaceae taken from a culture collection. At 24 h, the API 20E correctly identified by genus and species 229 of 291 (78.7%) of the strains, using Salmonella and Shigella serotyping where indicated. At 48 h, 95.2% were correctly identified by using additional biochemical tests as recommended by the manufacturer. The API 20E misidentified eight (2.7%) strains; these strains were not limited to any particular genus. When 81 of these Enterobacteriaceae strains were arranged into a weighted assortment correlating to the frequency with which they might be found in a clinical laboratory, the API 20E correctly identified 71 (87.7%) at 24 h and 78 (96.3%) at 48 h. This evaluation concluded that the accuracy of the identification of Enterobacteriaceae strains at 24 h (78.7%) may be significantly lower than that of earlier evaluations. However, there is no significant difference in the ability of the API 20E to correctly identify "challenge" type organisms (229 of 291) versus routine hospital isolates (71 of 81) (P greater than 0.05), but the system is not as accurate as the conventional biochemical method of identification.