Evaluation of the Vitek EPS enteric pathogen screen card for detecting Salmonella, Shigella, and Yersinia spp

Manual and automated instrumentation for identification of Enterobacteriaceae and other aerobic gram-negative bacilli

Identification of gram-negative bacilli, both enteric and nonenteric, by conventional methods is not realistic for clinical microbiology laboratories performing routine cultures in today's world. The use of commercial kits, either manual or automated, to identify these organisms is a common practice. The advent of rapid or "spot" testing has eliminated the need for some commonly isolated organisms to be identified with the systems approach. Commercially available systems provide more in-depth identification to the species level as well as detect new and unusual strains. The answers obtained from these systems may not always be correct and must be interpreted with caution. The patient demographics, laboratory workload and work flow, and technologist's skill levels should dictate the system of choice. Cost considerations introduce another variable into the equation affecting choice. Each system has its own strengths and weaknesses, and each laboratory must decide on the level of sophistication that fulfills its particular needs.

Comparison of CHROMagar Salmonella medium and hektoen enteric agar for isolation of salmonellae from stool samples

CHROMagar Salmonella (CAS), a new chromogenic medium, was retrospectively compared to Hektoen enteric agar (HEA) with 501 Salmonella stock isolates and was then prospectively compared to HEA for the detection and presumptive identification of Salmonella spp. with 508 stool samples before and after enrichment. All stock cultures (100%), including cultures of H2S-negative isolates, yielded typical mauve colonies on CAS, while 497 (99%) isolates produced typical lactose-negative, black-centered colonies on HEA. Following overnight incubation at 37 degreesC, a total of 20 Salmonella strains were isolated from the 508 clinical samples. Sensitivities for primary plating and after enrichment were 95% (19 isolates) and 100% (20 isolates), respectively, for CAS and 80% (16 isolates) and 100% (20 isolates), respectively, for HEA. The specificity of CAS (88.9%) was significantly higher than that of HEA (78.5%; P < 0.0001). On the basis of its good sensitivity and specificity, CAS medium can be recommended for use for primary plating when human stool samples are screened for Salmonella spp.

Rapid biochemical screening for Salmonella, Shigella, Yersinia, and Aeromonas isolates from stool specimens

Four screens for the rapid (4 to 6 h) biochemical detection of pathogens from enteric isolation media are described. The Salmonella screen consisted of Kligler iron agar (KIA), motility-indole-urea-tryptophan-deamination semisolid medium (MIU-TDA), and the o-nitrophenyl-beta-D-galactopyranoside (ONPG) test; the Shigella screen consisted of KIA, MIU-TDA, the ONPG test, and the lysine decarboxylation-indole test; the Yersinia screen consisted of a rhamnose broth; the Aeromonas screen consisted of a xylose agar plate. When tested on 2,102 fresh isolates and 71 stock strains, the screens correctly detected 212 enteric pathogens (sensitivity, 100%), with a specificity of 98.1%.

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.