Escherichia,Shigella, andSalmonella

Members of the genus Shigella are phenotypically similar to Escherichia coli and, with the exception of Shigella boydii serotype 13, would be considered the same species by DNA-DNA hybridization analysis and whole-genome sequence analysis. The dynamic nature of the Shiga toxin-converting phages has implications for diagnostic testing for Shiga toxin-producing E. coli (STEC). Since STEC strains can lose critical virulence genes, some researchers have proposed that multiple virulence-associated genes, as well as conserved genes, be used to diagnose infections by these bacteria. This concept would also apply to other pathotypes of E. coli, as most of them carry critical virulence genes on mobile genetic elements. A preliminary report can be issued as soon as a presumptive identification of Salmonella is obtained. In most situations, a presumptive identification is based on phenotypic traits determined by either traditional or commercial systems or by reactivity with Salmonella O grouping antisera. A confirmed identification requires both phenotypic identification and O group or serotype determination. As national surveillance systems depend on the receipt of serotype information for Salmonella strains isolated in the United States, laboratories should follow the procedures recommended by their state health departments for submitting Salmonella isolates for further characterization, including complete serotyping. The antimicrobial susceptibilities of typhoidal Salmonella strains and strains from normally sterile sites should be determined, and the strains should be forwarded to a reference or public health laboratory for complete phenotypic identification and serotyping.