Purpose: The purpose of this study was to characterize STEC O5 strains from the United States for their virulence genes, phenotypic markers, and genetic similarity.
Methods: STEC O5 isolates submitted to CDC by state and county public health laboratories were tested by PCR for Shiga toxins 1 and 2 (stx1 and stx2), intimin (eae) and enterohemolysin (ehxA) and serotyped. All isolates were inoculated to urea agar slants (Christensen’s) using an 18-24 hour culture. The slants were incubated at 35-37°C and examined at 24 hours, 48 hours, and 7 days for the development of an intense pink-red color on the slant, indicating urease activity. Genetic variation among isolates was assessed by PFGE analysis.
Results: During the past two years, CDC received 75 STEC O5 strains that were all non-motile from 29 states. The majority of these, 70/75 (93%), hydrolyzed urea within 24 hours, and an additional strain within 48 hours, while the remaining strains showed no activity within 7 days. 73 strains (97.3%) were positive by PCR for stx1, eae, and ehxA genes, while two strains (2.7%) possessed the genes for stx1, stx2, and ehxA and one strain (1.3%) was positive for stx1, stx2, eae, and ehxA. By PFGE analysis, 57 strains were separable into 48 unique XbaI patterns.
Significance: STEC strains typically lack urease activity. These findings demonstrate the urelytic nature of a subset of genetically diverse STEC strains. Continued surveillance is important to detect phenotypic variants that may impact identification schemes.