Shiga Toxin Subtypes, Serogroups, Phylogroups, RAPD Genotypic Diversity, and Select Virulence Markers of Shiga-Toxigenic Escherichia coli Strains from Goats in Mid-Atlantic US

Abstract

Understanding Shiga toxin subtypes in E. coli from reservoir hosts may give insight into their significance as human pathogens. The data also serve as an epidemiological tool for source tracking. We characterized Shiga toxin subtypes in 491 goat E. coli isolates (STEC) from the mid-Atlantic US region (stx1 = 278, stx2 = 213, and stx1/stx2 = 95). Their serogroups, phylogroups, M13RAPD genotypes, eae (intimin), and hly (hemolysin) genes were also evaluated. STEC-positive for stx1 harbored Stx1c (79%), stx1a (21%), and stx a/c (4%). Those positive for Stx2 harbored stx2a (55%) and Stx2b (32%), while stx2a/stx2d and stx2a/stx2b were each 2%. Among the 343 STEC that were serogrouped, 46% (n = 158) belonged to O8, 20% (n = 67) to 076, 12% (n = 42) to O91, 5% (n = 17) to O5, and 5% (n = 18) to O26. Less than 5% belonged to O78, O87, O146, and O103. The hly and eae genes were detected in 48% and 14% of STEC, respectively. Most belonged to phylogroup B1 (73%), followed by D (10%), E (8%), A (4%), B2 (4%), and F (1%). M13RAPD genotyping revealed clonality of 091, O5, O87, O103, and O78 but higher diversity in the O8, O76, and O26 serogroups. These results indicate goat STEC belonged to important non-O157 STEC serogroups, were genomically diverse, and harbored Shiga toxin subtypes associated with severe human disease.

Keywords: STEC; Shiga toxin subtypes; goats; phylogroups; serogroups; virulence genes.

Figure 1

Percentage of stx1 and stx2 subtypes in non-O157 (STEC) strains isolated from goats. Note: stx1a, stx1c, and stx1a/c—percentage of each subtype out of all STEC-positive for stx1; stx2a, stx2b, stx2d, and stx2a/stx2d—percentage of each variant/subtype out of all stx2-positive STEC.

Figure 2

Percentage of stx subtypes in STEC harboring stx1/stx2 genotype.

Figure 3

Phylogroups of STEC isolated from goats in the mid-Atlantic US.

Figure 4

Dendrogram showing overall M13 RAPD genotype clustering of STEC from goats. 1–23 STEC clusters at 95% similarity. Labels in order—{stx1; stx2; eae; hly (virulence genes: Y = present, N = none); phylogroups: A, B1, B2, D, E, F, and ND=not determined; serogroups: O5, O8, O26, O76, O78, O87, O91, O103, and NT=non-serogrouped}. Stx1: Y = stx1c unless indicated. Stx2: Y = stx2a unless indicated. Bolded black and red-ATCC reference strains (STEC-Red; Non-STEC-black). The highest number of STEC detected in this study belonged to the O8 serogroup. Overall, all the O8 STEC typed in this study shared about 40% genotype patterns based on the M13 RAPD typing, except for two unique isolates (Figure 5). The isolates could be discriminated into six clusters and four unique genotypes at a higher similarity index of 60%. The three clusters at 95% similarity (13, 14, and 15) carried over 40% of the 08 isolates typed. It is worth noting that these three cluster isolates were all shiga toxin 2a-positive but differed in other virulence gene composition and most belonged to the B1 phylogroup. Additionally, four of the six isolates in cluster 14 harbored both stx2a and stx2d shiga toxin genes. However, no unique virulence gene combination was associated with any of the clusters.

Figure 5

Dendrogram showing overall M13 RAPD genotype clustering of O8 serogroup STEC from goats. 1–18 O8 STEC clusters at 95% similarity. Labels in order—{stx1; stx2; eae; hly (virulence genes: Y=present, N=none); phylogroups: A, B1, B2, D, E, F, and ND=not determined; serogroups: O5, O8, O26, O76, O87, O91, O103, and NT=non-serogrouped}. Stx1: Y = stx1c unless indicated. Stx2: Y = stx2a unless indicated.