Purpose: Our objective was to develop and utilize several sets of multiplex PCR assays to identify STEC isolates from feedlot cattle feces that were negative for the Top 7 STEC.
Methods: A total of 359 E. coli strains, positive for stx1 (Shiga toxin 1; n=65), stx2 (Shiga toxin 2; n=306), stx1 and stx2 (n=16), and/or eae (intimin; n=68) genes and negative for the Top 7 STEC were used. One hundred eighteen isolates were from fecal samples collected from a single feedlot in 2013 and 241 strains ertr isolated from pen-floor fecal samples collected from eight feedlots in the central United States. Several sets of multiplex PCR assays targeting serogroup-specific genes were used to identify the serogroups.
Results: Of the 118 strains from the single feedlot, 84 (71.2%) belonged to seven serogroups and 34 (28.8%) were unidentifiable. The seven serogroups included: O168 (29.7%), O109 (20.3%), O171 (9.3%), O104 (4.2%), O2 (4.2%), O8 (2.5%), and O175 (0.8%). Among the 241 strains from eight feedlots, 164 strains (68.0%) belonged to 13 serogroups and 77 (32.0%) were unidentifiable. The 13 serogroups included O168 (29.9%), O109 (16.2%), O2 (8.3%), O104 (5.0%), O8 (2.9%), O171 (1.7%), O136 (1.2%), O178 (0.8%), O76 (0.4%), O113 (0.4%), O118 (0.4%), O175 (0.4%), and O98 (0.4%).
Significance: Cattle harbor a number of STEC serogroups other than the those included in the Top 7. This illustrates the diversity of STEC shed in the feces. The two most predominant serogroups identified were O168 (107 of 359: 29.8%) and O109 (63 of 359; 17.6%).