Inhibition of Escherichia coli O157:H7 Growth Through Nutrient Competition by Non-O157 E. coli Isolated from Cattle

Escherichia coli O157:H7 is a major food safety concern through contamination of beef and produce. Non-antibiotic interventions to minimize O157:H7 in food animals and products are highly desired and one strategy to improve food safety is to reduce O157:H7 in cattle, a main asymptomatic reservoir, through probiotic administration. Non-pathogenic E. coli populating the intestine represent a compelling probiotic source, as these strains are already host-adapted. The challenge is to identify non-pathogenic strains capable of competing with O157:H7 for nutrients or those producing compounds to inhibit O157:H7 growth. Here, E. coli isolated from cattle fecal and recto-anal junction swab samples were sequenced and screened for the ability to compete with O157:H7. Fourteen genetically distinct, non-Shiga toxin-encoding, non-O157:H7 E. coli strains were recovered, and individual isolates representative of each strain were assessed. Nearly all strains possessed complete genetic pathways for the utilization of carbon and nitrogen sources required for O157:H7 colonization of the cattle intestine. Growth curve assays were conducted, and growth metrics were compared between bovine non-O157:H7 E. coli isolates and two O157:H7 isolates. While no strain outperformed both O157:H7 strains for all nutrients tested, at least one strain outperformed O157:H7 for each of the carbon sources tested. No strain grew significantly better than O157:H7 in media supplemented with ethanolamine. A "highly competitive" consortium of 4 non-O157:H7 isolates that grew significantly better than O157:H7 reduced O157:H7 counts CFU/mL by 1.53 log₁₀ and >0.72 log₁₀ under anaerobic and aerobic conditions, respectively, in competition assays. A consortium of "low-competitive" strains reduced O157:H7 counts by >0.47 log₁₀ and >0.51 log₁₀ under anaerobic and aerobic conditions. These results suggest that cattle harbor non-O157:H7 E. coli strains capable of limiting O157:H7 growth in vitro. Surveys of commensal non-O157:H7 isolates from cattle using growth-based phenotypic assays may be useful in identifying E. coli strains capable of outcompeting O157:H7 in the bovine intestine for further in vivo testing as probiotics.