Characterization of Australian Escherichia coli O111 Isolates from Human and Cattle Sources

Introduction: Shiga toxin-producing Escherichia coli (STEC) O111 rose to prominence in Australia following the Garibaldi brand mettwurst outbreak in 1995. Between 2001 and 2009, STEC O111 were found to be associated with 13.7% of all notified STEC infections. Cattle are a reservoir of STEC, however little is known about the relationship between O111 isolates from cattle and human clinical sources.

Purpose: This study genomically characterized a selection of STEC O111 isolates from cattle and human clinical sources.

Methods: A total of 33 O111 isolates (11 from cattle and 22 humans), collected between 1995 and 2015, were included in the study. Isolates were characterized using next generation sequencing based analysis for multi-locus sequence type (MLST), comparative genome fingerprinting, pangenome content, and single nucleotide polymorphisms (SNPs).

Results: STEC O111 from cattle sources were either O111:H8 ST294 (45%) or O111:H- ST16 (55%). Human isolates were, mainly, O111:H8 of either ST294 (64%) or ST16 (18%) and O111:H11 ST21 (14%). Theta intimin was identified in 30/33 (91%) of all isolates. Carriage of stx correlated well with serotype with O111:H8 likely to harbor stx_1a and stx_2a; whereas O111:H- were typically stx_1a only, regardless of source. The three O111:H11 isolates from human clinical sources carried stx_1aand beta intimin, a profile typical of STEC O26 isolates. Further genetic analysis confirmed that these strains closely resembled typical STEC O26 isolates from Australia.

Significance: Characterization of isolates from cattle and human clinical sources determined that there are strong relationships between O111 isolates from cattle and humans. However, O111 isolates from humans demonstrated a greater tendency to harbor stx_2a. This study also reported the first description of a hybrid O111/O26 clone present in human clinical samples. The ability of the E. coli genome to undergo recombination creates challenges for those aiming to establish STEC detection protocols with greater discriminatory power.