In the United States, Shiga toxin-producing Escherichia coli (STEC) outbreaks cause >265,000 infections and cost $280 million annually. We investigated REPEXH01, a persistent strain of STEC O157:H7 associated with multiple sources, including romaine lettuce and recreational water, that has caused multiple outbreaks since emerging in late 2015. By comparing the genomes of 729 REPEXH01 isolates with those of 2,027 other STEC O157:H7 isolates, we identified a highly conserved, single base pair deletion in espW that was strongly linked to REPEXH01 membership. The biological consequence of that deletion remains unclear; further studies are needed to elucidate its role in REPEXH01. Additional analyses revealed that REPEXH01 isolates belonged to Manning clade 8; possessed the toxins stx2a, stx2c, or both; were predicted to be resistant to several antimicrobial compounds; and possessed a diverse set of plasmids. Those factors underscore the need to continue monitoring REPEXH01 and clarify aspects contributing to its emergence and persistence.

In both molecular epidemiology and microbial ecology, it is useful to be able to categorize specific strains of microorganisms in either an ingroup or an outgroup in a given population, e.g. to distinguish a pathogenic strain of interest from its non-virulent relatives. An "ingroup" refers to a group of microbes that are the primary focus of study or interest. Conversely, an "outgroup" consists of microbes that are closely-related to, but have evolved separately from, the ingroup. While whole genome sequencing and downstream phylogenetic analyses can be employed to do this, these techniques are often slow and can be resource intensive. Additionally, the laboratory would have to sequence the whole genome to use these tools to determine whether or not a new sample is part of the ingroup or outgroup. Alternatively, polymerase chain reaction (PCR) can be used to amplify regions of genetic material that are specific to the strain(s) of interest. PCR is faster, less expensive, and more accessible than whole genome sequencing, so having a PCR-based approach can accelerate the detection of specific strain(s) of microbes and facilitate diagnoses and/or population studies.

To understand genome evolution in a group of microbes, we need to know the timing of events such as duplications, deletions and horizontal transfers. A common approach is to perform a gene-tree / species-tree reconciliation. While a number of software packages perform this type of analysis, none are geared toward a complete reconstruction for all families in an entire clade. Here we describe an update to the xenoGI software package which allows users to perform such an analysis using the newly developed DTLOR (duplication-transfer-loss-origin-rearrangement) reconciliation model starting from genome sequences as input.