In 2013, foodborne outbreaks in Japan were linked to non-O1, non-O139 . However, laboratory tests have detected several serogroups, making it difficult to determine the causative agent. Therefore, whole-genome analyses revealed that only serogroup O144 possesses a genomic island with a type III secretion system (T3SS). A T3SS-deficient mutant was subsequently generated, and its pathogenicity was assessed using a rabbit ileal loop test. This led to the conclusion that serogroup O144 with T3SS was the causative agent of foodborne outbreaks. This study provides an illustrative example of the utilization of whole-genome data for pathogenicity and molecular epidemiological analyses in outbreak investigations.