Abstract
Campylobacter jejuni has caused several campylobacteriosis outbreaks via raw milk consumption. This study reports follow-up of a milk-borne campylobacteriosis outbreak that revealed persistent C. jejuni contamination of bulk tank milk for seven months or longer. Only the outbreak-causing strain, representing sequence type (ST) 883, was isolated from milk, although other C. jejuni STs were also isolated from the farm. We hypothesized that the outbreak strain harbors features that aid its environmental transmission or survival in milk. To identify such phenotypic features, the outbreak strain was characterized for survival in refrigerated raw milk and in aerobic broth culture by plate counting and for biofilm formation on microplates by crystal violet staining and quantification. Furthermore, whole-genome sequences were studied for such genotypic features. For comparison, we characterized isolates representing other STs from the same farm and an ST-883 isolate that persisted on another dairy farm, but was not isolated from bulk tank milk. With high inocula (105 CFU/ml), ST-883 strains survived in refrigerated raw milk longer (4–6 days) than the other strains (≤3 days), but the outbreak strain showed no outperformance among ST-883 strains. This suggests that ST-883 strains may share features that aid their survival in milk, but other mechanisms are required for persistence in milk. No correlation was observed between survival in refrigerated milk and aerotolerance. The outbreak strain formed a biofilm, offering a potential explanation for persistence in milk. Whether biofilm formation was affected by pTet-like genomic element and phase-variable genes encoding capsular methyltransferase and cytochrome C551 peroxidase warrants further study. This study suggests a phenotypic target candidate for interventions and genetic markers for the phenotype, which should be investigated further with the final aim of developing control strategies against C. jejuni infections.
Highlights
Campylobacter jejuni, which is the leading cause of bacterial gastroenteritis worldwide, is asymptomatically carried in the digestive tract of numerous wild and domesticated bird and mammal species
The outbreak and the persisting milk contamination were caused by a single clone of sequence type ST-883 (ST-21 clonal complex (CC)), which was prevalent in the dairy herd and shed in feces repeatedly by two cows
In addition to the outbreak clone, five other clones were sporadically detected in the herd (ST-45, ST-58, and ST-61; CCs other than ST-21 CC) and in an udder cloth (ST-50; ST-21 CC), but not in bulk tank milk
Summary
We aimed to determine whether and, if so, why certain C. jejuni strains pose a higher health risk in milk production settings to aid the development of enhanced control strategies
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