Purpose: We characterized one cantaloupe isolate representing each of the outbreak strains with genomic and phenotypic methods to identify traits that may explain the L.monocytogenes strains’ association with cantaloupe and processing equipment.
Methods: Whole genome sequencing (WGS) of five outbreak strains from cantaloupe was performed using the IonTorrent™ PGM. SNP-based phylogenetic methods were implemented to group these strains with previously sequenced L. monocytogenes. Outbreak strains were compared to 24 sequenced L. monocytogenes strains to find unique SNPs and genetic loci. Swarming in semi-soft BHI at 30°C and 37°C, adherence to polyvinyl chloride (PVC) at 30°C, sensitivity to chlorine sanitizer, and invasion in Caco-2 cells were also assessed.
Results: A phylogeny on 195 SNPs separated the outbreak strains into three clades. Four 1/2a strains separated into two clades of two strains each. One clade contained 1 unique SNP and 7 genetic loci (> 200 bp) not found in other sequenced L. monocytogenes, the other clade contained 1,059 SNPs and 14 loci. The 1/2b strain contained 4 unique SNPs and 10 loci. No differences in swarming were observed at 30°C (P > 0.05), however the 1/2b strain demonstrated higher swarming at 37°C than the other outbreak strains (P < 0.05). The 1/2b strain showed significantly lower attachment than three of the four 1/2a strains on PVC after five days (P < 0.05). Outbreak strains tolerated recommended concentrations of chlorine in TSB, yet were susceptible to sub-inhibitory concentrations of sanitizer in 1:10 dilute TSB. No differences in invasiveness were observed (P > 0.05).
Significance: WGS identified features unique to the cantaloupe outbreak strains, making it a useful tool in outbreak investigation. Interestingly, the 1/2b strain showed increased swarming at 37°C which may contribute to survival in certain niches.