Purpose: This study examined the diversity of Listeria spp. and characterized L. monocytogenes isolates to determine if waterways could serve as a potential reservoir of this pathogen.
Methods: Water (n = 329) and point source (n = 30) samples from an urban and an agricultural source watershed in Nova Scotia (Canada) were collected over 18 months. After enrichment in LEB/Fraser broths, samples were plated on Palcam agar followed by speciation on RAPID’Lmono agar of eight typical isolates from each positive sample. L. monocytogenes was also detected using Taqman quantitative PCR (qPCR). L. monocytogenes isolates were serogrouped, pulsotyped and compared with clinical isolates (n = 19) obtained from Nova Scotia during the study period.
Results: Listeria spp. was isolated from 72.1% of the rural water samples while 35.4% of the urban water samples tested positive for the genus. The L. innocua (56.3%) and L. welshimeri (43.1%) groups dominated in the rural and urban areas, respectively. Prevalence of L. monocytogenes was significantly (P < 0.05) higher when water samples were analyzed by qPCR with 67.8% positives vs. 31.6% by the culture method. L. monocytogenes co-occurred with other Listeria spp. in 45% of the culture-based positive samples, with serogroup IIa dominating (67.7%), followed by IVb (16.1%), IIb (15.8%) and IIc (0.4%). L. monocytogenes was detected by qPCR in 43, 100 and 0% of the cow feces (n = 24), raw sewage (n = 3) and septic tank (n = 3) samples, respectively. Absence of distinct clusters in a multidimensional scaling plot based on the AscI and ApaI pulsotypes of the environmental watershed and human L. monocytogenes isolates suggested the presence of shared genetic characteristics among the isolates.
Significance: Listeria spp. are common in rural and urban surface waters, which could potentially act as a reservoir for L. monocytogenes outbreak strains.