Distribution and Genetic Diversity ofSalmonella entericain the Upper Suwannee River

Abstract

The Suwannee River spans the Florida/Georgia border to the Gulf of Mexico, and contributes to regional irrigation and recreational activities. Association of Salmonella enterica with these resources may result in the contamination of produce and disease outbreaks. Therefore, surface water was examined for the distribution of S. enterica at multiple time points from 4 sites on the upper Suwannee River. Isolates were confirmed by detection of the invA gene, and 96% of all samples were positive for the bacterium. Most probable number enumeration ranged from <18 to 5400 MPN/100 mL. Genetic diversity of these isolates (n=110) was compared to other environmental (n=47) or clinical (n=28) strains and to an online library (n=314) using DiversiLab rep-PCR. All strains showed >60% similarity and distributed into 16 rep-PCR genogroups. Most (74%) of the Suwannee River isolates were clustered into two genogroups that were comprised almost exclusively (97%) of just these isolates. Conversely, 85% of the clinical reference strains clustered into other genogroups. However, some Suwannee River isolates (12%) were clustered with these primarily clinically-associated genogroups, supporting the hypothesis that river water can serve as a disease reservoir and that pathogenic strains may persist or possibly originate from environmental sources.

Distribution and Characterization of Salmonella enterica Isolates from Irrigation Ponds in the Southeastern United States

“…Among multidrug-resistant (MDR) strains (resistant to Ͼ2 antibiotics), genotype DT 23 was overrepresented, accounting for 27% (3/11) of this group, although the 3 strains were collected from different ponds and showed different resistance profiles. The DiversiLab profiles in this study were compared to a DT reference library comprised of 313 entries provided by DiversiLab, which was derived from the 55 most frequently identified serotypes from clinical and nonclinical sources; they were also compared to DT profiles from our previous study with Suwannee river isolates (9). Based on the Top Match module results in the DiversiLab classification report, unique DTs were identified for 22% of the strains in this study that did not match (i.e., had Ͻ95% similarity) any reference profile (Fig.…”

“…S. enterica serovar Typhimurium strain LT2 (ABC Research, Inc.) and Vibrio vulnificus strain CMCP6 (Wright laboratory strain collection) were used as quality controls for each rep-PCR assay. The reference Salmonella strains (n ϭ 485) included isolates from the Suwannee River (n ϭ 110), Florida lakes (n ϭ 14), and other environmental (n ϭ 20) or clinical (n ϭ 28) sources, as previously described by Rajabi et al (9). DiversiLab rep-PCR online profiles from its Salmonella strain library (n ϭ 313) were also used for comparison.…”

“…DTs from pond strains were evaluated using the Top Match module in the DiversiLab classification report to identify the highest values for similarity of reference strains to pond strains. Reference strains included those from the DiversiLab strain library and the publication of Rajabi et al (9).…”

“…These outbreaks confirm that environmental transmission of Salmonella from produce can lead to human illness (6,7). Recent investigations have focused on irrigation water as a potential environmental source of Salmonella for preharvest contamination of produce (8)(9)(10)(11)12). Furthermore, the static nature of some irrigation ponds may sustain persistent populations of Salmonella (13,14).…”

“…A 12-month survey of river water in this area showed Salmonella in 79.2% of samples, with an increased prevalence during summer months (62%) that was positively correlated with the water temperature (r ϭ 0.49, P Ͻ 0.05) and precipitation levels (r ϭ 0.68, P Ͻ 0.05) (8). Even higher prevalence (96% of samples) and levels (up to 5,400 MPN/100 ml) of Salmonella were reported in surface water samples of the lower portion of the Suwannee River watershed (9). Both surveys identified serotypes that were associated with human disease in this region.…”