Salmonella enterica recovery from river waters of the Maryland Eastern Shore reveals high serotype diversity and some multidrug resistance

Authors: CALLAHAN, MARY THERESA - University Of Maryland; Van Kessel, Jo Ann; MICALLEF, SHIRLEY - University Of Maryland

Submitted to: Environmental Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/11/2018
Publication Date: 9/12/2018
Citation: Callahan, M., Van Kessel, J.S., Micallef, S.A. 2018. Salmonella enterica recovery from river waters of the Maryland Eastern Shore reveals high serotype diversity and some multidrug resistance. Environmental Research. 168:7-13. https://doi.org/10.1016/j.envres.2018.09.012.
DOI: https://doi.org/10.1016/j.envres.2018.09.012

Interpretive Summary: The Delmarva Peninsula is a major agricultural and recreational region for the U.S. states Delaware, Maryland and Virginia. Foodborne outbreaks of salmonellosis have been traced back to this region and it seems that Salmonella enterica, a zoonotic bacteria that can make animals and humans sick, is persistent in this area. Previous research and analyses have pointed to the waterways as a reservoir for this pathogen. The objective of this research was to determine if Salmonella enterica could be isolated from the four main rivers of the Maryland Eastern Shore on the Delmarva Peninsula. Water was collected from twenty four sites along the Choptank, Nanticoke, Pocomoke and Wicomico Rivers during the spring and during the fall. The two time periods were chosen so that differences due to season could be identified. Salmonella enterica was isolated from all the rivers in both seasons and was detected in 65% of all the collected water samples, with greater recovery in spring (70%) than in fall (61%). There are many (>3000) serotypes of Salmonella enterica and there are many differences between serotypes, including their pathogenicity; some serotypes are more likely to make people or animals sick than others. In total, 18 different serotypes were identified, and there were differences in the serotype diversity between the fall and spring samplings. Some of the Salmonella enterica isolates were serotypes that are among the top serotypes that cause disease in humans (i.e. Typhimurium). The isolate salmonellae were characterized for their sensitivity to antimicrobial agents. Most of the isolates (84%) were sensitive to all of the antibiotics tested. However there were some isolates that were resistant to multiple antibiotics. Antimicrobial resistance is a public health concern due to the impact of treatment options when an infection is caused by resistant bacteria. The widespread presence and diversity of S. enterica in rivers in this region is a significant concern given the frequent use of rivers and tributaries as a source of irrigation and recreational waters. The results of this study can be used in future research to determine the source(s) of the Salmonella enterica in the river waters.

Technical Abstract: The Delmarva Peninsula is a major agricultural and recreational region for the U.S. states Delaware, Maryland and Virginia. Recurrent salmonellosis disease from this region suggests a persistent environmental reservoir of Salmonella enterica, and previous studies point to water environments. To evaluate this hypothesis, water from the four main rivers of the Maryland Eastern Shore on the Delmarva Peninsula was tested for the presence of S. enterica, and recovered isolates were characterized for antimicrobial resistance. Sampling was performed in autumn and spring to evaluate temporal persistence at twenty four sites along the Choptank, Nanticoke, Pocomoke and Wicomico Rivers. Water (10 L) was filtered through sterile modified Moore swabs at the sampling sites. The swabs were selectively enriched for S. enterica and presumptive salmonellae were confirmed by PCR amplification of the Salmonella-specific invA and hilA genes. The serogroup of 402 isolates was determined and the serotype was determined for 157 isolates chosen to represent all the identified serogroups. S. enterica was isolated from all the rivers in both seasons and was detected in 35/46 (65%) of surface water samples, with greater recovery in spring (70%) than in fall (61%). In total, 18 serotypes of S. enterica were identified, and serotype diversity was different between the fall and spring samplings. Newport was the most frequently isolated serotype, both overall and in the fall sampling, and was identified in 8 of 46 samples (17%). Typhimurium was the predominant serotype in spring. Other commonly isolated serotypes included 4,[5],12:i:, Bareilly and Enteritidis. Some temporal and biogeographic patterns were observed in S. enterica recovery, but 6 of the 18 serotypes were identified in both fall and spring. The majority (84%) of the 157 isolates were pan-susceptible, including all those tested from the Wicomico River (N=19). Twenty five isolates from 9 samples were resistant to at least one antimicrobial, including serotypes Typhimurium, Newport, Litchfield, III 17:z10:e,n,x,z15, III 60:I and IV_40:z4:z32:-. Of the isolates that were resistance to a single antimicrobial, the most common were resistance to streptomycin or sulfisoxazole. Thirteen of the isolates were multidrug resistant, nine exhibiting resistance to ampicillin, sulfisoxazole, tetracycline, amoxicillin/clavulanic acid, cefoxitin and ceftriaxone, and four to sulfisoxazole and tetracycline. The widespread presence and diversity of S. enterica in rivers in this region is a significant concern given the frequent use of rivers and tributaries as a source of irrigation and recreational waters. Further research is needed to determine specific potential point sources of S. enterica for surface river waters, and risks associated with acquisition of antimicrobial resistance traits.