Seasonal distribution and genotyping of antibiotic resistant strains of Listeria innocua isolated from a river basin categorized by ERIC-PCR

Listeria innocua retains many conserved homologous domains with Listeria monocytogenes, which is a food-borne and water-borne diarrhea-causing bacterium. Studies of antimicrobial resistance in L. innocua showed that this microbe is more prone to acquire resistance than other bacteria in the genus Listeria. However, little is known about the seasonal population distribution and antimicrobial resistance patterns of L. innocua in natural water environments. The aims of the study were: (1) to investigate the occurrence of L. innocua isolates in a subtropical watershed and reconstruct the population structure and (2) to analyze the antibacterial resistance patterns of the identified L. innocua isolates according to ERIC type. A total of 288 water samples was collected from the Puzi River basin (23° 28’ N, 120° 13’ E) between March 2014 and March 2015, and 36 L. innocua isolates were recovered from 15 positive water samples. With regard to seasonal variation, L. innocua was only detected in the spring and summer. Eighteen enterobacterial repetitive intergenic consensus (ERIC)-PCR types were identified, and two genogroups with four subgroups were reconstructed in a minimum spanning tree. Isolates from different sampling areas that were located near each other were genetically different. All L. innocua isolates (including 41.7% of the multidrug-resistant (MDR) isolates) were resistant to oxacillin and showed high minimum inhibitory concentrations of tetracycline. These findings demonstrate the seasonal variations and differing geographical distributions of L. innocua in this subtropical water environment, as well as the existence of strong population structures and MDR and antimicrobial resistance patterns. Phylogenetic analysis based on ERIC-type showed that the Cluster A isolates were resistant to more antibiotics, and two types, ERIC8 and ERIC15 were multidrug resistant. The more commonly detected types, such as ERIC1 and ERIC12, were also more likely to be resistant to two or more antibiotics. Close monitoring of drug resistance in environmental L. innocua is warranted due to its potential for transferring antimicrobial resistance determinants to pathogenic Listeria.