Emergence of colistin resistance and characterization of antimicrobial resistance and virulence factors ofAeromonas hydrophila,Salmonellaspp., andVibrio choleraeisolated from hybrid red tilapia cage culture

Abstract

BackgroundTilapia is a primary aquaculture fish in Thailand, but little is known about the occurrence of antimicrobial resistance (AMR) inAeromonas hydrophila,Salmonellaspp., andVibrio choleraecolonizing healthy tilapia intended for human consumption and the co-occurrence of these AMR bacteria in the cultivation water.MethodsThis study determined the phenotype and genotype of AMR, extended-spectrumβ-lactamase (ESBL) production, and virulence factors ofA. hydrophila,Salmonellaspp., andV. choleraeisolated from hybrid red tilapia and cultivation water in Thailand. Standard culture methods such as USFDA’s BAM or ISO procedures were used for the original isolation, with all isolates confirmed by biochemical tests, serotyping, and species-specific gene detection based on PCR.ResultsA total of 278 isolates consisting of 15A. hydrophila, 188Salmonellaspp., and 75V. choleraeisolates were retrieved from a previous study. All isolates ofA. hydrophilaandSalmonellaisolates were resistance to at least one antimicrobial, with 26.7% and 72.3% of the isolates being multidrug resistant (MDR), respectively. AllA. hydrophilaisolates were resistant to ampicillin (100%), followed by oxytetracycline (26.7%), tetracycline (26.7%), trimethoprim (26.7%), and oxolinic acid (20.0%). The predominant resistance genes inA. hydrophilaweremcr-3(20.0%), followed by 13.3% of isolates havingfloR,qnrS,sul1,sul2, anddfrA1. Salmonellaisolates also exhibited a high prevalence of resistance to ampicillin (79.3%), oxolinic acid (75.5%), oxytetracycline (71.8%), chloramphenicol (62.8%), and florfenicol (55.3%). The most common resistance genes in theseSalmonellaisolates wereqnrS(65.4%),tetA(64.9%),blaTEM(63.8%), andfloR(55.9%). AllV. choleraeisolates were susceptible to all antimicrobials tested, while the most common resistance gene wassul1(12.0%). One isolate ofA. hydrophilawas positive forint1, while all isolates ofSalmonellaandV. choleraeisolates were negative for integrons andintSXT. None of the bacterial isolates in this study were producing ESBL. The occurrence ofmcr-3(20.0%) in these isolates from tilapia aquaculture may signify a serious occupational and consumer health risk given that colistin is a last resort antimicrobial for treatment of Gram-negative bacteria infections.ConclusionsFindings from this study on AMR bacteria in hybrid red tilapia suggest that aquaculture as practiced in Thailand can select for ubiquitous AMR pathogens, mobile genetic elements, and an emerging reservoir ofmcrand colistin-resistant bacteria. Resistant and pathogenic bacteria, such as resistance to ampicillin and tetracycline, or MDRSalmonellacirculating in aquaculture, together highlight the public health concerns and foodborne risks of zoonotic pathogens in humans from cultured freshwater fish.