Risky business: eating shrimp can be bad for your health where Vibrio parahaemolyticus is concerned

Vibrio parahaemolyticus is a marine and estuarine bacterium that has been linked with acute gastroenteritis in humans, from consumption of raw or undercooked seafood. The bacterium has two virulence factors, thermostable direct hemolysin (TDH) and thermostable-related hemolysin (TRH), which play a key role in disease pathogenesis.

Molecular techniques were used to identify the virulence factors tdh+ and trh+ in a study by Letchumanan et al., (2015). Antimicrobial resistant V. parahaemolyticus strains were found in shrimps from wetmarkets and supermarkets in Malaysia. Some of these shrimp contained the trh gene which can cause food borne illness. It is worth noting that not all strains are pathogenic and environmental strains often do not have these pathogenic tdh or trh genes.

The study used banana prawn (Penaeus indicus) and red prawn (Solenocera subnuda). Samples were homogenized with a stomacher bag and the total Vibrio spp. were calculated using spread plate technique. A tox-R based assay was used to identify V. parahaemolyticus. Isolates were tested for antibiotic susceptibility using the disc diffusion method. PCR enabled the examination of resistant phenotypes and the presence of antibiotic resistant genes. The tested antibiotics are recommended by the Centre for Disease Control and Prevention (CDC) for treating Vibrio spp. infections. PCR was used to amplify tdh and trh genes from the isolates. In the resistant V. parahaemolyticus strains, plasmid curing was used to locate the antibiotic resistant genes.

The study showed V. parahaemolyticus was present in all 320 shrimp samples and 185 samples were toxR-positive. Importantly, wetmarket samples were more contaminated and had higher Vibrio counts than supermarket samples. This is because the shrimps are stored at ambient temperature for longer rather than being kept on ice. The study highlighted that banana prawn isolates displayed greater multiple antibiotic resistance (MAR) than red prawn isolates, perhaps because banana prawn species are exposed to a greater range of antimicrobials. A small percentage of shrimp samples contained virulence factors. 10% of the samples were trh-positive and none were tdh-positive. This is not surprising as research has shown that a small proportion of environmental strains possess either one or both of the virulence factors.

In terms of antibiotic resistance, most V. parahaemolyticus isolates were susceptible to chloramphenicol and tetracycline. Rather worryingly, 82% of the isolates were resistant to ampicillin in this study and this aligns with similar research. In the resistant V. parahaemolyticus isolates, chloramphenicol (catA2) and kanamycin (aphA-3) resistance genes were detected Antibiotic resistant genes can be transferred or exchanged with other bacteria by horizontal gene transfer through plasmids. Hence, plasmid curing is a way to eliminate antibiotic resistance and this can be achieved by chemical agents or treating with UV light and increased temperature.

This study highlights the public health concern about multidrug resistance strains in environmental bacteria such as Vibrio spp. as an indication of the excessive and widespread use of antibiotics in aquaculture. As a result, there is a greater urgency for measures to protect consumers, such as the importance of refrigeration to prevent proliferation of Vibrio bacteria and continued monitoring of pathogenic Vibrio spp. in seafood, to identify virulent markers.

Letchumanan, V., Yin, W.F., Lee, L.H. and Chan, K.G. (2015) Prevalence and antimicrobial susceptibility of Vibrio parahaemolyticus isolated from retail shrimps in Malaysia, Frontiers in Microbiology, 6, (33), 1-11.

http://www.ncbi.nlm.nih.gov.plymouth.idm.oclc.org/pmc/articles/PMC4311705/