Popularity of shrimps as seafood has encouraged shrimp farming in South Asia and Latin America. We have been frequently coming across Vibrio spp. in recent lectures as very diverse group of bacteria, very dominant in the marine environment, associated with a number of diseases of corals, fish, crustaceans, marine mammals etc. and also of humans. As Colin explained in the last lecture, three Vibrio species Vibrio cholerae, Vibrio parahaemolyticus and Vibrio vulnificus are of prime importance as they are causative agents of various disease outbreaks of humans. In intensive shrimp farming, antibiotics are heavily used, stimulating development of resistance among bacteria, including Vibrio spp. Similarly, antibiotics are also used to treat human diseases; for example, antibiotic treatment becomes necessity for patients of septicaemia caused by V. vulnificus. Here, aquaculture induced problem of antibiotic resistance, especially among human pathogens is notable and studies have shown resistant Vibrio spp. isolates from farmed shrimps.
Thailand has a huge aquaculture industry. Shrimps are farmed both in marine and inland waters. Producing shrimps also in inland waters with the use of antibiotics have had many ecological impacts, including impacts on bacteria like Vibrio spp. and their susceptibility to antibiotics. This study examined occurrence and antibiotic susceptibility of Vibrio pathogens of humans (V. cholerae, V. parahaemolyticus & V. vulnificus) in farmed shrimps from intensive farms of inland waters of Thailand.
A method called three-tube most-probable-number was used for isolating Vibrio species. Agar plating technique was used to culture the Vibrio pathogens and then colonies of bacteria were selected for biochemical tests, species-specific PCR and 16S rRNA sequencing. Total, 12 different antibiotics were tested in this study. Antibiotic resistant isolates were screened for resistant genes and those genes were sequenced using PCR.
The authors found that 94% of shrimp samples had V. cholerae. But, remarkably, none of them had cholera toxin. Thus, all the V. cholera isolated from farmed shrimps were non-choleragenic. Non-choleragenic strains of V. cholerae are abundant in estuarine environments. This study proposes that same is true for inland waters with shrimp aquaculture activity. Nevertheless, high frequencies and densities of V. cholera were found to be associated with the shrimps, capable of causing sporadic diarrhoea in humans.
V. parahaemolyticus was isolated from 38% of samples, of which one was positive for thermostable direct hemolysin (TDH). As Colin mentioned in the last lecture, TDH is produced by human disease producing strains of V. parahaemolyticus. This pathogen is found commonly in seafood and its contamination is a big problem in Asia. The authors also suggested a link between non-native species of shrimp (grown on farms in Thailand) which grows faster but found to have higher affinity for this pathogen.
The authors did not find notable concentrations of V. vulnificus from their samples. Nevertheless, they reported a previous study finding high concentration of V. vulnificus in farmed shrimps, sold in a market in China.
The authors discussed that shrimp aquaculture expansion in inland waters (and related lower salinity) might be a limiting factor for V. parahaemolyticus and V. vulnificus whereas this would be favourable for growth of V. cholerae. Nevertheless, shrimps grown in inland waters are at high risk of being contaminated with V. cholerae and V. parahaemolyticus. Other thing I want to mention here is that it is perhaps not very surprising to see Vibrios associated with shrimps because, as Colin have mentioned many times in the lectures that Vibrios prefer to colonise on chitinous organisms/chitin and have enzymes to breakdown chitin.
V. cholerae and V. parahaemolyticus mostly showed resistance to ampicillin and oxytetracyclin. V. vulnificus showed resistance to nalidixic acid. Farmed shrimps from inland waters can act as reservoirs of numerous oxytetracyclin-resistance genes, originating from both marine and inland waters. The resistance to oxytetracycline was related to plasmids which are passed from one bacteria to other in a process called horizontal gene transfer.
In summary, this study provides an experimental evidence for risk of contamination of Vibrio pathogens in farmed seafood. This paper attracted my attention because it links aquaculture, prevalence of bacteria (Vibrios) in these environments and seafood-borne bacterial diseases in humans. In this linkage, a worrisome factor is heavy use of antibiotics in aquaculture which is moving antibiotics in marine, freshwater and terrestrial environments and driving widespread antibiotic-resistance development among various bacteria (including many pathogens) from different environments to existing antibiotics.
Yano, Y., Hamano, K., Satomi, M., Tsutsui, I., Ban, M., & Aue-umneoy, D. (2014). Prevalence and antimicrobial susceptibility of Vibrio species related to food safety isolated from shrimp cultured at inland ponds in Thailand. Food Control, 38, 30-36.