WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2015 3 Editor’s Note Scientists from China recently published a study that found evidence of resistance to polymixins, a last-resort class of antibiotics, in pigs and humans. Colistin is widely used in livestock farming in China to promote animal health and growth. The gene for resistance was found on highly-mobile plasmids that allow transfer of the gene to different types of bacteria. For now, the gene appears to be confined to bacterial populations in China, but global spread appears to be inevitable. This is the latest example of a ‘superbug,’ or a multi-drug resistant microbe, that has public health officials and veterinarians alarmed about the prospects of a global health crisis. What is most concerning is that veterinary use of colistin has led to drug resistance of bacteria found in slaughtered animals, meat in the food supply and in humans. Another example of the mobility of plasmids was demonstrated recently in a paper that identified a strain of Vibrio harveyi that causes AHPND/EMS in shrimp in Vietnam. Until now, the diseasecausing agent has been identified as V. parahaemolyticus. This demonstrates that toxin genes and their related plasmid sequences could be transferred to different Vibrio species. So, what are the implications of these findings for aquaculture? Examples of resistant strains of Aeromonas, Edwardsiella and Vibrios to tetracycline and sulfonamide in aquaculture settings have been demonstrated. Affected species include Atlantic salmon, penaeid shrimps, pangasius, tilapia, rainbow trout, channel catfish, eels and prawns, among others. In these cases, antibiotic resistance is genetically transferable. In some cases (e.g. Vibrio), horizontal transfer of genes for antibiotic resistance on plasmids can occur from fish pathogens to human pathogens. In a short review of antibiotic use in aquaculture, Felipe Cabello stated that overuse of antibiotics in aquaculture, especially prophylactic use, has resulted in “the emergence of antibioticresistant bacteria in aquaculture environments, in the increase of antibiotic resistance in fish pathogens, in the transfer of these resistance determinants to bacteria of land animals and to human pathogens, and in alterations of the bacterial flora both in sediments and in the water column.” Of course, there is also justifiable concern about the safety of seafood produced in aquaculture using antibiotics. A Consumer Reports study from April 2015 indicated that “of 205 raw farmed imported shrimp samples [to the US], 11 samples from Vietnam, Thailand, and Bangladesh tested positive for one or more antibiotics: Nine tested positive for oxytetracycline, three contained enrofloxacin, and two contained sulfa antibiotics.” Vietnam continues to struggle with antibiotic residues in exports of shrimp and pangasius. The US food retailer giant Costco recently decided to reduce purchases of farmed salmon from Chile because of concerns about antibiotic use. Chilean salmon farmers use quinolones prophylactically to manage Piscirickettsiosis. In contrast to the salmon farming sector in Chile, antibiotic use in Norway is much less. Salmon farmers in Chile use about 630 g/t while their counterparts in Norway use 1.4 g/t. In the late 1980s in Norway, furunculosis was routinely managed by mixing antibiotics with feed. By the mid-1990s, salmon producers had shifted to controlling this disease through vaccination, leading to a dramatic decline in antibiotic use. Vaccination is clearly an effective approach to avoid concerns about food safety and the development of antibiotic resistance. In addition, genetic selection for disease resistance and management practices such as keeping a single generation at a site and site fallowing contribute to the reduced need for antibiotics. There are other encouraging trends in aquaculture, notably the increasingly widespread use of probiotics. Feed additives such as enzymes, organic acids, phospholipids, seaweed extracts and immunologically-active compounds can promote good health and growth of cultured animals. There are also emerging techniques to design antimicrobials that target specific genes that code for virulence factors. So-called decoy proteins that mimic antigens produced by pathogenic bacteria can be used to modulate the immune response. Viruses that attack pathogenic bacteria — bacteriophages — can be genetically engineered. Another important trend is the rise of certification schemes, which limit or proscribe antibiotic use. One new, exciting and emerging technique is gene editing with a tool called CRISPR. With CRISPR, an enzyme that cuts DNA (a nuclease) is guided by a short strand of RNA (easily synthesized in the laboratory) that can locate a specific gene. The technique allows the easy change, replacement or deletion of genes with great accuracy. Think of it as gene microsurgery. There are a number of ethical concerns about using CRISPR, especially in editing genes of germlines, but the promise and potential seem enormous. It’s only a matter of time before this technique is applied in the context of aquaculture. It’s easy to envision applications of this gene therapy technique to enable fish to better fight infectious diseases. The techniques used to develop the recently approved genetically engineered salmon will soon seem primitive. CRISPR has also been found to play a role in bacterial virulence, infection and evasion of the host immune system. In some bacteria, the absence of CRISPR was associated with antibiotic resistance and is thus valuable in bacterial typing. Although there are certainly examples of antibiotic resistant bacteria developing in the context of aquaculture, there have not been the superbugs with multi-drug resistance that have arisen from prophylactic or indiscriminant use of antibiotics in terrestrial animal production. Despite shortcomings, it seems likely that superbugs will not occur as the result of current aquaculture practices, although the risk should not be dismissed altogether. With best practices and new approaches, including the potential afforded by gene editing, the likelihood of multi-drug resistant superbugs in aquaculture can be minimized. — John A. Hargreaves, Editor-in-Chief Superbugs and Aquaculture
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