* By: Stephen Newman
Antibiotics are chemicals that interfere with microbial (bacterial and fungal) metabolic processes, altering their fitness to survive. The proper use of antibiotics, largely ignored not just for aquaculture but in the treatment of humans as well, entails isolating the bacteria responsible for a given disease outbreak. Using the wrong antibiotic for the specific problem with an improper dosage ensures that selection pressures on the bacterial population drive it towards resistance. Many aquaculturists abuse antibiotics although it is not done everywhere and by everybody.
Bacteria are complex organisms. We are continuing to learn about how important they are in health and disease. Studies suggest that there is a complex arrangement between the host and the microbiome (the assemblage of bacteria that are present in a given environment). Some believe that the specific composition of the microbiome is what makes animals healthy.
Others believe that this composition is not responsible for “good” health. These assemblages allow bacteria to act as groups that differ from how they act singularly. These are known as biofilms. Biofilms are collections of bacteria that are present at high densities and are protected from many environmental impacts by being in a biofilm including some antibiotics and disinfectants.
Shrimp farmers are often heavily focused on the role of vibrio species in shrimp disease. This is for good reason. This genus of bacteria is ubiquitous in marine and freshwater aquatic environments where they play an important role in the recycling of chitin, a major component of the carapace of shrimp and all other crustaceans and insects.
Cellulose, a biopolymer found in plants is the most abundant and chitin is the second. There are approximately 150 Vibrio species that have been identified to date. While three species are responsible for most of the impact on humans, Vibrio cholerae (yellow on thiosulphate citrate bile salts sucrose, TCBS), V. parahaemolyticus (green on TCBS) and V. vulnificus (green blue on TCBS), only specific strains cause disease.
Many strains are benign because they do not contain the toxins that cause the disease. This could be due to the lack of the specific genes, loss of gene integrity, production of a defective gene product, etc. A dozen or so species, again specific strains, cause disease in shrimp and fish.
These include V. alginolyticus (yellow on TCBS), V. campbellii, V. parahae- molyticus, and several others. Some are obligate and others opportunistic. Obligate pathogens usually produce acute disease, and it can take a very low level of bacteria to initiate the process in healthy animals. Opportunistic pathogens typically require weakened animals, usually as a result of other infectious disease processes although there are many things that can weaken animals.
Any given bacterial population is composed of many different strains. Those strains we can culture are not always the strains that are causing the impact that we are attempting to correlate with the presence of a specific pathological process. Strains vary in their genes and gene expression.
Given genera (like Vibrio) and species (like V. parahaemolyticus) share sufficient traits for them to be considered related to each other. Yet individual strains within the species often vary considerably. This causes considerable confusion among laymen. For vibrios, it has resulted in several widespread myths.
Perhaps the one that does the most harm ultimately is based on the ability to digest sucrose, a sugar molecule that is composed of two sugars, glucose and fructose. This sugar is abundant and linked to a variety of health issues in humans. When vibrio’s utilize sucrose as a nutrient they produce organic acids as a by-product.
These change the pH and on the selective, differential media, TCBS agar, results in yellow colonies. If they do not readily utilize it, one sees green colonies. The ability to use sucrose is not related in any way to the ability to produce disease. Focusing on yellow versus green is not going to eliminate vibrio disease. I have written elsewhere about this and what can be done to lessen the incidence and severity of vibrio disease processes.
Antibiotics are chemicals that interfere with microbial (bacterial and fungal) metabolic processes, altering their fitness to survive. They are not antivirals (although there are some compounds that have antiviral as well as antibacterial activity). Many dozen have been approved for use in humans.
A handful only have been approved for use in aquaculture and in the US (https://www.fda.gov/animal-veterinary/aquaculture/approved-aquaculture-drugs), which also regu- lates what antibiotics can be used on product imported into the US, only a few are approved for very specific usages.
For a detailed explanation of how they work, how resistance occurs, etc. we suggest you visit this link (Saloni Dattani, 2024 – “How do antibiotics work, and how does antibiotic resistance evolve?” Published online at OurWorldinData.org. Retrieved from: ‘https://ourworldindata.org/how-do-antibiotics-work’). There seems to be an inordinate amount of focus on the abuse of antibiotic usage in aquaculture despite the fact that the majority of the abuse is in human therapy and other terrestrial agriculture sectors.
Aquaculture is still largely an immature industry. The use of antibiotics is not as much of a concern for domestic consumption as it can be for export. Much of the global production takes place in less developed countries with some notable exceptions such as the farming of salmon mainly produced in Norway and Chile.
Farmed shrimp production, largely for export, takes place in Ecuador, India, Vietnam, Indonesia and others. There is no one consistent approach and the paradigms are, it seems, in a constant state of change. Disease is a major impediment to sustainable production. Many diseases are viral in origin with secondary infections, often from vibrios.
Primary disease from bacteria is still of course a challenge. Most experienced aquaculture pathologists will tell you that much of what is killing shrimp are mixed infections.
The proper use of antibiotics, largely ignored not just for aquaculture but in the treatment of humans as well, entails isolating the bacteria responsible for a given disease outbreak. If this is a new unreported strain or species, there must be a clear-cut science-based correlation between its presence and the observed disease process.
The isolates are screened using standard methods for their susceptibility to a range of antibiotics. This will determine what the best antibiotic to use is. The antibiotic needs to be used at the appropriate dosage and dura- tion for optimum effectiveness. For aquaculturists, the problem with all of this is time.
Some diseases appear to move through the population very rapidly making the time between seeing moribund animals (birds can be important in alerting farmers to this) and the population going off feed very short. Panic sets in and the farmers use the antibiotics that are readily available without determining whether they are appropriate.
As with human medicine where many of the “older” antibiotics have little to no effectiveness, the tendency is to use the latest most potent antibiotics. Using the wrong antibiotic for the specific problem with an improper dosage ensures that selection pressures on the bacterial population drive it towards resistance.
While biofilms can function to protect bacteria from the action of a wide range of antibiotics, resistance to antibiotics is natural. I refer you to the above (Saloni Dattani, 2024) link which discusses the various mechanisms by which this can occur. All bacteria produce antimicrobial compounds. This is an essential component of their ability to survive and thrive in an environment where there are vast numbers of bacteria competing for nutrients.
Antibiotics can leave residues in the flesh of treated animals. This is a major reason why antibiotics need to be tested and evaluated not just for efficacy against specific organisms, but for the ability of the animals that they are being used on to metabolize them. This impacts dosages, the duration of treatment, and withdrawal times. This is the amount of time needed after the antibiotic treatment is ended for the levels of metabolic residues to be below legal threshold levels.
Only a few antibiotics and specific residues are routinely tested for in farmed shrimp at the time of harvest/processing. The vast majority of them are not. Most of the antibiotics that are used in acts of desperation are typically not legal for use in aquatic animals in the countries that the final product is being exported to.
Should regulators determine that the risks of these residues are a serious issue then the list of antibiotics and metabolites being tested for could be expanded dramatically. In general, when ex- porting shrimp treated with antibi- otics have been used, it is important to ensure that the specific antibiotic being used is approved for use in the destination country or at the very least that sufficient time has passed to ensure that there are no detectable residues.
Responsible use of antibiotics is something that benefits all of us. It ensures that resistance develops slower. For the most part, resistance is inevitable because of the very nature of bacteria. Even tolerance is problematic because it requires higher dosages for the desired impact. Many aquaculturists abuse antibiotics although it is not done everywhere and by everybody.
The term sustainability has become a marketing phrase with little to no meaning. For aquaculture it means being able to produce a product economically without negatively impacting the production environment. It encompasses waste stream management and progressive disease prevention and mitigation programs among other things. Having effective antibiotics that are used in a responsible manner is an essential component of this process.
* Stephen G. Newman has a bachelor’s degree from the University of Maryland in Conserva- tion and Resource Management (ecology) and a Ph.D. from the University of Miami, in Marine Microbiology. He has over 40 years of experience working within a range of topics and approaches on aquaculture such as water quality, animal health, biosecurity with special focus on shrimp and salmonids. He founded Aquaintech in 1996 and continues to be CEO of this company to the present day. It is heavily focused on providing consulting services around the world on micro- bial technologies and biosecurity issues. sgnewm@aqua-in-tech.com www.aqua-in-tech.com www.bioremediationaquaculture.com www.sustainablegreenaquaculture.com.
