Probiotics are classically defined as "living microbes that when administered orally to an animal, colonize the animal’s intestinal tract and impact the animal’s health in a favorable manner. "
This word is frequently misused in aquaculture and an analysis of the literature (gray and peer-reviewed) suggests that the term may not even be remotely applicable for many types of aquatic animals. There are very few reproducible accounts of bacterial attachment in a consistent, long-term manner to the intestinal walls of fish or shrimp that have been shown to prevent the attachment of pathogenic bacteria to these same sites. The scientists who have spent the most time in this field are of the opinion that many of the effects attributed to the application of the various bacterial preparations that have been looked at are in fact simply a result of stimulation of the innate or non-specific immune system, an indirect effect. This would not require living cells.
Some 30 plus years ago, an early vendor of liquid products hijacked the term probiotic for use in aquaculture in an effort to sell their product and make it appear distinctive from what it really was, a microbial tool for bio-remediation. This is unfortunate as the term stuck and to this day contributes to what is already a dismal environment for farmers to find true solutions for many of the problems that plague them.
Many commercially available products contain low levels of bacteria, bacterial species that are likely dead at the time the product is being used, bacterial species that cannot possibly do what the vendors claim that they do, may include substances that appear to cause an effect (such as antibiotics or other adulterants), etc. While it is possible to sell bacterial preparations that do not consist of bacterial spores from gram-positive bacteria, many of these products would be costly, require refrigeration, have extremely short shelf lives or be acid stabilized liquids that involve selling products that are largely water.
Microbial "Probiotics" functionality falls generally into the following categories:
Bioremediation: Bacterial species are added to ponds to degrade organic matter and limit nutrients that other bacteria such as vibrios and cyanophytes (blue-green algae) require for growth inhibiting them.
Immune Stimulation: Bacteria are fed to the fish or shrimp to enhance their ability to non-specifically resist a subsequent bacterial onslaught.
True Probiotics: Based on the definition above this is not straightforward. Shrimp and fish are colonized by bacteria present in their environment.
There is no evidence to support stable colonization much less a reproducible impact on the health of the host.
As a marine microbiologist with a Ph.D., my perspective on the current state of the science has evolved over 40 years of experience in the field and the lab. One of my conclusions, reflected in our approach to this market is:
Each pond is different and effective programs need to take this into account.