The previous posts of this blog have gone over what microbes are, how one will find microbes in their pond, and finally a few methods in which to eliminate microbes. Though it is impossible to completely eliminate microbes from a pond or any environment, the numbers can be reduced by the use of cleaning agents (How to Properly Use a Cleaner – Things One Should Know about Disinfectants). There is also another way in which microbes can be controlled if not necessarily eliminated. This may be with the use of probiotics.
Lately, there has been a lot of buzz about probiotics. Many attribute health benefits to their ingestion, such as those that may be found in some buttermilk, sauerkrauts, fermented cereals, salamis and yoghurts. Probiotics have been found to reduce the incidence and duration of rotavirus-caused diarrhoea in infants and diarrhoea that is caused by taking antibiotics in adults. The effects of probiotics and how this is caused are recently coming to light in science, but what can the pond enthusiast gather from this? How can a pond be cleaned and maintained by adding more germs?
As always, first comes defining what a probiotic is and the terms associated with the subject. The term “probiotic” was first used in 1965 to describe “substances secreted by one microorganism which stimulates the growth of another”. This definition was used mainly to contrast antibiotics which, instead of stimulating, inhibit the growth of another microbe. A more modern definition is that a probiotic is a viable mono- or mixed culture of microorganisms which applied to animals or humans, beneficially affects the host by improving the balance of the bacteria found in the gut. Some though deem that probiotics may not necessarily be viable, but can be simply any microbial preparation that would benefit the host upon ingestion. Generally, though, probiotics are comprised of live microbes and therefore the constituents of microbes such as polysaccharides, proteins and DNA, and dead microbes are often excluded from the term “probiotic”. Another term may encompass the non living components of probiotics, but only if they fulfill the requirement of being able to select for the growth of specific microbes in the gut of a host. This is a prebiotic. A prebiotic is a non-digestible food ingredient that beneficially affects the host by selectively stimulating the growth of one or a limited number of bacteria in the colon to improve the health of the host. Many prebiotics can come from bacterial preparations, but some come from plant, and yeast preparations. The prebiotic is often mixed with the probiotic in a single preparation so the microbe which the prebiotic is intended to stimulate is already present for maximal results. This is referred to as a symbiotic, which alludes to the synergism one hopes to obtain by adding both products simultaneously.
Prebiotics and probiotics, therefore, are intended to increase a certain microbe in numbers in order to elicit a beneficial effect. Not everything is yet known on how the increased presence of probiotics can elicit this effect, but some ideas are in place. The possible modes of action include:
1. The production of inhibitory compounds:
Some microbes produce antibiotics, hydrogen peroxide, siderophores (compounds that bind to iron reducing its availability for other microbes which require it), organic acids, ammonia, diacetyl, enzymes which can interfere with the functioning of other microbes and degrade them (proteases, lyzozymes), bacteriocins (protein toxins produced by bacteria that inhibit the growth of similar or closely related bacterial strains), and can alter the pH of the environment. All of these products of probiotics can make an environment unfavourable to other microbes while ensuring the probiont has a replicative advantage. The idea of microbes competing in the same environment was already discussed in the second post “No Microbe is an Island: Biofilms”, so the idea of microbes having a competitive advantage in a mixed culture over others is no new concept. Probiotics simply would utilize these bacterial assets to the benefit of the host or pond enthusiast.
2. Competition for chemicals, available energy and adhesion sites:
Adding probiotics to increase their numbers may limit nutrients and energy sources for bacteria that may later cause a problem such as pathogenic bacteria in the gut. In the instance of gut bacteria, they may also take up the limited adhesion sites needed to permit the colonization of a host.
3. Alteration of microbial metabolism:
The introduction of probiotics to an environment has been shown to alter the way bacteria already present utilize nutrients, but how this produces favourable outcomes has yet to be determined.
4. Stimulation of host immunity:
The addition of probiotics may result in increased resistance of the host to pathogens. Some believe that this may be because the presence of probiotics can prime a host’s immune system for when the actual pathogen will present itself, though further research must be done to elucidate these mechanisms.
5. Enzymatic contribution to digestion:
A way that probiotics may be beneficial is that their enzymes, which may not be present in the host, can digest certain compounds that would otherwise be indigestible. This would therefore provide a new source of nutrients. Enzymatic digestion from probiotics may also contribute to the breakdown of toxic compounds.
6. Source of macro- and micronutrients:
Enzymatic digestion may assist in creating a source of nutrients, as mentioned in the previous reason, which can be a source of micronutrients for the host and the pond. Macronutrients, such as vitamins, may also be synthesized by probiotics which may produce positive results.
Most of the examples given so far have been with respect to probiotics being ingested to benefit a host like humans, but how do probiotics and their possible uses apply to a pond? In aquaculture, or in a pond, the microbial habitat undergoes many more alterations than in the more sheltered habitats within the host gut. In a pond, depending on the time of year, weather and life forms that may already be present, factors can fluctuate like the salinity, temperature and oxygen concentration. Also, microbes present in ponds will differ much more than those in the gut of terrestrial animals and those that may reproduce the most efficiently and colonize the pond may simply be a matter of being in the right place at the right time. The definition of a probiotic in the case of aquaculture changes slightly though. It is still an additive of lives microbes, but rather than the result of improving the health of a host, they serve the purpose of improving the quality of the ambient environment or the water by modifying the microbial community already present. Probiotics can be used to improve the health of a host too, such as fish or other aquatic animals, but this is by changing the ambient microbial community in the case of ponds or aquaculture. For example, probiotics have been used since the 1990s in shrimp hatcheries. In this case, probiotics may also reduce disease, improve the use of feed and enhance its nutritional value. Therefore, in the pond, depending on its purpose and what resides in it, such as whether it is a decorative pond with just water or a few plants or whether it is a more natural pond with fish or other animals, the kinds of probiotics used may vary.
Since adding microbes to a pond does not directly affect a host due to the aquatic nature of the environment and serves more to alter the ambient microbial environment of a pond, the addition of probiotics may also be referred to as biocontrol. Biocontrol is the limitation or elimination of parasites or specific pathogens by the introduction of adverse microorganisms. This is often microbial treatments that target other microorganisms, like algae. Probiotics in a pond may also assist in bioremediation or bioaugmentation, when microbes are added to a pond to treat and eliminate waste and pollutants (Figure 1).
Figure 1: Biocontrol, bioremediation and bioaugmentation in a pond. Live microbial preparations, when added to a pond, can beneficially affect a host in a pond like shrimp. However, unlike the case with probiotics, this is not done by directly ingesting the probiont but rather by affecting the aquatic microbial community. So, the addition of these microbes to a pond is often referred to as biocontrol. Bioremediation, or bioaugmentation, is when the microbes added improve the water quality by breaking down pollutants and waste.
There are several effects one may expect when adding microbes to a pond. Adding certain microbes may enhance decomposition of organic matter created by plants or fish among other life forms, reduce the concentration of nitrogen and phosphorous which often enhances algal growth therefore algal blooms may be stabilized, and the availability of oxygen may be increased. Nitrifying bacteria may be added to ponds to control and reduce the concentrations of ammonia or nitrite by converting these compounds to nitrate, which is less toxic for a pond. Some reports claim that many bacterial strains may be able to reduce algal populations like those of red tide plankton and cyanobacteria (described in the last post “Phycology: The Forgotten Field of Study and the Pond”).
With all the potential functions of biocontrol in ponds, many types of preparations exist. One of the most common preparations is live bacteria supplemented with yeast extracts and extracellular enzymes in order to ensure plenty of nutrients for the bacteria and host, if necessary. Some preparations contain only the enzymes, while others contain extracts of plants such as citrus seed and yucca extracts. These preparations generally have to be added at regular intervals though because a pond already carries a well established or stable microbial community. Adding the preparation on a regular basis ensures the desired microbe’s continual dominance over others. It is also unlikely that a single bacterial species will become dominant unless added continuously in huge quantities; therefore, many preparations have several bacterial species in order to better ensure that the microbial community of choice will remain dominant.
Probiotics, biocontrol, and bioremediation are all attractive methods to improving the day to day lives of pond enthusiasts, among other people. However, there is still much to be learned about the microbial candidates and how they would produce a desired effect. Some unanswered questions include what is the best way to introduce probiotics and their optimal dose, as well as which microbes would do the best job for the specific task at hand. There are some promising candidates though, and if you speak to us at Village Pond and Garden we can determine whether or not a probiotic regimen or biocontrol is right for your pond.
In the next instalment →Physical Methods to Kill Microbes or Taking Microbes by Force
• A probiotic is a viable mono- or mixed culture of microorganisms, which applied to animals or humans, beneficially affects the host by improving the balance of bacteria in the gut.
• A prebiotic is a non-digestable food ingredient that beneficially affects the host by selectively stimulating the growth of one or a limited number of bacteria in the gut to improve the health of the host.
• Probiotics and prebiotics can be added together in order to obtain a synergistic effect of having the selective compound and the bacterium for which it will select present. This is referred to as a symbiotic.
• Possible mechanisms for probiotics include the production of inhibitory compounds; the competition for nutrients, energy sources and adhesion sites; the alteration of the microbial metabolism, the stimulation of host immunity, the enzymatic contribution to digestion, and probiotics may be a source of macro- and micronutrients.
• In the instance of ponds, adding live microbes in order to control a microbial population is referred to as biocontrol. This is because the microbes are not directly added to the gut of the host due to the nature of the aquatic environment, and some treatments may not even target animal populations but may target other microbial populations such as algae.
• Probiotics in a pond can assist in bioremediation/bioaugmentation, which is when microbes are added in order to treat and eliminate waste and pollutants.
• In the case of ponds which generally have established microbial communities, microbe preparations will have to be added on a regular basis in order to ensure the desired microbe’s continual dominance.
• To ensure the desired continual dominance of select microbes, preparations often contain several bacterial species for biocontrol.
• There is still much to be learned about probiotics, biocontrol and bioremediation. What remains to be seen is the most efficient way to introduce them and the optimal doses. The best microbial candidates for specific tasks have yet to be determined.
Boyd, CE., Gross, A. 1998. Department of Fisheries and Allied Aquacultures Auburn University, Alabama USA 36849.
Gatesoupe, FJ. 1999. The use of probiotics in aquaculture. Aquaculture 180. 147–165.
Irianto, A., and Austin, B. 2002. Probiotics in aquaculture. Journal of Fish Diseases, 25, 633–642.
Rowland, I., Capurso, L., Collins, K., Cummings, J., Delzenne, N., Goulet, O., Guarner, F., Marteau, P., and Meier, R. 2010. Current level of consensus on probiotic science. Gut Microbes. 1(6): 436–439.
Schrezenmeir, J., and de Vrese, M. 2001. Probiotics, prebiotics, and synbiotics—approaching a definition. Am J Clin Nutr 2001;73(suppl):361S–4S.
Shanahan, F. 2010. Probiotics in Perspective. Gastroenterology. 139 (6): 1808-1812.
Verschuere, L., Rombaut,G., Sorgeloos, P., and Verstraete, W. 2000. Probiotic Bacteria as Biological Control Agents in Aquaculture. Microbiology and Molecular Biology Reviews, p. 655–671.