How do probiotic in fish food work?

Probiotic in fish food are becoming increasingly common. However, many aquarists still wonder whether using them in fish diets is truly beneficial. Do fish even need probiotics?

What is a probiotic?

Probiotics are live microorganisms that, when administered in the right quantities, have a positive impact on health. However, in fish, the microorganisms present in the digestive tract are the same as those found on their skin, gills, and in the surrounding water. Another important aspect to consider is that, in most fish species that lay eggs in the environment, hatchlings have no direct contact with the parent’s microflora. Instead, their eggs are colonized by bacteria from the environment, as are the digestive tract, gills, and skin of the fry, which lack their own microflora after hatching.

For this reason, the definition of a probiotic for aquatic animals should be slightly different: a probiotic is a live microorganism that, when administered in appropriate amounts, has a beneficial effect on health by modifying the bacterial flora of both the host and the surrounding environment.

The most commonly used probiotics in fish include Bacillus velezensis (formerly Bacillus subtilis), Lactobacillus acidophilus, Lactobacillus plantarum, Lactobacillus lactis, and Streptococcus thermophilus. Additionally, fungi such as Saccharomyces cerevisiae and Saccharomyces exiguus are also used.

Characteristics of probiotic organisms

In 1989, Roy Fuller outlined the key characteristics that an organism must have to be considered probiotic:

• Resistance to low stomach pH, digestive enzymes, and bile acids in the duodenum.
• Ability to survive and remain metabolically active in the colon.
• Capacity of adherence* to intestinal epithelial cells and colonize the gastrointestinal tract, either permanently or temporarily.
• Beneficial effects on the host organism.
• Durability and viability during storage and in unfavorable environments (e.g., low pH, digestive enzymes, and other factors that inhibit growth or adhesion).
• Origin from humans, fish, or the target animal.

How do probiotic organisms work?

Now that we understand what probiotics are and their key characteristics, how do they function in the host organism? A closer look at their abilities reveals that they are highly sophisticated bacteria-fighting agents, using both physical and chemical strategies to eliminate competing bacteria.

• They compete for surface attachment. To colonize the digestive tract, bacteria must attach to the intestinal epithelium. Probiotic bacteria often exhibit stronger adhesion capabilities than pathogenic bacteria, forming a protective barrier that prevents harmful bacteria from settling.
• They alter the environment by modifying pH levels. Many probiotic bacteria produce organic acids, such as lactic acid, which lower the pH of the digestive tract. This creates unfavorable conditions for pathogens like Escherichia coli, Clostridium sp., and Salmonella sp.
• They release antibacterial compounds. Probiotics produce hydrogen peroxide (H₂O₂), which disrupts the proteins of harmful bacteria by breaking disulfide bonds.
• They deploy biological weapons. Probiotic bacteria secrete bacteriocins, natural antibiotic-like substances that eliminate harmful bacteria. They also produce proteolytic enzymes that break down bacterial cell walls.
• They outcompete harmful bacteria for nutrients. Probiotics thrive in the intestinal environment, gradually dominating the space and limiting the nutrients available to less successful bacterial strains.
• They stimulate the host’s immune system. Probiotics activate the lymphatic system associated with the gastrointestinal tract, triggering the production of immune cells (lymphocytes) and immunoglobulin A (IgA) antibodies. IgA helps neutralize pathogens by preventing bacterial adhesion, aggregating harmful bacteria, and neutralizing toxins and enzymes.

Probiotics in fish food for live-bearing fish

Tropical Pro Defence Size S fish food contains the probiotic bacterium Bacillus velezensis DSM 15544 (formerly known as Bacillus subtilis C-3102). Studies conducted in 2008 examined the effects of this strain in ornamental fish diets, focusing on four common live-bearing fish species: guppy (Poecilia reticulata), platy (Xiphophorus maculatus), swordtail (Xiphophorus helleri), and molly (Poecilia sphenops). The study tested four different probiotic doses, along with a control group.

Analyzed probiotic doses:

• 5 × 10¹¹ CFU Bacillus velezensis/ kg of food
• 5 × 10¹⁰ CFU Bacillus velezensis/ kg of food
• 5 × 10⁹  CFU Bacillus velezensis/ kg of food
• 5 × 10⁸  CFU Bacillus velezensis/ kg of food

	Poecilia reticulata	Poecilia sphenops	Xiphophorus helleri	Xiphophorus maculatus
Number of fish	400	400	400	400
Number of tanks	20 (4×5)	20 (4×5)	20 (4×5)	20 (4×5)
Tank volume	150 liters	150 liters	150 liters	150 liters
Number of fish in a tank	20	20	20	20
Feeding	4% of body weight	4% of body weight	4% of body weigth	4% of body weigth
Duration of the experiment	90 days	90 days	90 days	90 days

Following this study, the researchers conducted an additional experiment in which they infected the test fish with the gram-negative bacterium Aeromonas hydrophila, a common pathogen responsible for ulcerative disease and mass fish mortality in aquaculture.

The results demonstrated that B. velezensis successfully colonized the fish’s digestive tract, leading to greater growth rates in all four species at doses of 5 × 10⁹ – 5 × 10¹¹ CFU/kg of food, along with increased resistance to Aeromonas hydrophila.

A separate study conducted on koi carp reached similar conclusions. Fish fed with Bacillus velezensis exhibited improved growth and greater resistance to the pathogenic bacterium Aeromonas veronii.

How long does the probiotic remain active in feed?

Another critical aspect examined was the stability of the probiotic in stored fish food. The study found that while the number of Bacillus velezensis bacteria gradually declined over time, the reduction was much slower compared to other probiotic strains. This is because Bacillus velezensis forms spores, known as endospores, which provide exceptional resistance.

The sporulation process is highly complex despite involving a single-celled organism. During this process, the nucleoid becomes encapsulated by layers of protective structures, including the cortex (the thickest layer) and a three-layered wall. These endospores can withstand extreme temperatures (over 100°C), drought, UV radiation, and unfavorable pH levels. The germination process, which activates the endospore, is equally intricate and independent of external energy sources.

Probiotics in fish food and reproduction

Interestingly, probiotics have also been shown to enhance fish reproduction. In a year-long study on guppies, platies, swordtails, and mollies, researchers observed increased fertility, a higher gonado-somatic index (GSI), improved fry survival rates, fewer deformities, and faster fry growth.

The effect of probiotic feed on fish immunity

In 2014, Telli et al. investigated the effects of Bacillus velezensis on fish immunity using Nile tilapia (Oreochromis niloticus). The study subjected fish to high-density conditions (50 fish per 800-liter tank) and examined their immune responses.

The number of fish	520 fish
Starting weight of fish	32.63 g
Volume of the aquarium	800 liters
The number of tanks	16
The number of fish in the aquarium (low and high density)	15 50
Feeding	3 times a day to the full
Amount of probiotics in the food	500 mg/kg
Duration of the experiment	84 days

It has been clearly established that probiotics in food have a positive effect on the nonspecific immunity of fish under stress conditions. Fish receiving probiotics exhibited a higher phagocytic index and increased lysozyme activity compared to the control group.

Just for explanation of difficult terms, phagocytes play a crucial role in the innate immune system, recognizing and eliminating pathogen cells. An increase in the phagocytic index in fish fed probiotic food and kept under high-density conditions is a response to environmental stress. Lysozyme, on the other hand, is an enzyme that breaks down the cell walls of pathogenic bacteria. Probiotic organisms can stimulate the host’s immune system to produce more lysozyme and may also produce it themselves.

Summary

As the research shows, probiotics in fish food offer numerous benefits, promoting fish growth, health, reproduction, and immune function. If you want to learn more about choosing the right probiotic fish food, check out Probiotic Food for Aquarium Fish.

Aleksandra Kwaśniak-Płacheta (Ph.D.Eng)

Literature:

Fuller R. (1989) Probiotics in man and animals. J. Appl. Bacteriol., 66, 365-78.

Ghosh S., Sinha A., Sahu C. (2007) Effect of probiotic on reproductive performance in female livebearing ornamental fish. Aquaculture Research, 38, 518-526.

Ghosh S., Sinha A., Sahu C. (2008) Dietary probiotic supplementation in growth and health of live-bearing ornamental fishes. Aquaculture Nutrition, 14, 289-299.

He at al. (2011) Evaluation of probiotic strain Bacillus subtilis C-3102 as a feed supplement for koi carp (Cyprinus carpio), Aquaculture Research & Development, January.

Nowak A., Śliżewska K., Libudzisz Z. (2010) Probiotyki – historia i miechanizmy działania. ŻYWNOŚĆ. Nauka. Technologia. Jakość, 4 (71), 5-19.

Telli G. i inni. (2014) Dietary administration of Bacillus subtilis on hematology and non-specific immunity of Nile tilapia Oreochromis niloticus raised at different stocking densities. Fish & Shellfish Immunology, 39, 305-311.lopment of ovaries and testicles.