Probiotics for larvae; Bacillus as a beneficial gut-bug

Rearing organisms in the laboratory can be a delicate process. Some are hardier than others but there are ways of boosting survivability rates. The larval period is often the most vulnerable of the life cycle and poor environmental conditions or food can negatively affect body size and development. Marine organisms are especially at risk due to their osmoregulatory function. The constant ingestion of the water around them means they don’t have much choice in what microflora they’re taking in. In fact, the bacteria they first ingest is what primarily colonises their once-sterile digestive tract. And they better be happy about what they’ve got because, even after metamorphosis occurs, those are the ones that stick around. Therefore, having a healthy microflora gives the developing larvae a higher chance of survival compared with those that don’t.

This is where probiotics come into play, Bacillus sp. being of particular interest to Hauville et al. (2015) due to its spore-forming ability. Spores are sensitive to environment changes despite their inactive metabolism and are resistant to physical/chemical stress, are heat stable and can survive in low pH. This coupled with their low productive cost means that it’s very cheap and easy to produce lots in labs with minimal effort. Previous studies have found that introduction of Bacillus can cause an increase in gene expression associated with growth metabolism and animal welfare, whilst others show the potential for increased resistance to vibrios. In this one, Hauville et al. (2015) test the effects of a commercial mix of Bacillus on growth and digestive enzyme in Florida pompano, common snook and red drum larvae.

In the control, all three organisms were fed rotifers enriched with Algamac 3050. In treatment 1 (PB) the rotifers were enriched with Algamac 3050 and also a commercial mix of 0.5g of Bacillus spp. per litre. Treatment 2 (PB+) consisted of the same as treatment 1 but with an additional 5g m^-3 of probiotics.

The results showed no significant difference in survival between treatments and species, but survival was significantly higher in pompano and red drum compared with snook larvae. PB and PB+ pompano larvae had significantly greater standard length and body depth than the pompano control and the same occurred for snook larvae although there was no significant difference in body length. Red drum showed no differenced observed between the treatments. Although for some reason, the authors don’t go on to describe what difference occurs in common snook which is odd. For pompano, it was found that the counts of colony forming units (CFU) per larvae were higher on marine agar media for PB and PB+ when compared with the control. The lack of description occurs again for snook and red drum where no comparisons are made and in table 2 it shows only pompano being tested for CFU. It seems incomplete to only include one species. The number of vibrios on the TCBS media were low and there was no significant difference between treatments. In terms of enzyme activity, pompano and snook showed trypsin specific activities which were significantly higher in PB and PB+ in comparison to the control. Amylase activity was different only in snook, with PB+ higher than in the control. Alkaline phosphatase (AP) of pompano (PB and PB+) and snook (PB+) was higher than their controls.

The results show that Bacillus as a probiotic has beneficial effects on growth and digestive enzyme activity. The Bacillus mix given for this study involved three different strains; B. pumilus, B. lichenformis and B. amyloliquefaciens and are all known to be closely related to B. subtilis. These strains have different metabolic abilities. B. pumilus produces amylase and cellulose whilst also having strong inhibition factors against Vibrio sp. and B. lichenformis has antiviral properties and B. amyloliquefaciens is closely related to B. subtilis. Hauville et al. (2015) discuss previous studies where B. subtilis, B. lichenformis or B. pumilus were added to the diet of olive flounder. B. subtilis enhanced growth, B. subtilis and B. pumilus increased survival rate and B. pumilus and B. lichenformis increase superoxide dismutase activity and disease resistance. This showed how beneficial it was to supplement the diets with multiple strains of the bacteria simultaneously.

This paper is looking at the benefit of using Bacillus sp. as a probiotic in three different organisms, but in many of their tables and figures they only include comparisons with either one, or two and use a different table or figure for another. This seems a very convoluted way of presenting the information rather than just using separate comparison tables for e.g. standard body length and specific activities separately. However, it could be due to the fact that there were no statistical differences observed in red drum larvae as opposed to snook and pompano. 

Ref; Hauville, M. R., Zambonino-Infante, J. L., Gordon Bell, J., Migaud, H. & Main, K. L. (2015) Effects of a mix of Bacillus sp. as a potential probiotic for Florida pompano, common snook and red drum larvae performances and digestive enzyme activities. Aquaculture nutrition. 

doi: 10.1111/anu.12226