WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2023 49 While the term ‘probiotic’ was originally restricted to Grampositive lactic acid bacteria (including Lactobacillus, Bifidobacterium or Streptococcus), transient, pathogenic Grampositive bacteria (such as Salmonella, Listeria and Escherichia coli) were also identified in the gastrointestinal tract (GI) of aquatic animals, in addition to probiotic bacteria (Bacillus, Carnobacterium, Enterococcus, Lactobacillus) and Gramnegative bacteria such as Vibrio and Pseudomonas, along with fungi (e.g., Debaryomyces), yeasts (e.g., Saccharomyces) and algae (e.g., Tetraselmis) (Iranto and Austin 2002, Burr et al. 2005, Vine et al. 2006). Of these, Lactobacillus sp. inhibit the adhesion of fish pathogens to the gut walls in vitro (Candela et al. 2008), increase immune functions in brown trout, groupers and zebrafish, decrease fish stress levels (Rollo et al. 2006, Hunt von Herbing et al. 2017), improve growth rate and disease resistance (Macey et al. 2005) and reduce skeletal deformities from malnutrition (Avella et al. 2010). Investigations of the intestinal microflora of wild caught Atlantic cod Gadus morhua have shown that wild cod, after having been starved post-capture and subsequently fed typical farmed cod feed for six weeks, experienced a significant reduction in the number of bacterial species in their intestines compared to freshly caught fish (Dhanasiri et al. 2011). These findings suggest that beneficial bacteria are being lost in farmed fish, leaving them susceptible to malnutrition and disease, leading then to the question “Can probiotics be used as an alternative to antimicrobials?” Using Microbial Ecology for Globally Sustainable Aquaculture Ione Hunt von Herbing, Wren Busby, Michael Anderson, Victoria Youngblood and Bailu Zhang What are Direct-fed Microbials? To mitigate the impact on the environment and improve fish welfare in aquaculture, new technologies such as direct-fed microbials can be employed to reduce the impact of stressors at the farm. Probiotics are “good bacteria,” i.e. live microorganisms beneficial to the host organism, and are an example of direct-fed microbials. Fuller (1989) defined probiotics as “live microbial food supplements that benefit the host (human and animal) by improving the microbial balance of the body.” Gatesoupe (1991) further defined probiotics as “microbial cells administered in a certain way, which reaches the gastrointestinal tract and remains alive with the aim of improving health.” Definitions of probiotics expanded as knowledge about their effectiveness and function were documented. Aquatic animal health is intrinsically linked to the natural microbial populations of the intestines. Application of this knowledge to animal food production systems is limited because we still don’t know details of how the intestinal microbial community works. What Do Gut Microbes Do? Investigations of the gut flora have revealed high diversity. Estimations of the total number of different microbial genes in the gut are 100 times that encoded by the human genome (Gill et al. 2006). An animal could therefore be considered a ‘supra-organism’ or holobiont (Greek, from holos (όλος), whole; bios (βίος), life), living in a dynamic, commensal relationship with their microbes (Turnbaugh et al. 2007). Given there are so many microbes intimately associated with the gut we can ask “what do they actually do?” FIGURE 1. A) Juvenile Mozambique tilapia Oreochromis mossambicus at 28 days after hatching from three treatments, T0 = 0 CFU/kg (control, no probiotics), T1 = 5 x 1011 CFU/kg (manufacturer’s recommended dose) and T2 = 10 x 1011 CFU/kg (double the manufacturer’s recommended dose). B) Changes in mean wet mass (g) of juveniles raised for 28 days with five probiotic treatments. T0 = 0 CFU/kg (control, no probiotics), T1= 5 x 1011CFU/kg (manufacturer’s recommended dose), T2 = 10 x 1011CFU/kg (double the manufacturer’s recommended dose), T3 = 15 x 1011CFU/kg (triple the manufacturer’s recommended dose) and T4 = 20 x1011CFU/kg (four time the manufacturer’s recommended dose). From: Hunt von Herbing (2017 and Anderson (2019. (CONTINUED ON PAGE 50)
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