Page 13 - New additives and ingredients in the formulation of aquafeeds

Natural growth promoters in aquaculture practices 11
bacteria load, enhanced disease resistance, healthier absorptive surface
area, and consequently better nutrient utilization. These host benefits
may be explained by different mechanisms of action such as secretion
of exo-enzymes, organic acids and other antimicrobial compounds, as
well as by competition for nutrient and attachment sites of intestinal
cells (Kesarcodi-Watson et al., 2008). Most recent information reveals an
immune stimulating effect of some Bacillus spp. by increasing the activity
of phagocityc cells and lysozyme (Yung-Zhang et al., 2010).
Water and soil quality management is considered one of themost important
aspects of pond aquaculture. While water quality parameters such as
dissolved oxygen, pH, or ammonia concentrations affect essential body
functions, the condition of pond bottoms and the exchange of substances
between soil and water strongly affect water quality. Low dissolved oxygen
level is the major limiting water quality parameter (Boyd and Tucker, 1998).
Waste accumulation will increase oxygen demand by bacteria in order to
performorganicmatter break down, consequently favoring the development
of anaerobic patches. These anoxic sediments will contact the more oxygen
enriched water column leading to disruption of the adequate environmental
conditions supporting healthy growth. Additionally ammonia may diffuse
into thewater column not only as a result of fish excretions but also following
microbial decomposition of organic material. In conditions of high pH and
temperature, ammonia accumulation can cause sub-lethal effects as poor
growth or disease proneness (Hargreaves and Tucker, 2004). Therefore, it is
of key importance to maintain pond bottom under aerobic conditions and to
avoid organic matter and slime. In this sense, Bacillus spp. is considered a
bioremediation agent, concept referring to the use of microorganisms that
help to maintain proper pond environment by the secretion of exo-enzymes
that break down organic matter (Moriarty, 1997). In addition, Bacillus spp.
may use ammonia nitrogen as nitrogen source for their growth (Ghosh et
al., 2010). Many of the benefits reported by Table 1, particularly increased
survival, are the consequence of improved pond bottom conditions as a
result of reduced concentration of organic matter and ammonia. These
improvements are generated not only by direct application into water but
also when Bacillus spp. are included in feed, as a significant portion of
the probiotic leaches from the pellet and feces becoming a component of
the pond ecology. While water quality is also assessed by pathogen load,
it has been reported that Bacillus spp. can improve water quality reducing
pathogenic bacteria load and consequently balancing bacterial population in
water (Vaseeharan et al., 2004). This is achieved by competitive exclusion
based on secretion of antimicrobial compounds as well as competition
with other pathogens for nutrients (Merrifield et al., 2010). Likewise, the
direct application of probiotics into the water results in reduced improved
microbiota balance at the host’s intestinal level, indicating the ability of
probiotics to reach the animal’s gastrointestinal tract (Li et al., 2007).
Norel manufactures and markets probiotic products containing Bacillus
spp. as B. amyloliquefaciens, B. licheniformis , and B. cereus. Although the
combination of these bacteria is also commercially available for aquaculture