Page 20 - New additives and ingredients in the formulation of aquafeeds

(1976) the stomach pH of channel catfish ranges approximately from 1 to
3 proximally and from 3 to 4 distally, while pKa values for butyric, lactic
and formic acids are 4.82, 3.83 and 3.77, respectively (Dibner and Buttin,
2002). These values suggest that while the degree of undissociation, and
consequently antimicrobial efficacy, in proximal stomach of catfish is similar
for the three acids, it is greater for butyric as pH alkalinity increases further
down the digestive tract.
Table 6. Minimum inhibitory concentration (in mM) of SCFA against common
pathogens affecting fish and shrimp species
Pathogen Butyric Propionic Valerianic
V. anguillarum 22.2 33.3 33.3
V. vulnificus 22.2 33.3 33.3
V. harveyi 22.2 22.2 33.3
V. parahaemolyticus 22.2 22.2 33.3
V. alginolyticus 22.2 22.2 33.3
A. hydrophila 22.2 33.3 33.3
A. caviae 22.2 33.3 33.3
A. salmonicida 22.2 22.2 33.3
F. columnare 22.2 22.2 33.3
Y. ruckeri 33.3 33.3 50
Note that pH reduction can also determine survival of bacteria; however, the similar pKa value among
butyric, propionic and valerianic (4.82 vs. 4.88 vs. 4.82) indicates a greater inhibitory activity of butyric
by diffusing through the membrane of gram - bacteria
The increasing pH alkalinity as going distally along the intestine leads to
a reduced presence of the butyric forms and the subsequent efficacy to
diminish pathogenic flora but at the same time favours the presence
of butyrate -or dissociated form of the acid-. The stimulatory effect of
butyrate on intestinal epithelium development in different animal species
has been demonstrated both in vivo and in vitro conditions, elucidating
greater epithelial cell proliferation with the administration of sodium
butyrate when compared to other salts as sodium acetate and sodium
propionate (Sakata and Tamate 1978, 1979; Sakata, 1987). In a study
conducted with pigs, dietary supplementation of 0.17% sodium butyrate
increased cell proliferation in ileum by 33%, resulting in 30% increase in villi
length (Galfi and Bokori, 1990). This stimulatory effect can be interpreted
as a combination of direct and indirect mechanisms along the different
sections of the intestine (Sakata and Yajima, 1984). While the caloric
content of butyrate is higher than that of propionate and acetate, and it is
preferentially oxidized by the epithelial cells when compared to other energy
sources (Roediger, 1982; Roediger and Rae, 1982; Ardawi and Newsholme,
1985), butyrate also stimulates insulin secretion, which consecutively exerts
a stimulatory effect on intestinal epithelium (Jordan and Phillips, 1978). As
18 Natural growth promoters in aquaculture practices