Effects of dietary organic acids on growth, nutrient digestibility and gut microflora of red hybrid tilapia, Oreochromis sp., and subsequent survival during a challenge test with Streptococcus agalactiae

A 14-week feeding trial was conducted to determine the effects of dietary organic acids. The experimental diets were added with 0, 1, 2 or 3 g kg(-1) of a novel organic acid blend or with 2 g kg(-1) of potassium diformate and fed to triplicate groups of red hybrid tilapia (Oreochromis sp.). Upon completion, tilapia were challenged by immersion with Streptococcus agalactiae. There was no significant difference (P > 0.05) in the growth, feed utilization and nutrient digestibility among treatment groups despite a trend towards improved results with fish fed organic acid-supplemented diets. Diet pH decreased, causing a reduction in the digesta pH of the stomach and gut. Total bacteria per gram of faeces were significantly (P < 0.05) reduced from 1.81 x 10(8) colony-forming units (CFU) (control group) up to 0.67 x 10(8) CFU in the fish fed organic acid diets. A similar trend was observed for adherent gut bacteria. Cumulative mortality of fish fed no organic acids was higher compared with fish fed organic acid-supplemented diets at 16 days post challenge. The data showed that dietary organic acids can exert strong anti-microbial effects and have the potential to exert beneficial effects on growth, nutrient utilization and disease resistance in tilapia.