Cold stress‐induced changes in the aerobic heterotrophic gastrointestinal tract bacterial flora of red hybrid tilapia

Abstract

Fifteen juvenile red hybrid tilapia Oreochromis mossambicus×O. macrochir averaging 50 g body weight were cold‐stressed by immersion in 18° C sea water; control fish (n= 15) were kept at the acclimation temperature of 26° C. Three fish from each group were killed 0, 24, 48 and 72 h after the start of the experiment. Gastrointestinal tracts were removed and dissected into the stomach, anterior gut and posterior gut regions. Aerobic heterotrophic bacteria were identified and enumerated relative to temperature, exposure time and gastrointestinal tract region. Gram negative genera included Alcaligenes, Flavobacterium, Photobacterium, Pseudomonas and Vibrio; the latter three were predominant. Mean bacterial numbers and taxonomic composition of the microflora varied significantly (P<0·05) in response to the three test variables. There were greater mean total bacterial numbers at 18 than at 26° C, due primarily to proliferation of Vibrio spp. Mean bacterial numbers after 24 h were greater than those at both the earlier and later sampling periods. Mean bacterial numbers in the stomach were less than those in the anterior and posterior gut, which were not significantly different. The relative abundance of Vibrio spp. was negatively correlated with that of Flavobacterium, which may have reflected competition between pathogenic and nonpathogenic species. Such stress‐induced changes in the bacterial microflora may contribute to the onset of disease because several species of Vibrio are known primary or opportunistic fish pathogens.