To test the effects of probiotic bacteria against
crowding stress, juvenile Nile tilapia Oreochromis niloticus
were fed commercial feed supplemented with 1 9 1010 cfu/
g pellets of Lactobacillus rhamnosus for 3 weeks. The fish
were confined and subjected to crowding for 7 and 14 days
after which their physiological condition was investigated.
The administration of probiotic prophylaxis improved
growth performance even in the stressed fish. After 7 days
of stress exposure, fish receiving probiotic-supplemented
feed showed proactive behavior and coping responses to the
stressor, as evidenced by elevated plasma glucose levels
and osmolality, stabilized plasma electrolytes, and a higher
RNA:DNA ratio. Fish stressed for 7 days and fed normal
commercial diet showed impairment of plasma electrolytes;
after 14 days, the plasma osmolality, electrolytes, glucose,
and RNA:DNA ratio decreased, indicating physiological
maladaptation to the stressor. The growth rate was also
found to be reduced, suggesting that energy demand
exceeded the energy available from metabolism and dietary
uptake. This latter effect was not observed in fish fed the
probiotic-supplemented diet. The results suggest that probiotic
bacteria prophylaxis may increase energy availability
for metabolic support of the crowding stress response and
improve the stress coping capacity of fish