Contaminated meal intake (Aeromonas hydrophila) does not elicit hormonal or immune modulation in bullfrogs (Lithobates catesbeianus)

Abstract

The gastrointestinal tract (GIT) is colonized by resident microbiota but contact with foreign microbiota during feeding can impair GIT functions. During meal digestion, several vertebrates modulate the systemic immune function and immunoregulatory hormones concentration. However, in ectothermic animals, it is not known if this hormonal and immune modulation during the postprandial period is affected by the presence of pathogenic microbiota in the food. This study aimed to investigate the effects of contaminated meal ingestion on hormonal and innate immune responses in bullfrogs (Lithobates catesbeianus). Bullfrogs were divided into three treatments: fed three times with sterilized fish feed (control group), fed twice with sterilized fish feed and once with fish feed containing live bacteria (Aeromonas hydrophila, 109 UFC/mL), and fed three times with fish feed containing live bacteria. Blood and GIT tissues were collected 24 h after treatments to measure plasma and tissue corticosterone levels, NL ratio, and plasma bacterial killing ability. The ingestion of contaminated meal did not affect the hormonal and immune parameters. In conclusion, ingestion of contaminated food was not capable of intensifying the hypothalamic–pituitary–interrenal axis activation and the consequent hormonal and immune responses observed after feeding in bullfrogs. However, our results suggest that the ingestion of three contaminated meals tended to decrease stomach corticosterone levels (nonstatistically significant), possibly contributing to preventing the transmigration of the bacteria to organs outside the GIT.