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It is postulated that the effect of antibiotics on intestinal commensal microbiota can differ with the category of the antibiotics. These antibiotics, added into the piglet diet for the therapy of bacterial respiratory infections, can differently affect the composition of the microbiota and also can eventually have a growth promoting effect in healthy weaned pigs. The aim of this study was to determine the effects of 3 antibiotics used for pulmonary pathologies added in the feed of weaned pigs on growth performance, commensal microbiota, and immune response. At weaning, a total of 72 pigs were randomly assigned by BW and litter to 1 of the following diets: control (typical weaning diet), control + 400 mg of tilmicosin/kg, control + 600 mg of amoxicillin/kg, and control + 300 mg of doxycycline/kg. Individually penned pigs were slaughtered after 3 weeks (wk) (12 pigs/treatment) or 4 wk (6 pigs/treatment). During the fourth week, all pigs received the control diet to test the residual effect of the antimicrobial supplementation.
The antibiotic supplementation increased growth and feed intake during the first week (P < 0.01) and over the first 3 wk combined (P < 0.05). Gain-to-feed ratio tended to improve during the first week (P = 0.076) by the antibiotics compared with the control. Among the antibiotic treatments, no difference was observed in ADG and feed intake, which were also unchanged by the diet in the fourth week. The fecal enterobacteria counts were increased by amoxicillin on d 14 and 21 (P < 0.05 and 0.01, respectively) and were decreased by tilmicosin (P < 0.001) compared with the control. Amoxicillin decreased lactic acid bacteria (P < 0.01) counts compared with the control. The antibiotic supplementation tended to decrease total bacteria variability in the jejunum (Shannon index, P = 0.091) compared with the control. The antibiotic treatment decreased the mean total serum IgM concentration (P = 0.016) after 3 wk and did not change the mucosal histomorphometry of the small intestine.
For tilmicosin, the observed positive action on piglet performance and feed intake can originate by the decreased costs of immune activation determined by the action on intestinal microbiota. For amoxicillin and doxycycline, the observation on intestinal and fecal microbiota seems to be not sufficient to explain their growth-promoting effect.
P Bosi, G Merialdi, S. Scandurra, S. Messori, L. Bardasi, I. Nisi, D. Russo, L. Casini, P Trevisi. Feed supplemented with 3 different antibiotics improved food intake and decreased the activation of the humoral immune response in healthy weaned pigs but had differing effects on intestinal microbiota. 2011. Journal of Animal Science, 89:4043-4053. doi:10.2527/jas.2010-3311.
