Effects of a natural feed additive and an in-feed antibiotic on abundance of antibiotic resistance genes in feces of weaned piglets

Antibiotics such as tetracyclines have been extensively used in swine production in the last decades. As a consequence, levels of antibiotic resistant bacteria in livestock have increased, which is discussed as major risk for public health. Thus, alternatives to antibiotics are urgently needed to reduce the amounts of antibiotics used in food producing animals. Natural feed additives, such as acids, phytogenics or pre/probiotics, are known to have a positive influence on gut health and performance. Here, we evaluated the effect of a natural feed additive, that combines a permeabilizing complex, a phytochemical and a blend of organic acids, on the abundance of antibiotic resistance genes in feces of weaned piglets.

After weaning, 24 Large White piglets with no history of antibiotic treatment were allocated to three groups (n=8): control group (basal diet; CG), antibiotic group (basal diet supplemented with 40 mg oxytetracycline-HCl/kg b.w./day for 10 days, thereafter basal diet; AG) or product group (basal diet supplemented with 3 kg/t natural feed additive; PG). Piglets received treatment diets for 28 days ad libitum and were housed pairwise. On day 0, 10 and 28, feces samples from individual piglets were collected. The animal experiment was approved by the State Veterinary and Food Administration of the Slovak Republic (No 1485/15-221).

For DNA extraction, the QIAamp DNA Stool Kit (Qiagen) was used according to the manufacturer´s instructions. Abundance of genes conferring resistance to tetracycline (tetA, tetB), sulfamethoxazole (sul1, sul2) and streptomycin (strA, strB) was determined by qPCR.

Administration of the in-feed antibiotic significantly increased the levels of tetA in feces (p < 0.05). On day 10 and 28, mean tetA gene copies/g feces were 9.24E+06 and 7.43E+06 in the AG, respectively, while levels in the PG and CG remained between 6.34E+05 and 1.22E+06. Independent of the treatment, tetA was detected in all samples at all time points. This was not the case for tetB, which was less frequently found in feces of the PG and CG. Interestingly, levels of sul2, strA and strB decreased over time in the AG. A similar pattern was observed in the CG. Neither treatment nor time had a significant influence on copy numbers of sul1 and tetB.

In contrast to in-feed supplementation with oxytetracycline, the use of the natural feed additive did not increase abundance of the resistance gene tetA in feces of weaned piglets. As this product has previously been shown to enhance performance in pigs, its application might contribute to a reduced use of antimicrobials in swine production.

Disclosure of Interest: None Declared.