Antibiotic resistance problem in aquaculture : the role of Com gene products in DNA uptake as a nutrient source for bacteria

Abstract

Natural transformation is one of three mechanisms by which genetic material can be exchanged horizontally between bacteria and the only mechanism that the DNA is actively taken up by the recipient cell. It is also one of the mechanism by which bacteria acquire resistance against antibiotics and thus creating a growing concern worldwide for aquaculture industries. This study uses Acinetobacter baylyi BD413/wild strain to investigate the role of different com genes in DNA uptake as a nutrient source and thus strengthening the hypothesis that transformation evolved as a nutrient-uptake system. Since competition is induced by nutritional limitation in Haemophilus influenzae and Bacillus subtilis, model bacteria Acinetobacter baylyi BD413 were first grown in different concentrations of nitrogen (N) and phosphorus (P) to find out their nutritional limiting conditions. Afterwards, different concentrations of DNA were added into nutritionally limiting concentrations of N and P for these bacteria. The bacteria were then grown with and without DNA under these nutritional limiting conditions to see whether or not the DNA may be taken up by these bacteria as a nutrient source. A. baylyi was also grown without any carbon source in the medium and with DNA as a carbon source in the S2 medium to find out the role of DNA as the sole source of carbon and energy supporting microbial growth. A. baylyi growth was not found to be enhanced in phosphorus limiting conditions (below 5% P concentrations) but managed to grow even without any nitrogen sources in the medium. This suggests that A. baylyi could possibly get the nitrogen from dying cells during its growth phase. Adding different concentrations of DNA without any nitrogen sources in the media did not cause any significant increase in growth of A. baylyi suggesting that these bacteria do not take DNA as a nitrogen source. On the other side, adding DNA in phosphorus limited growth condition showed to induce significant increase in growth of A. baylyi suggesting that competence might be induced by phosphorus limited growth conditions for these bacteria. Like nitrogen, competence was not found to be induced by carbon limiting growth conditions since A. baylyi was not influenced by DNA and it may be grown even without any source of carbon.