Agriculture, Antibiotic Use, and Genes in the Environment

Who would ever think that 'wastewater lagoons'--lakes full of animal shit--could be so interesting? And important in understanding how agricultural antibiotic use increases the frequency of antibiotic resistance genes.

A recent study looked at the abundance of tetracycline resistance genes in eight lagoons that stored cattle waste. Two of the lagoons were used with cattle that were never exposed to tetracycline. Two were 'mixed use', and four high use, where the cattle have been exposed to tetracycline (an aside: the lagoon from which the waste is used as fertilizer is a high use lagoon. Just saying...).

The investigators then tracked the numbers (and type) of tetracycline resistance genes in each lagoon using real-time PCR. They also determined the number of bacteria by using real-time PCR to amplify 16S-RNA (all bacteria have this gene). They could then use the 16S-RNA gene abundances to determine the relative frequencies of tetracycline resistance genes in each of the lagoons. Here's what they found:

(from the paper) Fig. 2. Tetracycline resistance gene copy numbers over time for tet(O), tet(W), tet(Q), tet(M), tet(L) and tet (B). Absolute gene copy numbers are presented for (A) no-use, (B) mixed-use and (C) high-use feedlot lagoons, whereas copy numbers normalized to ambient 16S-rRNA gene level are presented for (D) no-use, (E) mixed-use and (F) high-use feedlot lagoons respectively. Note that the Y-axis scales for absolute gene copy numbers and normalized gene copy numbers for the No-use lagoons (A and D) are two and three orders of magnitude lower respectively, than the mixed-use and high-use lagoons (B, C, E and F).

Essentially, the lagoons where tetracycline was used had 100-1000 times more resistance genes than the lagoons where no tetracycline was used (compare panels D versus F; keep in mind that the scale for D is 100 times smaller than panel F).

Granted, tetracycline is not a frontline antibiotic, although tetracycline resistance is often linked to antibiotic resistance genes of medical importance, so selection of tetracycline resistant organisms often drags along other resistances (particularly in the medically important organisms). At some point, we're going to have to get serious about the use of antibiotics in agriculture--and I don't only mean cefquinome.