In-feed administered sub-therapeutic chlortetracycline alters community composition and structure but not the abundance of community resistance determinants in the fecal flora of the rat

The impact of continuous sub-therapeutic chlortetracycline on community structure, composition and abundance of tetracycline resistance genes in the rat fecal community was investigated. Rats were fed a standard diet containing chlortetracycline at 15 μg g−1 diet for 28 days, followed by 30 μg g−1 diet to completion of the study on day-56. These levels are similar to those administered to swine during the grow-out phase. Sub-therapeutic chlortetracycline affected the fecal community as determined through change in the cultivable anaerobic community and through molecular-based analyses including denaturing gradient gel electrophoresis profiles of the variable 2–3 region community 16S rRNA genes over time and through comparative sequence analysis of 16S rRNA gene community libraries. Significant decreases in fecal phylotype diversity occurred in response to sub-therapeutic chlortetracycline, although total bacterial output remained constant over the entire feeding trial. Chlortetracycline at 15 μg g−1 diet resulted in significant change in community composition, but only modest change to the fecal community structure in terms of the distribution of individual phylotypes among the major fecal lineages. Chlortetracycline at 30 μg g−1 diet significantly altered the distribution of phylotypes among the major fecal lineages shifting the overall community such that Gram-negative phylotypes aligning within the phylum Bacteroidetes became the dominant lineage (>60% of total community). While chlortetracycline impacted both fecal community structure and composition, there was no significant effect on the abundance of community tetracycline resistance genes [tet(Q), tet(W), tet(O)] or on the emergence of a new putative tetracycline resistance gene identified within the fecal community. While sub-therapeutic chlortetracycline provides sufficient selective pressure to significantly alter the fecal community, the primary outcome appears to be the development of a community which may have a higher inherent tolerance to sub-therapeutic levels of chlortetracycline rather than an overgrowth of the tetracycline resistant bacteria already present within the community.