Diverse tetracycline resistance genotypes of Megasphaera elsdenii strains selectively cultured from swine feces

Abstract

A total of 30 Megasphaera elsdenii strains, selectively isolated from the feces of organically raised swine by using Me109 M medium, and one bovine strain were analyzed for tetracycline resistance genotypic and phenotypic traits. Tetracycline-resistant strains carried tet(O), tet(W), or a tet gene mosaic of tet(O) and tet(W). M. elsdenii strains carrying tet(OWO) genes exhibited the highest tetracycline MICs (128 to >256 microg/ml), suggesting that tet(O)-tet(W) mosaic genes provide the selective advantage of greater tetracycline resistance for this species. Seven tet genotypes are now known for M. elsdenii, an archetype commensal anaerobe and model for tet gene evolution in the mammalian intestinal tract.

FIG. 1.

Schematic depiction of PCR assays for differentiating M. elsdenii tetracycline-resistant genotypes. Strain and tetracycline resistance genotype designations are given at the left of the figure. Checkered regions of genes have high-level sequence identity with tet(O); open regions have high-level sequence identity with tet(W). Products of PCR amplification are depicted by double-headed arrows. Crossover regions of the genes were established by sequence analysis.

FIG. 2.

Phylogenetic analysis of a Tet(W) common region (365 amino acids) shared by various species. GenBank accession numbers for nucleotide sequences used to derive amino acid sequences are given in parentheses. The unrooted neighbor-joining tree was generated using Mega2 (11). A Poisson substitution model was applied to distance calculations. Branches are labeled by bootstrap values from 10,000 replicates. Bar, number of substitutions per amino acid.