title: Isolation of tetracycline-resistant Megasphaera elsdenii strains with novel mosaic gene combinations of tet(O) and tet(W) from swine.
abstract: Anaerobic bacteria insensitive to chlortetracycline (64 to 256 microg/ml) were isolated from cecal contents and cecal tissues of swine fed or not fed chlortetracycline. A nutritionally complex, rumen fluid-based medium was used for culturing the bacteria. Eight of 84 isolates from seven different animals were identified as Megasphaera elsdenii strains based on their large-coccus morphology, rapid growth on lactate, and 16S ribosomal DNA sequence similarities with M. elsdenii LC-1(T). All eight strains had tetracycline MICs of between 128 and 256 microg/ml. Based on PCR assays differentiating 14 tet classes, the strains gave a positive reaction for the tet(O) gene. By contrast, three ruminant M. elsdenii strains recovered from 30-year-old culture stocks had tetracycline MICs of 4 microg/ml and did not contain tet genes. The tet genes of two tetracycline-resistant M. elsdenii strains were amplified and cloned. Both genes bestowed tetracycline resistance (MIC = 32 to 64 microg/ml) on recombinant Escherichia coli strains. Sequence analysis revealed that the M. elsdenii genes represent two different mosaic genes formed by interclass (double-crossover) recombination events involving tet(O) and tet(W). One or the other genotype was present in each of the eight tetracycline-resistant M. elsdenii strains isolated in these studies. These findings suggest a role for commensal bacteria not only in the preservation and dissemination of antibiotic resistance in the intestinal tract but also in the evolution of resistance.
authors.affiliation: Pre-Harvest Food Safety and Enteric Diseases Research Unit, National Animal Disease Center, Agricultural Research Service, U.S. Department of Agriculture, PO Box 70, Ames, IA 50010, USA. tstanton@nadc.ars.usda.gov
authors.forename: Thaddeus B
authors.initials: TB
authors.lastname: Stanton
authors.forename: Samuel B
authors.initials: SB
authors.lastname: Humphrey
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D001421:Bacteria, Anaerobic / Q000145:classification / Q000187:drug effects / Q000235:genetics / Q000302:isolation & purification*; D001426:Bacterial Proteins / Q000235:genetics*; D002352:Carrier Proteins / Q000235:genetics*; D002432:Cecum / Q000382:microbiology; D002751:Chlortetracycline / Q000494:pharmacology; D003470:Culture Media; D004275:DNA, Ribosomal / Q000032:analysis; D019143:Evolution, Molecular; D006095:Gram-Positive Cocci / Q000145:classification / Q000187:drug effects / Q000235:genetics / Q000302:isolation & purification*; D007773:Lactates / Q000378:metabolism; D008826:Microbial Sensitivity Tests; D008969:Molecular Sequence Data; D016133:Polymerase Chain Reaction; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D011995:Recombination, Genetic; D017422:Sequence Analysis, DNA; D013552:Swine; D013753:Tetracycline Resistance / Q000235:genetics*
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D002352:Carrier Proteins; D003470:Culture Media; D004275:DNA, Ribosomal; D007773:Lactates; D012336:RNA, Ribosomal, 16S; C083139:Tet O resistance protein, Bacteria; D002751:Chlortetracycline