Submitted to: Applied and Environmental Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/25/2011
Publication Date: 10/15/2011
Citation: Stanton, T.B., Humphrey, S.B. 2011. Persistence of antibiotic resistance: evaluation of a probiotic approach using antibiotic-sensitive M. elsdenii strains to prevent colonization of swine by antibiotic-resistant strains. Applied and Environmental Microbiology. 77(20):7158-7166. Interpretive Summary: Megasphaera elsdenii is an obligately anaerobic bacterium commonly present in the rumens and lower GI tracts of mammals, including humans. Our previous studies demonstrated that different strains of M. elsdenii can be resistant to multiple antibiotics. In these studies we discovered that M. elsdenii is transmitted from mother sow to offspring pigs early after birth and the bacterium colonizes the intestines of the offspring shortly after weaning, or when mother’s milk is replaced with a solid food diet. The mother sow transmitted M. elsdenii strains that were resistant to three antibiotics - tetracycline, ampicillin, and tylosin. We fed cultures of five M. elsdenii strains that were antibiotic sensitive to offspring pigs to block the transmission of antibiotic resistant strains from the mother. This approach delayed but did not prevent the colonization of the offspring by antibiotic resistant strains from the mother. The sow strains showed a surprising diversity in their antibiotic resistance properties and in their molecular fingerprints. This unexpected subspecies diversity suggests there are numerous intestinal niches and habitats for M. elsdenii and provides an explanation why a few dosed antibiotic sensitive strains were unable to compete with the resistant strains.
Technical Abstract: Megasphaera elsdenii is a lactate-fermenting, obligately anaerobic bacterium commonly present in the gastrointestinal tracts of mammals, including humans. Swine M. elsdenii strains were previously shown to have high levels of tetracycline resistance (MIC = 64->256 micro g/ml) and to carry mosaic (recombinant) tetracycline resistance genes. Baby pigs inherit intestinal microbiota from the mother sow. In these investigations we addressed two questions. When do M. elsdenii strains from the sow colonize baby pigs? Can five antibiotic-sensitive M. elsdenii strains administered intragastrically to newborn pigs affect natural colonization of the piglets by antibiotic-resistant M. elsdenii strains from the mother? M. elsdenii natural colonization of newborn pigs was undetectable (< 10**4 cfu/gm feces wet wgt) prior to weaning (20 d after birth). After weaning, all pigs became colonized (4 x 10**5 to 2 x 10**8 cfu/gm feces). In a separate study, sixty-one percent (76/125) of M. elsdenii isolates from a gravid sow never exposed to antibiotics were resistant to chlortetracycline, ampicillin, or tylosin. The inoculation of the sow’s offspring with mixtures of M. elsdenii antibiotic sensitive strains reduced colonization by AR strains until at least eleven days post weaning. At 25 to 39 days postweaning, however, AR strains predominated. Antibiotic susceptibility phenotypes and SNP-based identities of M. elsdenii isolated from sow and offspring were unexpectedly diverse. These results suggest dosing newborn piglets with M. elsdenii antibiotic-sensitive strains delays but does not prevent colonization by maternal resistant strains. M. elsdenii subspecies diversity provides an explanation for the persistence of resistant strains in the absence of antibiotic selection.