Multiple Antibiotic Resistances of Enterococci from the Poultry Production Environment and Characterization of the Macrolide-Lincosamide-Streptogramin Resistance Phenotypes of Enterococcus faecium

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Enterococcus spp. were collected from poultry production and processing environments across a region of the eastern seaboard of the United States. Using a microtiter plate adaptation of traditional biochemical assays, 532 isolates were identified to the species level. E. faecalis was observed to be the predominant species recovered (64%), followed by E. faecium (24%), E. hirae (6%), E. gallinarum (5%), and less than 1% of E. avium, E. casseliflavus, E. durans, and indeterminate species. All isolates were tested for susceptibility to 28 antimicrobials using broth microdilution. Antimicrobial resistance was observed among the isolates with E. faecalis more resistant to lincosamide, macrolide, and tetracycline antimicrobials and E. faecium more resistant to fluoroquinolone and penicillin antimicrobials. Resistance to multiple antimicrobials was observed among all species, with a proportionately greater diversity of resistance phenotypes among isolates of E. faecium. The prevalence of resistance to an important antimicrobial used in human medicine, the streptogramin quinupristin-dalfopristin, was demonstrated among 63% of E. faecium isolates. These observations led to the investigation of the molecular determinants among those isolates of E. faecium that contributed to resistance to the group of antibiotics known as the macrolide-lincosamide-streptogramin (MLS) superfamily. Ribotype analysis demonstrated that four ribotypes constituted 65% of the observed population, but displayed diverse antibiograms, suggestive of the acquisition of multiple resistance elements to other antimicrobials. This was consistent with the absence of geographic clustering of MLS phenotypes or ribotypes. Colony PCR screening for the streptogramin resistance determinants erm(A), erm(B), mef(A), lnu(B), msr(C), vgb(A), vat(D), and vat(E) was performed. Resistance to streptogramin antimicrobials was largely unaccounted for by PCR screening for specific resistance determinants. Erythromycin methyltransferase determinants erm(A) and erm(B) were observed among 7.5% and 5% of resistant isolates whereas the efflux gene msr(C), the streptogramin B hydrolase vgb(A), or the streptogramin A acetyltransferase genes vat(D) and vat(E) were not detected among resistant E. faecium. Mef(A) was detected in only 4% of macrolide-resistant isolates whereas the screening for lnu(B) was not successful. These results indicate that streptogramin resistance is widespread among E. faecium from the poultry production environment but the mechanisms of resistance within this population remain largely uncharacterized.