Recovery and characterization of high-level gentamicin resistant strains of enterococci from commercial poultry

Abstract

Enterococci are normal enteric flora in humans and many animal species, and they are also opportunistic pathogens that have become increasingly responsible for many hospital-acquired infections. Antibiotic resistant enterococci have become an important public health concern; therefore, identifying potential sources of antibiotic resistant enterococci and/or factors contributing to their resistance has become a priority. During the raising of commercial poultry, chickens are often exposed to antibiotics in order to prevent infection and/or as growth-promoters. We have found that antibiotic exposure of chickens, even at subtherapeutic levels, encourages the growth of and selects for an antibiotic resistant enteric flora. To assess the level of antibiotic resistant enterococci in a typical commercial flock, we randomly sampled 1-, 2-, and 6-week old chickens of a large commercial poultry supplier. Antibiotic susceptibility assays of the recovered enterococcal isolates demonstrated that chickens harbor a multitude of multiply antibiotic resistant enterococcal species. It may be possible that enterococci are specially adapted for the intestinal tract of chickens and are not easily transmitted to humans. However, it may also be possible that there are subpopulations of enterococci that are more capable of transmission between animal species. Also, if enterococci from poultry are not capable of colonizing humans, they may still contribute to antibiotic resistance in humans by donating resistance genes to resident bacteria. Enterococci are model organisms to study antibiotic resistance because they readily acquire and transfer antibiotic resistance factors. Thus, the possibility that enterococci of human and chicken origin can exchange genetic elements coding for antibiotic resistance was addressed by filter-mating experiments to determine the transferability of gentamicin resistance. We found that transfer of gentamicin resistance (i) occurred between strains of enterococci derived from humans and chickens, (ii) was strain dependent, (iii) was most efficient when donor strains were of human origin, and (iv) was not enhanced with enterococci derived from like origins. Finally, the success of these in vitro transfer experiments prompted further experimentation of such transfers within an in vivo chicken model in which transfer of gentamicin resistance was observed but also appears to be strain dependent and fairly inefficient.