Original articles
Comparison of antimicrobial resistance phenotypes and resistance genes in Enterococcus faecalis and Enterococcus faecium from humans in the community, broilers, and pigs in Denmark

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Abstract

Enterococcus faecalis and E. faecium isolated from humans in the community (98 and 65 isolates), broilers (126 and 122), and pigs (102 and 88) during 1998 were tested for susceptibility to 12 different antimicrobial agents and for the presence of selected genes encoding resistance using PCR. Furthermore, the presence of vancomycin resistant enterococci was examined in 38 human stool samples using selective enrichment. Widespread resistance to chloramphenicol, macrolides, kanamycin, streptomycin, and tetracycline was found among isolates from all three sources. All E. faecium isolates from humans and pigs were susceptible to avilamycin, whereas 35% of isolates from broilers were resistant. All E. faecium isolates from humans were susceptible to vancomycin, whereas 10% and 17% of isolates from broilers and pigs, respectively, were resistant. A vancomycin resistant E. faecium isolate was found in one of the 38 human fecal samples examined using selective enrichment. All vancomycin resistant isolates contained the vanA gene, all chloramphenicol resistant isolates the catpIP501 gene, and all five gentamicin resistant isolates the aac6-aph2 gene. Sixty-one (85%) of 72 erythromycin resistant E. faecalis examined and 57 (90%) of 63 erythromycin resistant E. faecium isolates examined contained ermB. Forty (91%) of the kanamycin resistant E. faecalis and 18 (72%) of the kanamycin resistant E. faecium isolates contained aphA3. The tet(M) gene was found in 95% of the tetracycline resistant E. faecalis and E. faecium isolates of human and animal origin, examined. tet(K) was not observed, whereas tet(L) was detected in 17% of tetracycline resistant E. faecalis isolates and in 16% of the E. faecium isolates. tet(O) was not detected in any of the isolates from pigs, but was observed in 38% of E. faecalis isolates from broilers, in two E. faecalis isolates from humans and in three E. faecium isolates from broilers. tet(S) was not detected among isolates from animals, but was observed in 31% of E. faecalis and one E. faecium isolate from humans. This study showed a frequent occurrence of antimicrobial resistance and the presence of selected resistance genes in E. faecalis and E. faecium isolated from humans, broilers and pigs. Differences in the occurrence of resistance and tetracycline resistance genes were observed among isolates from the different sources. However, similar resistance patterns and resistance genes were detected frequently indicating that transmission of resistant enterococci or resistance genes takes place between humans, broilers, and pigs.

Introduction

Enterococci are part of the natural intestinal flora of most mammals and birds (Facklam et al., 1999) and have emerged as important nosocomial and community acquired pathogens in recent years Murray 1990, Schaberg et al 1991, Moellering 1992. Most infections are probably of endogenous origin and infections with enterococci can be difficult to treat because they are intrinsic resistant to several antibiotics Moellering 1990, Murray 1991, Spera and Farber 1994.

During the last two decades clinical isolates resistant to high levels of aminoglycosides, penicillin or vancomycin has emerged Chen and Williams 1985, Mederski–Samoraj and Murray 1983, Murray and Mederski–Samaroj 1983, Leclercq et al 1988. Previous studies have shown the presence of a reservoir of antimicrobial resistant enterococci in the community including vancomycin Klare et al 1995a, Bates et al 1994, Jordens et al 1994, van der Auwera et al 1996, Endtz et al 1997 and aminoglycoside resistant enterococci Nachamkin et al 1988, Coque et al 1995, McNamara et al 1995, Silverman et al 1998. Furthermore, a frequent occurrence of antimicrobial resistant enterococci have been observed among food animals Bates et al 1994, Klare et al 1995b, Thal et al 1995, Aarestrup et al 1998a, Aarestrup et al 1998b and food of animal origin Klare et al 1995b, Wegener et al 1997, Klein et al 1998 and it has been suggested that food animals may be a reservoir of resistant enterococci and resistance genes capable of transferring to humans through the food chain.

A large number of different resistance determinants have been identified in enterococci, and in several cases they are located on conjugative plasmids or transposons Shaw and Clewell 1985, Clewell and Gawron–Burke 1986, Clewell et al 1995, Rice 1998. Similar determinants have been observed among enterococci of widely different ecological origin Rollins et al 1985, LeBlanc et al 1986, Klare et al 1995b, Roberts 1995, Thal et al 1995, Aarestrup et al 1996, Hammerum et al 1998, Jensen et al 1998, Jensen et al 1999. Only limited information about the occurrence of different resistance genes among enterococci from humans and animals is available.

This study was conducted to study the occurrence of antimicrobial resistance and presence of selected resistance genes among Enterococcus faecalis and Enterococcus faecium isolated from humans in the community, broilers and pigs in Denmark during 1998.

Section snippets

Fecal samples from humans

Statens Serum Institut receives stool samples for diagnostic purposes from most regions of Denmark. In March 1998 a total of 254 stool samples submitted for examination of diarrheal pathogens were examined for the presence of enterococci. All samples were from humans with a history of diarrhea, with no history of recent stay in a hospital and no history of antimicrobial treatment.

Bacterial isolates from broilers and pigs

Bacterial isolates from broilers and pigs were obtained from the continuos surveillance program for antimicrobial

Results

E. faecalis was isolated from 98 and E. faecium from 65 of the 254 different human stool samples. From broilers 126 E. faecalis and 122 E. faecium isolates were included and from pigs 102 E. faecalis and 88 E. faecium isolates were included. Table 2, Table 3 show the MIC distribution of the isolates.

Discussion

Enterococci cause serious infections in humans, whereas infections in animals are rare. Enterococcal infections in humans are in most cases believed to be caused by enterococci of endogenous origin Moellering 1990, Murray 1990. Thus, the presence of antimicrobial resistant enterococci in the intestinal tract of humans has implication for the treatment. Furthermore, the presence of resistant enterococci in humans in the community might act as a reservoir for infections at hospitals.

In the

Acknowledgments

We are grateful to René Hendriksen, Mette Juul, Anne Lykke Lauritsen, Lena Folkman, Inge Hansen, Erika Widén, Jacob Reichhardt Svendsen, and Anja Dahl at the Danish Veterinary Laboratory and Jette Hansen at Statens Serum Institut for technical assistance. This study is a part of the Danish Integrated Antimicrobial Resistance Monitoring and Research Program (DANMAP) conducted in collaboration between Statens Serum Institut, the National Food Agency of Denmark and the Danish Veterinary Laboratory

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