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JAC Advance Access published online on July 1, 2003
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkg304
© 2003 by The British Society for Antimicrobial Chemotherapy
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© 2003 The British Society for Antimicrobial Chemotherapy

Original article

Gentamicin resistance in dairy and clinical enterococcal isolates and in reference strains

Maria de Fátima Silva Lopes 1 *, Tânia Ribeiro 1 , Maria Paula Martins 2 , Rogério Tenreiro 3 , and Maria Teresa Barreto Crespo 1

1 Instituto de Biologia Experimental e Tecnológica/Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Apartado 12, 2781-901 Oeiras
2 Estação Agronómica Nacional, Instituto Nacional de Investigação Agrária, 2781-505 Oeiras
3 Faculdade de Ciências/Departamento de Biologia Vegetal/Centro de Genética e Biologia Molecular, Universidade de Lisboa, 1749-016 Lisboa, Portugal

* Corresponding author. E-mail: flopes{at}itqb.unl.pt.

Received 4 February 2003 ; revised 17 March 2003 ; accepted 27 April 2003

Abstract

Enterococci isolated from Portuguese dairy products (milk and cheese) and clinical settings (hospitals and veterinary clinics), together with reference strains from the genus Enterococcus, were screened for low- and high-level gentamicin resistance using the standard disc diffusion method (10 and 120 µg gentamicin discs). MICs were also determined using both the macrodilution method and the Etest. Four genes [aac(6')-Ie-aph(2'')-Ia, aph(2'')-Ib, aph(2'')-Ic and aph(2'')-Id] responsible for high- and mid-level gentamicin resistance were sought using PCR. Although enterococci generally are regarded as being intrinsically resistant to low levels of gentamicin, results revealed that many dairy enterococci (around 30% of the isolates used) are not intrinsically resistant to gentamicin, showing MICs of £4 mg/L. High-level gentamicin resistance was not detected in any of the dairy isolates studied, except for aph(2'')-Ib, which was found in one. Therefore, gentamicin resistance should be monitored in dairy enterococci, although it does not seem to be a problem at present. In contrast, all clinical isolates studied were, as expected, intrinsically resistant to low levels of gentamicin, presenting MICs > 8 mg/L. Fifteen percent of these clinical isolates showed high-level gentamicin resistance (MICs > 512 mg/L), with the bifunctional gene aac(6')-aph(2'') being detected in four of them. However, discs with gentamicin 120 µg failed to detect some isolates with high-level gentamicin resistance.

Keywords: aminoglycosides, antibiotic resistance, enterococci
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