In vitro activity of daptomycin against enterococci from nosocomial and community environments in Portugal

Sir,

Daptomycin is a novel cyclic lipopeptide antibiotic with a unique mechanism of action that involves disruption of multiple aspects of the plasma membrane function without penetration into the cytoplasm. This antibiotic exhibits rapid concentration-dependent killing and the occurrence of spontaneous resistant mutants is rare. Although several studies have shown good in vitro and in vivo activity of daptomycin against clinical isolates of enterococci,^1–5 its activity against animal and environmental enterococci has not been studied. The objective of this work was to investigate the activity of daptomycin against enterococcal isolates from nosocomial and community environments, including human faecal flora, animal food samples, sewage and rivers in Portugal, the European country currently with the highest prevalence of vancomycin-resistant enterococci (VRE) (Annual Report of European Antibiotic Resistance Surveillance System, EARSS, 2002; http//:www.earss.rivm.nl). A high occurrence of VRE in the Portuguese community has also been shown by our group.⁶

We studied 1151 isolates from: (i) patients at three hospitals in the centre and north of Portugal collected during 1996–2003 (n=251); (ii) human faecal samples from healthy volunteers living in the centre and north of the country and collected in 2001 (n=338 isolates from 99 samples); (iii) swine faecal samples collected during 1997–1998 (n=17 isolates from six samples); (iv) raw poultry products corresponding to 93 chicken lots and six turkey lots from 10 different brands and purchased at two different butcher shops—the samples were recovered during 1999–2001 (n=397 isolates from 99 samples); (v) sewage water from Porto hospitals recovered in the period 2001–2002 (n=130 isolates from 26 samples); (vi) river water samples from the Porto area in 2003 (n=18 from six samples). A multiplex PCR assay was performed for species identification and detection of vancomycin resistance genes.⁷ Seven hundred and seventy-one isolates were resistant to three or more antibiotics (771/1151 isolates, 67%, data not shown). Thirty percent of the isolates (n=344/1151) were VRE of genotypes vanA (n=279/344, 81%), vanB (n=4/344, 1%) or vanC1 (n=61/344, 18%). Twenty-two percent (n=250/1151) showed high-level resistance (HLR) to gentamicin, 44% (n=504/1151) showed HLR to streptomycin and 66% (n=760/1151) were resistant to erythromycin. The MIC of daptomycin (Cubist Pharmaceuticals, Inc., Lexington, MA, USA) was determined by the agar dilution method following NCCLS guidelines and using Mueller–Hinton II agar (bioMérieux, Marcy l'Étoile, France), since the Ca^2 + concentration was at the required levels for daptomycin testing (50 mg/L). Enterococcus faecalis ATCC 29212 was used as a control strain. Daptomycin showed good activity against enterococci susceptible and resistant to different antibiotics from hospital and community Portuguese environments; all studied isolates were inhibited at concentrations ≤4 mg/L (Table 1). Previous studies have shown that clinical isolates from other European countries and from the USA are very susceptible to this antimicrobial agent^1,3–5 although isolates with an MIC₉₀ of 8 mg/L have been reported.⁴ In agreement with other investigators, Enterococcus faecium isolates were less susceptible than E. faecalis, with MIC₅₀ and MIC₉₀ values in the range 1–4 mg/L depending on the series.^1,4Enterococcus spp. were also less susceptible than E. faecalis isolates, although MIC₅₀s were lower than those of E. faecium (Table 1).

To our knowledge, this is the first study in which the activity of daptomycin has been evaluated against Enterococcus spp. isolated from animals and the environment. Daptomycin showed good activity against Portuguese enterococcal isolates susceptible and resistant to different antimicrobial agents and isolated from different settings, indicating that daptomycin could be a good therapeutic choice to treat infections caused by enterococci in an area with a high occurrence of VRE. The absence of resistance in the community environment mirrors the lack of a non-susceptible reservoir in this setting.

This work was supported by a grant from Cubist Pharmaceuticals, Inc. (Lexington, MA, USA). Carla Novais was supported by a fellowship from Fundação para a Ciência e Tecnologia (SFRH/BD/3372/2000). Members of the Portuguese Resistance Study Group are: Graça Ribeiro, Clementina Vital (Coimbra University Hospital); Isabel Marques, Ana M. Queirós (São Teotónio Hospital, Viseu); and Helena Ramos (Santo António Hospital, Oporto).

References

Author notes

1REQUINTE, Lab. Microbiologia, Faculdade de Farmácia, Universidade do Porto, Porto; 2Microbiologia, Faculdade de Ciências da Saúde, Universidade Fernando Pessoa, Porto, Portugal; 3Lab. de Microbiologia, Hospital Ramón y Cajal, Madrid, Spain