Servicio de Microbiología Clínica, Hospital Clínico San Carlos, 28040 Madrid, Spain
Sir,
Although viridans group streptococci are a part of the normal oropharyngeal and gastrointestinal flora in humans, these organisms are also the most common cause of infective endocarditis and sepsis syndrome in neutropenic patients. ß-Lactam agents are the treatment of choice for these infections, but increasing resistance to penicillin has been reported recently in many parts of the world. Macrolides and related drugs have been suggested as an alternative, but recent studies have shown that macrolide resistance may also be a problem.1
There are two recognized mechanisms of resistance to macrolide antibiotics in the genus Streptococcus: target site modification and active efflux of the antibiotic. Target site modification is mediated by erythromycin resistance methylases encoded by erm genes, and active efflux is mediated by a membrane-bound efflux protein encoded by mef genes. The mechanism of macrolide resistance in Streptococcus pyogenes and Streptococcus pneumoniae has been reported in many countries, but there are few reports and little information about this in viridans streptococci.2,3
We studied a total of 63 consecutive blood culture strains of viridans streptococci isolated between March 1998 and June 1999. All the isolates were identified by standard methods and the erythromycin-resistant isolates were identified at the species level using two standard phenotypic systems, API 20-Strep and ID 32-Strep (bioMérieux, Marcy lÉtoile, France), and then classified according to the review of Coykendall. MICs for the following antibiotics were determined by standard agar dilution technique using MuellerHinton agar supplemented with 5% sheep blood: erythromycin, roxithromycin, azithromycin, midecamycin, spiramycin and clindamycin.
The resistance phenotypes were determined by a modification of the double disc test with erythromycin (15 µg) and clindamycin (2 µg) discs on MuellerHinton agar supplemented with 5% sheep blood, and the DNAs of the erythromycin-resistant isolates were amplified with primers specific for the erm(A), erm(B), erm(C) and mef(A) genes.
This study showed high rates of erythromycin resistance (49%) among our viridans group streptococci, as have been noted previously in Spain (39%)1 and in other countries. The 31 erythromycin-resistant strains (MIC 1 mg/L) had cross-resistance to the other 14- and 15-membered macrolides tested (roxithromycin and azithromycin), whereas only 16 strains were resistant to 16-membered compounds and clindamycin. These results agree with the results obtained by the double disc diffusion test: 16 strains (52%) exhibited the MLSB phenotype, always constitutive, and 15 (48%) exhibited the M phenotype. The activities of the macrolide antibiotics and clindamycin in relation to the MLSB and M phenotypes are shown in the Table
. The incidence of the M phenotype in our erythromycin-resistant viridans group streptococci was higher than that reported elsewhere (20%).1,2
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The genotypic characterization of the 31 erythromycin-resistant viridans group streptococci showed that the erm(B) gene was always detected in MLSB phenotype strains, either alone (12 strains) or in combination with mef(A) gene (four strains). The mef(A) gene alone was found only in the 15 M phenotype strains. Erythromycin resistance was found to be greater in the isolates with an erm gene (MIC90 256 mg/L) than in those with a mef gene (MIC90 4 mg/L). As expected, none of the isolates exhibited erm(A) or erm(C) genes.
The molecular characterization of erythromycin resistance in viridans group streptococci is limited. Poutanen et al.3 found the mef gene in three S. mitis isolates displaying erythromycin resistance and clindamycin susceptibility. Seven isolates with erythromycin and clindamycin resistance were found to possess the erm(B) gene. They noted that although the erm(B) gene had previously been found in viridans group streptococci, the mef gene had not. Later, Arpin et al.2 studied eight strains (seven S. mitis and one S. salivarius) with the M phenotype and their results showed that all strains except for one carried the mef(A) gene. In our study, the isolates that had only the mef(A) gene were 10 S. mitis, four S. sanguis and one S. salivarius. To our knowledge, the mef(A) gene has not previously been found in S. sanguis. The erm(B) gene alone was present in five S. mitis, two S. sanguis, four S. bovis and one S. milleri. In S. bovis, the MLSB phenotype has previously been found4 but the gene has not yet been reported. A total of three S. mitis and one S. milleri possessed both the erm(B) and mef(A) genes, as reported by Nishijima et al.5 in erythromycin-resistant S. pneumoniae.
Research on the phenotypes and genotypes of macrolide-resistant viridans group streptococci is of concern, not only for this group, but also for other species of streptococci and other genera of Gram-positive bacteria, with which they share the oropharynx. The transfer of resistance genes may occur, as has already been described.6
Notes
* Corresponding author. Tel: +34-913-303-478; Fax: +34-913-303-478; E-mail: cravial{at}eucmos.sim.ucm.es
References
1 . Alcaide, F., Carratala, J., Liñares, J., Gudiol, F. & Martín R. (1996). In vitro activities of eight macrolide antibiotics and RP-59.500 (quinupristindalfopristin) against viridans group streptococci isolates from blood of neutropenic cancer patients. Antimicrobial Agents and Chemotherapy 40, 211720.[Abstract]
2
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Arpin, C., Canron, M. H., Maugein, J. & Quentin, C. (1999). Incidence of mef A and mef E genes in viridans group streptococci. Antimicrobial Agents and Chemotherapy 43, 23356.
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Poutanen, S. M., de Azavedo, J., Willey, B. M., Low D. E. & McDonald, K. S. (1999). Molecular characterization of multidrug resistance in Streptococcus mitis. Antimicrobial Agents and Chemotherapy 43, 15057.
4 . Renneberg, J., Niemann, L. L. & Gutschik, E. (1997). Antimicrobial susceptibility of 278 streptococcal blood isolates to seven antimicrobial agents. Journal of Antimicrobial Chemotherapy 39, 13540.[Abstract]
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Nishijima, T., Saito, Y., Aoki, A., Toriya, M., Toyonaga, Y. & Fujii, R. (1999). Distribution of mefE and ermB genes in macrolide-resistant strains of Streptococcus pneumoniae and their variable susceptibility to various antibiotics. Journal of Antimicrobial Chemotherapy 43, 63743.
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Luna, V. A., Coates, P., Eady, E. A., Cove, J. H., Nguyen, T. T. & Roberts, M. C. (1999). A variety of Gram-positive bacteria carry mobile mef genes. Journal of Antimicrobial Chemotherapy 44, 1925.