Streptococcus pyogenes: penicillin and erythromycin susceptibility in the cities of Neuquen and Cipolletti.

Penicillin resistance has not yet been detected in Streptococcus pyogenes. However macrolide-resistant streptococci have emerged in several countries. Only low rates of erythromycin-resistant S. pyogenes were reported in Arge ntina, with the exception of a 11.1% observed in Mendoza. The aim of the pr esent study was to determine the susceptibility to penicillin and to erythr omycin of 251 consecutive clinically-significant isolates of S. pyogenes ob tained from four centers of Cipolletti and Neuquen during the period April- December 1998. The double disk test with erythromycin and clindamycin disks was employed as a screening method to detect ERY-resistant streptococci an d to determine the phenotype of macrolide resistance. Disk diffusion was al so employed for determining penicillin susceptibility. Macrolide-resistant isolates were also tested for penicillin, ceftriaxone, erythromycin, clinda mycin and azithromycin susceptibility by the agar dilution method. Addition ally they were also tested for erythromycin susceptibility by E-test (AB Bi odisk, Solna, Sweden). All streptococci studied were susceptible to penicil lin and thirty of them (12.0%) were resistant to erythromycin. All these re sistant isolates were also resistant to azithromycin but susceptible to cef triaxone and clindamycin. They showed the phenotype M (probably efflux-medi ated mechanism) and the MICs of erythromycin ranged between 8 and 16 mu g/m l. According to these results we conclude that in spite of universal suscep tibility to penicillin in S. pyogenes, macrolide resistance is a matter of concern in Neuquen and Cipolletti. At least in those cities ii appears to b e necessary to routinely perform macrolide susceptibility tests in beta-hem olytic streptococci.