Antimicrobial susceptibility of invasive Streptococcus pneumoniae in Italyby agar dilution method and E test

Few data on antimicrobial susceptibility of invasive Streptococcus pneumoni ae isolated in Italy are available. Ninety-two invasive isolates from all o ver the country collected from January 1997 to April 1998 were tested for s ensitivity to penicillin, erythromycin, ceftriaxone, chloramphenicol, tetra cycline, and trimethoprim/sulfamethoxazole by the agar dilution method. Fiv e (5.4%) strains were resistant to penicillin tone highly, four intermediat ely resistant), 8 (8.7%) to chloramphenicol, 27 (29.3%) to erythromycin, 17 (18.5%) to tetracycline (16 highly, one intermediately), and 21 (22.8%) to trimethoprim/sulfamethoxazole (14 highly, 7 intermediately). All strains w ere susceptible to ceftriaxone, although the penicillin-resistant strain ha d the highest minimal inhibitory concentration (MIC) value (0.5 mu g/ml); t hree penicillin-resistant strains were also resistant to erythromycin. Eigh t strains were multi-drug resistant, being also resistant to at least three antibiotics. The commercially available E test was compared with the stand ard agar dilution method for the determination of MIC of penicillin, erythr omycin, ceftriaxone, chloramphenicol, and trimethoprim/sulfamethoxazole. E test established the same susceptibility categories for 100% of the strains tested for penicillin and ceftriaxone, 99% for chloramphenicol, 97% for er ythromycin, and 74% for trimethoprim/sulfamethoxazole. According to our res ults, E test was simple to perform, easy to interpret, and a valid method f or susceptibility testing of S. pneumoniae. Our study shows that in Italy t he rate of penicillin resistance in invasive isolates of S. pneumoniae is o ne of the lowest in Europe (5.4%), while the rate of erythromycin is very h igh (29.3%) and is reaching the highest rates of other Southern European co untries.