Penicillin resistance in Streptococcus pneumoniae strains

: In this study a total of 87 Streptococcus pneumoniae strains were tested for their susceptibility to penicillin with disk diffusion method by using 5 micrograms methicillin disks. In 68 strains susceptibility to penicillin was also determined by using agar dilution method performed in Mueller-Hinton agar supplemented with 5% sheep blood. Of 63 adult isolates tested with disk-diffusion method 35 strains (55.6%) were susceptible to penicillin, 24 strains (38.1%) had low level resistance and 4 strains (6.3%) had high-level resistance to penicillin. Of 24 S. pneumoniae isolates recovered from children 5 (20.8%) were penicillin susceptible, while 11 (45.8%) had low level resistance and 8 (33.4%) had high level resistance to penicillin. These values were 40 (45.9%), 35 (40.3%) and 12 (13.8%) in total isolates respectively. Of 44 isolates in which MIC values of penicillin were determined by agar dilution method, 34 strains (77.3%) were penicillin susceptible and 10 strains (22.7%) had low level resistance while none of these strains had high level resistance to penicillin. Of 24 children isolates tested with agar dilution method 11 strains (45.8%) were susceptible to penicillin, 8 strains (33.4%) showed low level resistance and 5 strains (20.8%) showed high level resistance to penicillin. For total of 68 isolates these values were 45 (66.1%), 18 (26.4%) and 5 (7.3%) respectively. These findings indicate the need to perform antibiotic susceptibility testing of all pneumococcal isolates to avoid therapeutic failure.