Changes in the drug resistance of Haemophilus influenzae and Streptococcus pneumoniae isolated between 1997 and 2006

We studied changes in the drug resistance of 606 strains of Haemophilus influenzae (H. influenzae) and 502 strains of Streptococcus pneumoniae (S. pneumoniae) isolated from our patients between 1997 and 2006. The incidence of beta-lactamase nonproducing ampicillin-susceptible H. influenzae (BLNAS) in 1997-1998 and 1999-2000 was 72.0 and 69.6%, respectively. In 2005-2006, the incidence of BLNAS decreased to 31.0%, while that of beta-lactamase nonproducing ampicillin-resistant H. influenzae (BLNAR) and intermediate-resistant H. influenzae increased to 65.5%. Remarkably early development of BLNAR and intermediate-resistant H. influenzae was found in patients younger than 3 years, as compared to patients older than 3 years. The proportion of penicillin-susceptible S. pneumoniae (PSSP) in 1999-2000 was 18.4%. In 2005-2006, the proportions of penicillin-resistant S. pneumoniae (PRSP) and penicillin-intermediate S. pneumoniae (PISP) were lower, while that of PSSP increased to 38.2%. An early increase in the proportion of PSSP was found in patients older than 3 years, as compared to patients younger than 3 years. The difference between age groups may be attributed to entrance into nursery school, frequent administration of antibiotics, and the immature immunological state of patients younger than 3 years. Therefore, changes in the drug resistance of H. influenzae and S. pneumoniae should be investigated separately, depending on the age of the patients. The minimum inhibitory concentrations of antibiotics, including cefditoren and cefcapene, against BLNAR and PRSP did not increase. The marked increase in intermediate-resistant H. influenzae and BLNAR mandates a re-evaluation of the directions for antibacterial agents.