New susceptibility breakpoints in antimicrobial resistance rates of invasive pneumococcal strains

To evaluate the impact of new penicillin susceptibility breakpoints on resistance rates of pneumococcal strains collected from children with pneumonia.

Pneumococcal strains collected from patients admitted with pneumonia were isolated at the clinical analysis lab of Hospital de Clínicas de Uberlândia, Uberlândia, Brazil, and sent to Instituto Adolfo Lutz, São Paulo, Brazil, for further identification, serotyping and determination of antimicrobial susceptibility.

From April 1999 to December 2008, 330 strains of pneumococcus were sent to Instituto Adolfo Lutz; of these, 195 (59%) were collected from patients with pneumonia. One hundred strains collected from patients < or = 12 years old were analyzed. The patients' age ranged from 1 to 12.6 years old (with mean age of 2.4 and median of 1.7 years). Forty-seven patients were male. The strains were isolated from blood (42%) and pleural fluid (58%). There were 35 oxacillin-resistant strains: according to the criteria defined by the Clinical and Laboratory Standards Institute (CLSI) in 2007 [minimum inhibitory concentration (MIC) < or = 0.06 microg/mL for susceptibility (S), 0.12 to 1 microg/mL for intermediate resistance (IR), and > or = 2 microg/mL for full resistance (FR)], 22 strains had IR and 11 strains had FR. According to the current breakpoints defined by the CLSI in 2008 (< or = 2 microg/mL for S, 4 microg/mL for IR and > or = 8 microg/mL for FR), only one strain had IR to penicillin. There was resistance to co-trimoxazole (80%), tetracycline (21%), erythromycin (13%), clindamycin (13%), and ceftriaxone (one strain simultaneously resistant to penicillin).

When the new breakpoints for in vitro susceptibility were applied, penicillin resistance rates dropped 97%, from 33 to 1%.