Development and evaluation of a new selective agar plate for penicillin-resistant pneumococci (PRSP)

Penicillin-resistant pneumococci (PRSP) have been increasing recently in our hospital. The percentages of penicillin-resistant strains in isolated pneumococci were 38.5% in 1992, 33% in 1993 and 34% in 1994. We developed a new selective agar plate for PRSP (PRSP-SM). Briefly, we added oxacillin (0.1 mg/1), aztreonam (10 mg/1) and gentamicin (5 mg/1) to Tripticase soy II agar supplemented with 5% sheep blood. In these agar plates, 100% (70/70) of PRP grew and 81% (91/112) of penicillin-sensitive pneumococci (PSSP) did not grow. However, some gram-negative rods (Pseudomonas aeruginosa, Pseudomonas cepacia, Serratia marcescens, Xanthomonas maltophilia) also grew. Antibiotics in the agar plates stored at 4°C were active enough for 42 days (oxacillin) or 90 days (aztreonam, gentamicin). In these agar plates, PRSP in the clinical samples grew by overnight incubation, however major contaminant organisms, P.aeruginosa, Haemophilus influenzae, Staphylococcus aureus, Branhamella catarrhalis and PSSP almost never grow.