Gentamicin-blood agar for isolation of Streptococcus pneumoniae from respiratory secretions.

Previous studies have suggested that the yield of Streptococcus pneumoniae from respiratory secretions can be increased by using a 5% sheep blood agar plate supplemented with 5 microgram of gentamicin (GBA) per ml. We report our experience with 245 lower respiratory specimens in which this method was compared with 5% sheep blood agar (SBA) alone. Of 35 specimens with growth of S. pneumoniae on either plate, 21 were detected exclusively on SBA, whereas only 3 were detected on GBA alone (P less than 0.01). By subculturing representative alpha-hemolytic colonies from the final 169 specimens, the yield of S. pneumoniae was increased by 27% compared with the number of identifications that could be made directly from the primary culture. Minimal inhibitory concentrations of gentamicin for the last 25 isolates were greater than or equal to 8 microgram/ml. Our results do not substantiate the previous observations that S. pneumoniae from respiratory secretions gives an increased yield in cultures on GBA.