Detection of pneumococci in respiratory secretions: clinical evaluation of gentamicin blood agar

Abstract

The use of sheep blood agar containing 5 microng of gentamicin per ml has been suggested as a means of selectively isolating Streptococcus pneumoniae from respiratory secretions. We have tested this method, in parallel with standard methods, on 844 respiratory specimens in a clinical laboratory and have confirmed that the yield of pneumococci can be increased approximately 40% by using agar containing gentamicin. However, since the antibiotic suppresses the growth of staphylococci, group A streptococci, and gram-negative bacilli, gentamicin agar cannot be used as a replacement for the standard method. The requirement for duplicate plating raises the cost per additional pneumococcal isolate to prohibitive amounts. Although the method is useful in studies designed to isolate only pneumococci, it cannot be recommended for the routine clinical laboratory. An unanticipated observation from our study is that the yield of pneumococci in respiratory secretions can be increased 10-fold simply by screening sputum for the presence of leukocytes using the Gram stain. This is in agreement with results reported from other laboratories.