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

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Abstract
The use of sheep blood agar containing 5 .mu.g of gentamicin/ml was suggested as a means of selectively isolating Streptococcus pneumoniae from respiratory secretions. This method was tested in parallel with standard methods on 844 [human] respiratory specimens in a clinical laboratory and it was confirmed that the yield of pneumococci can be increased .apprx. 40% by using agar containing gentamicin. 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. The yield of pneumococci in respiratory secretions can apparently be increased 10-fold simply by screening sputum for the presence of leukocytes using the Gram stain.