Epidemiological studies of Streptococcus pneumoniae in infants: methods of isolating pneumococci

Abstract

A prospective study of the natural history of pneumococcal infection, which involves serial culture studies in healthy infants from 6 wk of age onward, is in progress. Results of a comparison of several methods for the isolation and identification of S. pneumoniae from the nasopharynges and throats of these infants were described. Sheep blood agar, sheep blood agar with gentamicin sulfate (gentamicin agar) and mouse inoculation with 4-h broth cultures were used. Gentamicin agar was superior to plain sheep blood agar as a solid culture medium, especially in enhancing the recovery of pneumococci from throat cultures. With gentamicin agar, similar carrier rates were found for both culture sites (nasopharynx and throat). Gentamicin agar was superior to mouse inoculation for the recovery of carrier strains from 131 nasopharyngeal culture samples processed by both methods. Of 131 samples, 60 were positive for pneumococci, 25% of which would have been missed had mouse inoculation alone been used. In only 3 instances was a strain recovered by mouse inoculation that failed to grow on gentamicin agar; conversely, 15 strains were isolated on gentamicin agar but could not be recovered from mice. The latter observation might be explained by the fact that certain carrier strains may be relatively mouse avirulent. The use of blood agar containing gentamicin appears to offer a simple and inexpensive method for the recovery of S. pneumoniae and provides an ideal method for the identification of pneumococcal carriers and for the recovery of these strains from clinical material such as sputum or ear exudates, where other and less fastidious organisms may also be present.