Comparison of four methods for identifying Streptococcus pneumoniae

Four methods (bile solubility, optochin, latex agglutination, and DNA probe) were compared for identification of clinical isolates of Streptococcus pneumoniae. Of 209 isolates tested, 151 (Group I) were selected based on typical colony morphology of S. pneumoniae, while 58 (Group II) were selected on the basis of alpha-hemolysis alone. Using the DNA probe as a reference method, 141 isolates from Group I and 10 from Group II were identified as S. pneumoniae. The optochin disk test and the latex agglutination test both exhibited a 100% sensitivity and specificity for Group I isolates; bile solubility identified all but 1 isolate in this group. For Group II isolates, the sensitivity and specificity of optochin testing was 100%, 80 and 94% for latex and 80 and 100% for bile. The results of this study indicate that all methods tested give reliable results for isolates with typical colony morphology of S. pneumoniae. Bile solubility and latex may not be as reliable when testing alpha-hemolytic colonies without colony morphology typical S. pneumoniae.