Optochin resistance in Streptococcus pneumoniae: mechanism, significance, and clinical implications.

Traditionally, Streptococcus pneumoniae is identified in the laboratory by demonstrating susceptibility to optochin. Between 1992 and 1998, 4 pneumococcal isolates exhibiting optochin resistance were recovered from patients at Children's National Medical Center. Three of the 4 isolates consisted of mixed populations of optochin-resistant and -susceptible organisms. Both subpopulations had identical antibiograms, serotypes, and restriction fragment profiles. The other isolate was uniformly resistant to optochin. Resistant strains had MICs of optochin 4-30-fold higher than susceptible strains, belonged to different serotypes, and had dissimilar restriction fragment profiles, indicating clonal unrelatedness. Resistance arose from single point mutations in either the a-subunit (W206S) or the c-subunit (G20S, M23I, and A49T) of H(+)-ATPase. There is speculation of a possible association between exposure to antimalarial drugs and evolution of optochin resistance. alpha-Hemolytic streptococci resistant to optochin, particularly invasive isolates, should be tested for bile solubility or with an S. pneumoniae DNA probe before identification as viridans streptococci.