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 pneumoco ccal isolates exhibiting optochin resistance were recovered from patients a t Children's National Medical Center. Three of the 4 isolates consisted of mixed populations of optochin-resistant and -susceptible organisms. Both su bpopulations had identical antibiograms, serotypes, and restriction fragmen t profiles. The other isolate was uniformly resistant to optochin. Resistan t strains had MICs of optochin 4-30-fold higher than susceptible strains, b elonged to different serotypes, and had dissimilar restriction fragment pro files, 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 isola tes, should be tested for bile solubility or with an S. pneumoniae DNA prob e before identification as viridans streptococci.