Optochin resistance in Streptococcus pneumoniae induced by frozen storage in glycerol

Susceptibility to optochin is frequently the only test used to differentiate Streptococcus pneumoniae from other α-hemolytic streptococci isolated from clinical specimens. The current study shows that storage of S. pneumoniae isolates in tryptic soy broth containing 15% glycerol at −70 °C can lead to optochin resistance. This optochin resistance was sometimes reversible by growing the bacteria in broth. Optochin-susceptible and optochin-resistant variants of individual S. pneumoniae isolates have similar pulsed-field gel electrophoresis pattern. However, optochin-resistant S. pneumoniae isolates exhibit differences in ultrastructure compared with optochin-susceptible variants. This study demonstrates that the frozen storages of S. pneumoniae in glycerol may affect the optochin phenotype. Thus, this characteristic should not be the only one used for identification of S. pneumoniae after frozen storage of isolates.