Sample abstract

Title: Frozen storage in glycerol induces optochin resistance in S. pneumoniae

Optochin susceptibility has been used for more than 50 years to differentiate Streptococcus pneumoniae from other alpha-hemolytic streptococcus species that occupy the respiratory tract. Classically, S. pneumoniae on blood agar will show a zone of inhibition of at least 14 mm around an optochin disk when incubated overnight at 37° C at 5% CO2, while other streptococci are resistant to optochin. This study shows that storage of S. pneumoniae in tryptic soy broth containing 15% glycerol at -70° C can induce optochin resistance in clinical isolates. This optochin resistance was sometimes reversible upon culturing of previously-frozen bacteria in broth. Optochin-susceptible and optochin-resistant variants of individual S. pneumoniae isolates have the same PFGE pattern, but optochin-resistant S. pneumoniae exhibit differences in ultrastructure compared to optochin-susceptible variants. Optochin susceptibility thus should not be used as the only means of differentiating S. pneumoniae from other alpha-hemolytic streptococci, and researchers should be aware that long-term frozen storage of S. pneumoniae isolates has the potential to affect their optochin susceptibility phenotype.

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