Optochin resistance in Streptococcus pneumoniae induced by frozen storage in glycerol

Area of Science:

• Microbiology
• Clinical Diagnostics
• Bacterial Genetics

Background:

• Optochin susceptibility is a primary method for differentiating Streptococcus pneumoniae from other alpha-hemolytic streptococci in clinical settings.
• Accurate identification of Streptococcus pneumoniae is crucial for appropriate patient treatment and epidemiological surveillance.

Purpose of the Study:

• To investigate the impact of frozen storage conditions on the optochin susceptibility of Streptococcus pneumoniae isolates.
• To determine if cryopreservation in glycerol affects the reliability of optochin testing for bacterial identification.

Main Methods:

• Streptococcus pneumoniae isolates were stored in tryptic soy broth with 15% glycerol at -70°C.
• Optochin susceptibility testing was performed on stored isolates.
• Pulsed-field gel electrophoresis (PFGE) and ultrastructural analysis were used to compare resistant and susceptible variants.

Main Results:

• Frozen storage of Streptococcus pneumoniae in 15% glycerol at -70°C induced optochin resistance in some isolates.
• Optochin resistance was sometimes reversible upon subculturing in broth.
• Optochin-resistant and susceptible variants showed similar PFGE patterns but differed in ultrastructure.

Conclusions:

• Frozen storage of Streptococcus pneumoniae in glycerol can alter its optochin phenotype, leading to false-resistant results.
• Optochin susceptibility alone is insufficient for definitive identification of Streptococcus pneumoniae following frozen storage.
• Additional diagnostic methods are recommended to confirm Streptococcus pneumoniae identification after cryopreservation.