Identification of Streptococcus pneumoniae: Development of a Standardized Protocol for Optochin Susceptibility...

Area of Science:

• Clinical microbiology
• Bacteriology
• Antimicrobial susceptibility testing

Background:

• Optochin susceptibility testing is a key method for identifying Streptococcus pneumoniae in laboratory settings.
• Currently, there is a lack of a single, standardized protocol for optochin testing.
• Optochin is not included in established antimicrobial susceptibility testing guidelines from EUCAST or CLSI.

Purpose of the Study:

• To establish an evidence-based protocol for optochin testing suitable for automated laboratory systems.
• To optimize the optochin susceptibility test for reliable identification of Streptococcus pneumoniae.

Main Methods:

• Evaluation of seven different agar types and four incubation time points (7, 12, 18, and 24 hours).
• Testing involved 99 Streptococcus pneumoniae strains representing 34 different serotypes to account for diversity.
• Analysis of 5544 inhibition zones using multivariable linear regression.

Main Results:

• Reading optochin inhibition zones was feasible for all strains at the 12-hour time point.
• Agar type and manufacturer significantly influenced inhibition zone sizes (up to 2mm variation).
• Serotype variation, particularly for serotype 3, also impacted zone sizes (up to 3mm variation).
• False-negative results (zones <14mm) occurred in up to 38% of tests, dependent on agar and incubation time.

Conclusions:

• Shortening the incubation time for optochin susceptibility testing from 24 hours to 12 hours is a viable option.
• Careful selection of agar medium and incubation duration is critical to minimize false-negative optochin test results.
• Standardization of optochin testing protocols is needed to improve the accuracy of Streptococcus pneumoniae identification.