Abstract
Purpose. Optochin susceptibility is one parameter used in the laboratory to identify Streptococcus pneumoniae. However, a single standardized procedure does not exist. Optochin is included neither in the current EUCAST breakpoint tables nor in the CLSI performance standards for antimicrobial susceptibility testing. We wanted to establish an evidence-based protocol for optochin testing for our Total Lab Automation. Methods. We tested seven different agars and four different reading time points (7 h, 12 h, 18 h, and 24 h). To accommodate for serotype diversity, all tests were done with 99 different strains covering 34 different serotypes of S. pneumoniae. We calculated a multivariable linear regression using data from 5544 inhibition zones. Results. Reading was possible for all strains at 12 h. Agar type and manufacturer influenced the size of the inhibition zones by up to 2 mm and they varied considerably depending on serotype (up to 3 mm for serotype 3). Depending on agar and reading time point, up to 38% of inhibition zones were smaller than the cut-off of 14 mm; that is, the result of the test was false-negative. Conclusions. Shortening incubation time from 24 h to 12 h for optochin susceptibility testing is feasible. Agar and incubation time have to be chosen carefully to avoid false-negative results.
Highlights
Even in 2017, the unequivocal identification of Streptococcus pneumoniae is still easier said than done
For all experiments the following media were used: Columbia agar with 5% sheep blood (COS): bioMerieux (COS_bmx), Becton Dickinson (COS_BD); Trypticase Soy agar with 5% sheep blood (TSA): bioMerieux (TSA_bmx), Oxoid (TSA_Ox); Müller-Hinton agar with 5% horse blood (MHF): bioMerieux (MHF_bmx), Oxoid (MHF_Ox); Müller-Hinton agar with 5% sheep blood (MHB): bioMerieux (MHB_bmx); all plates used were produced by commercial manufacturers and used within their regular shelf live
We modeled the influence of the categorical variables, agar, incubation time, and serotype, on the continuous outcome variable, diameters of inhibition zones (DOIZ)
Summary
Even in 2017, the unequivocal identification of Streptococcus pneumoniae is still easier said than done. Its derivative ethylhydrocuprein hydrochloride is better known as optochin It was tested as a treatment option for experimental pneumococcal infections in animals (e.g., for experimental pleuritis in mice) [2]. For reasons of solubility and tolerance, ethylhydrocuprein was never introduced as an i.v. drug for use in humans For some time, it was used as a local treatment for ocular infections [3]. Bowers and Jeffries described the laboratory diagnosis of S. pneumoniae using optochin susceptibility and bile solubility [4]. In Europe, EUCAST propagates Müller-Hinton agar with horse blood for susceptibility testing of fastidious organisms (like S. pneumoniae), but there are no data on the performance of the optochin disk on Müller-Hinton agar with horse blood. Taking into consideration the fact that there are more than different serotypes of pneumococci, is there a serotypedependent influence on the size of the optochin inhibition zone?
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