Controlling the proliferation of Zygosaccharomyces bailii in ephedrine anti-phlegm cough mixture

To explain a reported proliferation of Zygosaccharomyces bailii in a commercially available ephedrine anti-phlegm cough mixture preserved with 0.1% sodium benzoate, and to present a strategy for controlling its growth.

A yeast strain was isolated from the cough mixture and identified using biochemical tests, selective media and 18S rDNA sequencing. Preservative minimum inhibitory concentrations (MICs) were determined based on the broth microdilution technique. The cough mixture was reproduced using benzoate and a number of other candidate preservatives. Bottles were challenged with the yeast using the test for antimicrobial efficacy outlined in the monographs of the European Pharmacopoeia.

The contaminating yeast was identified as Z. bailii. The yeast MIC value for benzoate was close to or above the concentration used in the commercially available cough mixture. Reintroduction of the strain into bottles preserved with benzoate or sorbate, gave an initial reduction in the inoculum concentration (>1 log) followed by growth to values close to those found in the contaminated product. Furthermore, yeast cells taken from bottles at the end of the challenge test, suffered no initial reduction in numbers and grew in fresh bottles of the product, possibly suggesting adaptation to weak-acid preservatives. Two paraben-based preservative systems passed the challenge test.

Preservatives of the weak-acid type do not control the growth of Z. bailii in ephedrine cough mixture for reasons discussed in this article. If the raw juice used in production of the product cannot be treated to eliminate the yeast, other preservatives must be sought. We show that paraben-based systems are effective in this role, and these are discussed as possible replacements for benzoate in the cough mixture.