Direct spectroscopic (FTIR) detection of intraspecific binary contaminations in yeast cultures

Fourier transform infrared spectroscopy has proved to be a good method to identify and characterize microorganisms. This technique has been proposed as a tool to determine the level of contamination in binary mixtures of strains belonging to different species and even to diverse kingdoms, showing a good linear relationship between spectral outputs and contamination levels. The monitoring of intraspecific contamination is a critical point in both laboratory practice and industrial monitoring, but it is challenged by the difficulty to discriminate between very similar cultures belonging to the same species. In this paper we considered binary intraspecific mixtures of strains belonging to three species (Saccharomyces cerevisiae, Debaryomyces hansenii and Rhodotorula minuta). Results showed that contaminated and pure cultures can be discriminated on the basis of their infrared spectra and that different spectral areas respond to the contamination according to the species under test. Moreover, some spectral areas change linearly with the increase of contaminants, giving the possibility of using this procedure for preliminary estimations of the contamination in addition to the even more important opportunity to indicate the presence of contaminants of the same species at low levels in fermentation cultures.