Discrimination of lactic acid bacteria Enterococcus and Lactococcus by infrared spectroscopy and multivariate techniques

Abstract

Raw milk is often described as a major source of lactic acid bacteria for indigenous lactic starter. These indigenous starters contribute to the sensorial quality of cheese. Raw milk, rich in Lactoccocus lactis may therefore be very interesting for the cheese making. Currently, the most commonly used methods to differentiate lactic acid bacteria, and particularly the closely related phenotypes Lactococcus and Enterococcus, are based on DNA sequencing, but the cost and time required for these analytical methods hinder their use for rapid screening of raw material. The present study therefore proposes a simple alternative method to identify and discriminate against Lactococcus and Enterococcus, at the genus, but also at the species level, that is based on collecting near infrared spectra directly from bacterial colonies in Petri dishes. The infrared spectra of 280 strains of Lactococcus and Enterococcus cultured on solid media were collected by using a spectrometer with a wavelength range of 908 to 1684 nm and a remote probe. The best Classification And Regression Trees models for genus and species discrimination gave an excellent classification rate of 87% on an external validation set (30 strains). Loading line plots, with prominent bands at 900–960 and 1270–1390 nm, confirmed that the source of variation was due to changes in the polysaccharides.