Extracellular metabolites play a dominant role in near-infrared spectroscopic quantification of bacteria at food-safety level concentrations

Abstract

Near infrared spectroscopy (NIRS) is a promising new tool for quick and accurate bacterial quantification in water-rich samples like food. However, we still do not know what this spectroscopic method takes into account in order to measure bacterial counts at food safety limits. This study tries to identify which part of the bacterial culture has a more significant impact on spectral models for bacterial quantification in an aqueous environment. Two type strains of E. coli and S. aureus were evaluated, at a concentration range 0.1–8 log₁₀ CFU. Spectra from bacterial cells, separated by filtration, and extracellular metabolites were acquired separately and then combined in a reciprocal ratio in order to identify the spectral combination that would provide the best partial least squares quantification model for each strain. Spectral data were evaluated in the first wavelength overtone region of water 1300–1600 nm. The best model was achieved at a cells : extracellular metabolites spectral combination ratio of 40 : 60 for the E. coli strain and 20 : 80 for the S. aureus strain. While the best ratio was different for each species, extracellular metabolite spectra had a consistent significantly higher impact on PLS models. It can be concluded that species-specific extracellular metabolite spectra give more valuable information on bacterial quantification, and it is the spectral detection of changes in extracellular water conformations, due to extracellular bacterial metabolites, that represents the most significant factor for successful bacterial quantification. Finding out that bacteria influence the water matrix is an important step in understanding the mechanism of NIRS detection of low bacterial concentrations.