Application of surface enhanced Raman spectroscopy (SERS) for the rapid detection and enumeration of probiotics in yogurt

Probiotics are a critical component in fermented dairy products such as yogurt. An efficient method to enumerate probiotics in commercially produced yogurt is important, but traditional methods are time-consuming, sometimes taking up to 72 hours. The objective of this study was to develop a rapid method using surface-enhanced Raman spectroscopy (SERS) to identify and enumerate probiotic bacterial cells in yogurt. The identification and enumeration of three different probiotic bacterial strains, <i>Lactobacillus acidophilus</i>, <i>Lactobacillus bulgaricus</i>, and <i>Streptococcus thermophilus</i> were successfully conducted using a developed SERS protocol. <i>L. bulgaricus</i> and <i>S. thermophilus</i> were also identified from a yogurt matrix using a simple extraction procedure utilizing a Percoll gradient. Statistical spectral analysis including principal component analysis and partial least squares regression analysis allowed for the differentiation, characterization and quantification of probiotic strains. The limits of detection were 10⁵ CFU mL^-1 for <i>L. acidophilus</i>, 10⁴ CFU mL^-1 for <i>L. bulgaricus</i>, and 10⁶ CFU mL^-1 for <i>S. thermophilus</i>. Quantification capability was tested up to 10⁸ cfu mL^-1 and showed strong correlation coefficients (0.94, 0.98 and 0.95 respectively) between predicted and actual concentrations. The presence of probiotic mixtures were clearly detectable from a yogurt food matrix starting at 10⁵ CFU mL^-1. Sample analysis took less than three hours per sample set. Our results demonstrate the capability of SERS to detect and enumerate probiotics from a yogurt matrix rapidly.