Detection of sublethal thermal injury in Salmonella enterica serotype typhimurium and Listeria monocytogenes using fourier transform infrared (FT-IR) spectroscopy (4000 to 600 cm-1)

Fourier transform infrared (FT-IR) spectroscopy (4000 to 600 cm(-1)) was utilized to detect sublethally heat-injured microorganisms: Salmonella enterica serotype Typhimurium ATCC 14028, a Gram-negative bacterium, and Listeria monocytogenes ATCC 19113, a Gram-positive bacterium. A range of heat treatments (N=2) at 60 degrees C were evaluated:0D(control), 2D, 4D, 6D, and 8D using D-60 degrees C (S. enterica serotype Typhimurium ATCC 14028=0.30 min, L. monocytogenes ATCC 19113=0.43 min). The mechanism of cell injury appeared to be different for Gram-negative and Gram-positive microbes as observed from differences in the 2nd derivative transformations and loadings plot of bacterial spectra following heat treatment. The loadings for PC1 and PC2 confirmed that the amide I and amide II bands were the major contribution to spectral variation, with relatively small contributions from C-H deformations, the antisymmetric P=O stretching modes of the phosphodiester nucleic acid backbone, and the C-O-C stretching modes of polysaccharides. Using soft independent modeling of class analogy (SIMCA), the extent of injury could be predicted correctly at least 83% of the time. Partial least square (PLS) calibration analysis was constructed using 5 latent variables for predicting the bacterial counts for survivors of the different heat treatments and yielded a high correlation coefficient (R=0.97 [S. enterica serotype Typhimurium] and 0.98 [L. monocytogenes]) and a standard error of prediction (SEP=0.51 [S. enterica serotype Typhimurium] and 0.39 log(10) CFU/mL [L. monocytogenes]), indicating that the degree of heat injury could be predicted.