Changes in Ultrastructure and Fourier Transform Infrared Spectrum of Salmonella enterica Serovar Typhimurium Cells after Exposure to Stress Conditions

Abstract

The effect of exposure to acid (pH 2.5), alkaline (pH 11.0), heat (55°C), and oxidative (40 mM H 2 O 2 ) lethal conditions on the ultrastructure and global chemical composition of Salmonella enterica serovar Typhimurium CECT 443 cells was studied using transmission electron microscopy and Fourier transform infrared spectroscopy (FT-IR) combined with multivariate statistical methods (hierarchical cluster analysis and factor analysis). Infrared spectra exhibited marked differences in the five spectral regions for all conditions tested compared to those of nontreated control cells, which suggests the existence of a complex bacterial stress response in which modifications in a wide variety of cellular compounds are involved. The visible spectral changes observed in all of the spectral regions, together with ultrastructural changes observed by transmission electron microscopy and data obtained from membrane integrity tests, indicate the existence of membrane damage or alterations in membrane composition after heat, acid, alkaline, and oxidative treatments. Results obtained in this study indicate the potential of FT-IR spectroscopy to discriminate between intact and injured bacterial cells and between treatment technologies, and they show the adequacy of this technique to study the molecular aspects of bacterial stress response.Salmonella spp. are an important cause of bacterial foodborne disease all over the world, causing a diversity of illnesses that include typhoid fever, gastroenteritis, and septicemia (11). According to epidemiological data from the European Union, a total of 131,468 laboratory-confirmed salmonellosis cases were reported in 2008, with two serovars, Salmonella enterica serovar Typhimurium and Salmonella enterica serovar Enteritidis, being responsible for 79.9% of all cases (13). The detection and identification of pathogens in foods are a basic cornerstone of food safety, because they make it possible to identify sources of contamination, provide data on the evaluation of risk reduction measures, and identify the food chain operations, processes, batches, or products representing a threat to public health. Furthermore, they also are fundamental in the epidemiological investigation of food-borne diseases. The presence of stress-injured bacterial cells in foods represents a challenge to those involved in food quality assurance, as routine microbiological procedures may yield negative results for sublethally injured cells. Thus, food could be presumed to be safe and free from pathogenic cells but during storage become dangerous due to the recovery and growth of previously injured cells.Given the fact that bacterial cells react to the different environmental stress conditions by inducing structural and physiological changes, Fourier transform infrared (FT-IR) spectroscopy, which reflects the biochemical composition of the cellular constituents of bacteria that include water, fatty acids, proteins, polysaccharides, and nucleic acids (26), should be able to monitor the changes occurring in bacterial ce...

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