Examining <i>Listeria monocytogenes</i> Cell Envelope Physiology in Dairy Relevant Environments

In the food industry, many interventions used to control L. monocytogenes target the cell envelope, the outer barrier of the cell made up of various glycopolymers and wall teichoic acids. The cell envelope functions in protection of the cell and as an entryway for many treatment methods. It’s unclear if changes in the environment will affect the physiology of the cell envelope and in turn the cell’s sensitivity to antimicrobial interventions. For example, nisin is a common bacteriocin used as an antimicrobial agent against L. monocytogenes. However, previous studies have found that cell envelope changes are responsible for the acquisition of nisin resistance. It has also been shown that environmental conditions such as temperature and pH have had significant impact on nisin and bacteriophage susceptibility. The purpose of this study was to determine if typical dairy processing conditions affect the physiological state of L. monocytogenes cell envelope. Model laboratory, dairy outbreak, and mutant strains of L. monocytogenes were grown on skim milk agar media at different pHs (5.7, 6.0, 6.2, 6.5) and temperatures (6°C, 14°C, 22°C, 30°C) relevant to the dairy industry. The cells were collected, washed, and standardized to a specific optical density (OD₆₀₀). The cells were then added to either phage binding or cytochrome c binding assays. Phages LP-048 and LP-125 were used as they bind to specific cell surface residues (N-acetylglucosamine and rhamnose). Cytochrome c was used to measure cell wall charge as a negative charge indicator. Significant differences were found between conditions, but results varied by experiment and strain. It was concluded that the physiological state of the L. monocytogenes cell envelope is affected by the cell’s growth conditions. As the cell envelope is a major factor in resistance or susceptibility to antimicrobials and other control interventions, these data suggest that control interventions can be targeted to specific pH or temperature conditions for greater impact.