Flow Cytometry Analysis and Transmission Electron Microscopic Examination of Listeria monocytogenes Treated with Sodium Metasilicate

Introduction: Occurrence of Listeria monocytogenes in ready-to-eat (RTE) food products is a significant food safety concern. Sodium metasilicate (SMS) is an alkaline antimicrobial compound approved by USDA for use in RTE meat and poultry products. Sodium metasilicate has been found to be effective against various foodborne pathogens, but the mechanism by which it inactivates foodborne pathogenic bacteria needs investigation.

Purpose: To determine the antimicrobial mechanism of action of sodium metasilicate for inactivation of L. monocytogenes.

Methods: The effect of SMS on the membrane integrity and viability of L. monocytogenes (Scott A) was determined by use of fluorescent propidium iodide (PI) and SYTO9 nucleic acid stains with subsequent flow cytometry. Listeria monocytogenes cells were treated with 2% SMS solution (w/v) and high pH (0.1 N NaOH) and stained with PI and SYTO9 and subjected to flow cytometry analysis to differentiate live and dead bacterial cells based on the membrane integrity. Transmission electron microscopy (TEM) was performed to observe the changes at cellular level following exposure of L. monocytogenes cell suspensions to 2% SMS and 0.1 N NaOH treatments.

Results: The disruption in membrane integrity was observed by uptake of PI by cells treated with SMS with subsequent flow cytometry. Ultrastructural changes from corresponding transmission electron micrographs further verified the disruption in the cytoplasmic membrane and changes in the morphology of the cells treated with SMS and high pH.

Significance: The results from flow cytometry analysis and transmission electron microscopy examination indicated that after SMS treatment, the membrane integrity of L. monocytogenes was compromised leading to leakage of intracellular contents and subsequent cell death.