The Effectiveness of Solid-Phase Antimicrobials Against Gram-Positive Bacteria in Food Processing

Contamination of food products during production remains an immense risk to public health with foodborne pathogens causing outbreaks each year. The dairy industry is searching for methods to eliminate or mitigate bacterial growth, most prominently that of Listeria monocytogenes (LM), a gram-positive bacterium. LM infection is particularly concerning due to the alarmingly high mortality rate of 20-30% for young, elderly, and immuno-compromised individuals. In addition, consumer demands for lower levels of preservatives incentivize the discovery of novel antibacterial methods. Solid-phase antimicrobial surfaces represent one such method of targeting and destroying these gram-positive bacteria.

The feed spacer material of a spiral-wound membrane element represents an ideal surface to functionalize with a solid-phase antimicrobial agent. The feed spacer is composed of polypropylene (PP) and serves to induce turbulence and preserve flux. Furthermore, the spacer is in direct contract with the dairy process stream and can be implemented throughout the manufacturing process.

Compounds shown to possess antimicrobial properties in solution were selected to attach to the PP membrane to confirm any suspected solid-phase antimicrobial activity. Phenols and polyphenols were chosen as they have been shown to inhibit bacterial cell enzymes and disrupt their phospholipid bilayer leading to cell lysis. Quaternary ammonium compounds (QACs) were also chosen as their positive head and hydrophobic tail have been shown to heavily disrupt the integrity of the bacterial cell wall leading to cell death.

The surface chemistry of the PP feed spacer was altered with phenolic and quaternary ammonium compounds to confirm any solid-phase antimicrobial activity. Gram-positive bacteria served as surrogates to LM due to their similar cell wall structures and ease of use. The functionalized PP mesh was qualitatively assessed by dye characterization, with the absorption of dye confirming the successful alteration of the PP mesh. Any antimicrobial activity of phenolic modified mesh, represented by eugenol and tyrosine, was found to be negligible. This is suspected to be a dosing issue or due to phenols/polyphenols antibacterial method of action being inapplicable in the solid phase. The QAC altered mesh, however, was found to eliminate gram-positive bacteria below detection limits. These results suggest that PP membranes functionalized with a QAC could serve as an extremely effective antimicrobial surface, providing a novel solution against pathogenic bacteria in the dairy industry and food processing as a whole.