Bacteriostatic mechanisms of CO2 against high-oxygen packaged meat-borne Pseudomonas fragi during chilled storage: Membrane physiological and microstructural changes.

The CO2 inside high-oxygen modified atmosphere packaging (HiOx-MAP) greatly retards the predominant Pseudomonas fragi (P. fragi) growth in chilled meat, whereas the CO2-regulated mechanism on P. fragi cell membrane properties is still unclear. This study investigated the influences of CO2 on both type and wild-type P. fragi growth and cell membrane characteristics during the chilled storage of HiOx-MAP (TMAP = 50% O2/40% CO2/10% N2; CMAP = 50% O2/50% N2). Compared to CMAP, TMAP significantly reduced P. fragi counts by limiting its motility and disturbing cell membrane structure, composition and function, without causing membrane rupture. CO2 weakened P. fragi membrane function by inducing phospholipid perturbations, but promoted intracellular adenosine triphosphate production as a stress response. Although both packaged P. fragi membrane fluidity and permeability first decreased then increased during storage, a higher (P < 0.05) membrane fluidity and permeability was observed in TMAP P. fragi than in CMAP P. fragi over time, indicating P. fragi gradually adapted to a high-CO2 environment. The CO2-elevated ratio of unsaturated to saturated fatty acids significantly increased P. fragi membrane fluidity, and thus enhanced its membrane permeability. This study offers a new mechanistic basis for the CO2-bacteriostatic action in meat preservation targeted at bacterial cell membranes.