Purpose: To understand FA composition and cell membrane fluidity effects in desiccation-induced thermal resistance of Salmonella.
Methods: FA extraction and methylation of S. Tennessee cells were done by the Miller-Berger method. FA profiles were generated by GC-FID mass spectrometry (Midi Inc.). Anisotropy and membrane fluidity of fresh and desiccated cells (36% ERH, 24 h and 5 days) were measured by fluorimetry during thermal treatment from 36 to 80°C, using 1, 6-diphenyl-1,3,5-hexatriene. Data from three independent experiments were analyzed by one-way ANOVA.
Results: Compared to fresh cells, desiccated cells exhibited significantly increased percentage of FAs that are saturated (+2.8%), cyclic (+4.5%), and decreased unsaturated FAs (-4.5%). Longer desiccation significantly increased these changes (P = 7.4E-05). There was no significant difference between long- and short-chain FAs among all conditions, indicating that FA changes during desiccation did not result from catabolic pathways. Anisotropic measurements during thermal treatment showed that membrane fluidity of fresh cells remained unchanged until a sharp decrease at 58ºC, indicating a phase transition. Desiccated cells showed overall lower fluidity with a continual decrease from 36 to 50ºC, followed by a gradual increase to 62ºC, then a decrease. The reduction of unsaturated and increase of cyclic FAs in desiccated cells suggested an upshift of melting point, which explained the overall downshift of fluidity and its protection from cellular component leakage.
Significance: Thermal resistance of Salmonella after desiccation is influenced by changes in cell membrane FA composition and structure.