Combined effects of thermal treatments and monoglycerides on Listeria monocytogenes survival in model system

The antimicrobial activity of various fatty acids or monoglycerides is well known and it has been exploited mainly for medical purposes to control infection caused by Chlamydia trachomatis, Neisseria gonorrheae and Candida albicans. However, studies concerning their mechanisms of action towards microbial cells are lacking. In this work the influence of some monoglycerides on the viability and heat resistance of Listeria monocytogenes was investigated. In particular, the deactivation curves of three stains of Listeria monocytogenes, characterised by a different stress resistance, were obtained in model system with added different monoglycerides, i.e. monolaurin, monomyristin, monolinolein and monolinolenin (0, 5, 15, 50 and 300 ppm). Moreover, the combined effect of thermal treatments and exposition to monoglycerides on cellular structure has been evaluated by means of Scanning Electron Microscopy (SEM). The results obtained evidenced that all the monoglycerides tested have an immediate and intense activity towards outer cell membranes generating superficial damages. This resulted in a viability loss ranging between 1 and 3.5 Log CFU/ml following a 10 min-exposure. This effect was strengthened when the temperature was lowered from 25°C down to 2°C. All the molecules tested also strongly influenced the deactivation dynamics of the three strains of Listeria monocytogenes subjected to heat treatments at 70°C: in fact their addition to the system resulted in a sharper slope of the survival curves regardless of the strain tested. The highest antimicrobial efficacy was detected for monolinolenin and monolinolein that caused a 6 and 5 Log CFU/ml reduction, respectively, following a 20-second treatment when employed at the lowest concentration (15 ppm). The antimicrobial activity of these compounds could be attributed to their affinity and “emulsifying” action towards cellular membranes, particularly at the high temperature adopted for the thermal treatments. In fact, SEM micrographs of cells exposed to the monoglycerides during heat treatments evidenced severe surface damages and loss of cellular turgor that could also result in the discharge of cytoplasmic content