2.3. Hemolytic activity of L. monocytogenes
The hemolytic activity of Listeria strains was determined by using a procedure developed by Gallego et al. (2007). The isolates were inoculated in brain hearth infusion (BHI) and incubated at 37 C for 18 h. Serial twofold dilutions were made by mixing 50 mL
of each bacterial suspension with 0.85% ClNa in microtitle plates with U-form wells. Then, 100 mL of a 3% suspension of human erythrocytes, previously washed with a solution containing 0.85% ClNa, 0.01% gelatin and 0.43% NaN3,was added to each dilution. The
microplates were incubated at 37 C for 4 h. The hemolytic activity titer was expressed as complete hemolysis units (CHU: the reciprocal of the highest dilution at which a 100% hemolysis was observed) and minimum hemolysis units (MHU: the reciprocal of the highest dilution at which hemolysis was detected).