Author/Authors :
Abelardo Margolles، نويسنده , , Baltasar Mayo، نويسنده , , Clara G de los Reyes-Gavil?n، نويسنده ,
Abstract :
Listeria monocytogenes and Listeria innocua strains isolated from short-ripened cheeses and classified, respectively, into five (m1 to m5) and two (i1 and i2) REDP clusters (restriction enzyme digestion profiles by PFGE), were characterized on the basis of their carbohydrate fermentation profiles, growth in milk, haemolytic activity, sensitivity to sanitizing agents, tolerance to low pH, organic acids and NaCl, and resistance to pasteurization. All isolates fermented L -rhamnose and α-methyl- D -mannoside and most of them were lactose positive; melezitose was fermented by L. monocytogenes strains but not by L. innocua. The haemolytic activity of L. monocytogenes varied depending on the strain. The isolates were sensitive to all cleaning and disinfectant agents tested (acid, alkaline, chlorine and peroxide-containing agents), with the only exception of NaOH. Storage at refrigeration temperature enhanced the tolerance to NaCl of both species and the resistance of L. monocytogenes to the acid. Organic acids were more inhibitory than HCl and their effectiveness was greater at higher than at lower concentration and followed the order acetic>lactic>citric. M4, a group genetically related to certain pathogenic L. monocytogenes strains, was unable to ferment lactose although its growth in milk was not affected. The slightly reduced tolerance to NaCl of this group with respect to the others might have prevented a wider distribution in dairy environments. Although pasteurization was effective in most cases, two different L. monocytogenes strains isolated from the same cheese sample displayed a certain degree of thermotolerance; this resistance could have been induced by successive heat treatments of strains contaminating the dairy environment. All this data pointed to a relationship between several phenotypic characteristics and the ability of Listeria to colonize certain environments.
