Phenotypic characterization of Listeria monocytogenes and Listeria innocuastrains isolated from short-ripened cheeses

Listeria monocytogenes and Listeria innocua strains isolated from short-rip ened 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, gro wth in milk, haemolytic activity, sensitivity to sanitizing agents, toleran ce to low pH, organic acids and NaCl, and resistance to pasteurization. Aii isolates fermented L-rhamnose and alpha-methyl-D-mannoside and most of the m were lactose positive; melezitose was fermented by L. monocytogenes strai ns but not by L. innocua. The haemolytic activity of L. monocytogenes varie d depending on the strain. The isolates were sensitive to all cleaning and disinfectant agents tested (acid, alkaline, chlorine and peroxide-containin g agents), with the only exception of NaOH. Storage at refrigeration temper ature enhanced the tolerance to NaCl of both species and the resistance of L. monocytogenes to the acid. Organic acids were more inhibitory than HCl a nd their effectiveness was greater at higher than at lower concentration an d followed the order acetic > lactic > citric M4, a group genetically relat ed to certain pathogenic L. monocytogenes strains was unable to ferment lac tose although ifs growth in milk was not affected The slightly reduced tole rance to NaCl of this group with respect to the others might have prevented a wider distribution in dairy environments. Although pasteurization was ef fective 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 strain s contaminating the dairy environment Aii this data pointed to a relationsh ip between several phenotypic characteristics and the ability of Listeria t o colonize certain environments. (C) 2000 Academic Press.