Survival of osmotic and acid stress by Listeria monocytogenes strains of clinical or meat origin

The ability of 30 Listeria monocytogenes strains, 15 of meat origin and 15 of clinical origin, to use carnitine as an osmoprotectant and to resist aci d stress was determined. All strains examined were able to use carnitine as an osmoprotectant. indicating the importance of this characteristic to the survival of L. monocytogenes in natural environments. Clinical and meat st rains, however, differed with respect to this characteristic. Specifically, 73% of meat strains reached a lower maximum cell density in the presence o f carnitine with osmotic stress than in its absence with no stress. Only 33 % of clinical strains displayed the same feature whereas the remaining clin ical strains reached a higher maximum cell density in the presence of carni tine with osmotic stress than in its absence with no stress. The physiologi cal reasons and advantage of this difference are unclear. When exposed to c onditions of severe acid stress (pH 2.5) for 2 h, only two L. monocytogenes strains (L66 and L78), both of meat origin, displayed significant reductio ns (P < 0.05) in number (3.51 and 2.79 log cfu, respectively). Acid-sensiti ve strains were not found among the clinical isolates examined, highlightin g the importance of acid stress resistance in the infection process. (C) 20 00 Elsevier Science B.V. All rights reserved.