title: Acid and heat tolerance of persistent and nonpersistent Listeria monocytogenes food plant strains.
abstract: OBJECTIVE
  Acid and heat tolerance of 17 persistent and 23 nonpersistent Listeria monocytogenes strains, recovered from three meat-processing plants, were investigated.
  
  
  RESULTS
  The isolates were genotyped by pulsed-field gel electrophoresis and categorized into persistent strains according to the frequency of the strain and duration of the contamination. The persistent and nonpersistent strains were challenged to acidic conditions (pH 2.4 for 2 h, 1 mol l(-1) HCl were used to acidify the suspension) and to heat (55 degrees C for 40 min) to receive a reduction in cell count. Listeria monocytogenes strains showed large variation in acid tolerance (over 6 log units) and in heat tolerance (3 log units). The persistent strains showed higher tolerance to acidic conditions than the nonpersistent strains (Student's t-test, P = 0.02), but significant differences in heat tolerance between persistent and nonpersistent strains were not observed.
  
  
  CONCLUSIONS
  The results indicate that acid tolerance may have an effect on the persistence of L. monocytogenes contamination.
  
  
  CONCLUSIONS
  This study highlights the fact that there are great differences in acid and heat tolerances between L. monocytogenes strains, and the preventive measures should be designed to be effective against the most tolerant strains.
authors.affiliation: Department of Food and Environmental Hygiene, Faculty of Veterinary Medicine, University of Helsinki, Helsinki, Finland. janne.lunden@helsinki.fi
authors.forename: J
authors.initials: J
authors.lastname: Lundén
authors.forename: R
authors.initials: R
authors.lastname: Tolvanen
authors.forename: H
authors.initials: H
authors.lastname: Korkeala
mesh_terms: D000143:Acids / Q000494:pharmacology*; D000222:Adaptation, Physiological; D000818:Animals; D024881:Drug Resistance, Bacterial; D016521:Electrophoresis, Gel, Pulsed-Field; D005516:Food Microbiology; D005525:Food-Processing Industry; D005838:Genotype; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D008089:Listeria monocytogenes / Q000187:drug effects* / Q000235:genetics / Q000254:growth & development; D008461:Meat Products / Q000382:microbiology; D013045:Species Specificity
chemical_list: D000143:Acids