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Applied and Environmental Microbiology, December 2005, p. 8390-8396, Vol. 71, No. 12
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.12.8390-8396.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Contingency Locus in ctsR of Listeria monocytogenes Scott A: a Strategy for Occurrence of Abundant Piezotolerant Isolates within Clonal Populations

Kimon A. G. Karatzas,^1,2, Vasileios P. Valdramidis,^1,2, and Marjon H. J. Wells-Bennik^1,2*

Wageningen Centre of Food Sciences, P.O. Box 557, 6700 AN Wageningen, The Netherlands,¹ Agrotechnological Research Institute, P.O. Box 17, 6700 AA Wageningen, The Netherlands²

Received 25 May 2005/ Accepted 6 September 2005

In a recent study we demonstrated that a high-hydrostatic-pressure-tolerant isolate of Listeria monocytogenes lacks a codon in the class 3 heat shock regulator gene ctsR. This mutation in the region that encodes four consecutive glycines was directly responsible for the observed piezotolerance, increased stress resistance, and reduced virulence. The aim of the present study was to determine whether mutations in ctsR are frequently associated with piezotolerance in L. monocytogenes. Wild-type cultures of L. monocytogenes were therefore exposed to 350 MPa for 20 min, and the piezotolerance of individual surviving isolates was assessed. This rendered 33 isolates with a stable piezotolerant phenotype from a total of 84 survivors. Stable piezotolerant mutants were estimated to be present in the initial wild-type population at frequencies of >10^–5. Subsequent sequencing of the ctsR gene of all stable piezotolerant isolates revealed that two-thirds of the strains (i.e., n = 21) had mutations in this gene. The majority of the mutations (16 of 21 strains) consisted of a triplet deletion in the glycine-encoding region of ctsR, identical to what was found in our previous study. Interestingly, 2 of 21 mutants contained a codon insertion in this repeat region. The remaining three stable piezotolerant strains showed a 19-bp insertion in the glycine repeat region, a 16-bp insertion downstream of the glycine repeat area (both leading to frameshifts and a truncated ctsR), and an in-frame 114-bp deletion encoding a drastically shortened carboxy terminus of CtsR. In four instances it was not possible to generate a PCR product. A piezotolerant phenotype could not be linked to mutations in ctsR in 8 of 33 isolates, indicating that other thus-far-unknown mechanisms also lead to stable piezotolerance. The present study highlights the importance of ctsR in piezotolerance and stress tolerance of L. monocytogenes, and it demonstrates that short-sequence repeat regions contribute significantly to the occurrence of a piezotolerant and stress-tolerant subpopulation within L. monocytogenes cultures, thus playing an important role in survival.

Present address: University of Bristol, School of Clinical Veterinary Science, Veterinary Pathology and Infection and Immunity, Langford House, Langford, Bristol BS40 5DU, United Kingdom.

Present address: BioTeC-Bioprocess Technology and Control, Department of Chemical Engineering, Faculty of Applied Sciences, Katholieke Universiteit Leuven, W. de Croylaan 46, B-3001 Leuven (Heverlee), Belgium.

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