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Applied and Environmental Microbiology, November 2008, p. 6570-6583, Vol. 74, No. 21
0099-2240/08/$08.00+0 doi:10.1128/AEM.00997-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.
inlA Premature Stop Codons Are Common among Listeria monocytogenes Isolates from Foods and Yield Virulence-Attenuated Strains That Confer Protection against Fully Virulent Strains
,
K. K. Nightingale,1,2*
R. A. Ivy,1
A. J. Ho,1
E. D. Fortes,1
B. L. Njaa,3,4
R. M. Peters,3 and
M. Wiedmann1
Department of Food Science, Cornell University, Ithaca, New York 14853,1 Department of Animal Sciences, Colorado State University, Fort Collins, Colorado 80523,2 Department of Biomedical Sciences, Cornell University, Ithaca, New York 14853,3 Department of Veterinary Pathology, Oklahoma State University, Stillwater, Oklahoma 740784
Received 2 May 2008/ Accepted 30 August 2008
Previous studies showed that a considerable proportion of Listeria monocytogenes isolates obtained from foods carry a premature stop codon (PMSC) mutation in inlA that leads to production of a truncated and secreted InlA. To further elucidate the role these mutations play in virulence of L. monocytogenes, we created isogenic mutants, including (i) natural isolates where an inlA PMSC was reverted to a wild-type inlA allele (without a PMSC) and (ii) natural isolates where a PMSC mutation was introduced into a wild-type inlA allele; isogenic mutant sets were constructed to represent two distinct inlA PMSC mutations. Phenotypical and transcriptional analysis data showed that inlA PMSC mutations do not have a polar effect on the downstream inlB. Isogenic and natural strains carrying an inlA PMSC showed significantly reduced invasion efficiencies in Caco-2 and HepG2 cell lines as well as reduced virulence in oral guinea pig infections. Guinea pigs were also orally infected with a natural strain carrying the most common inlA PMSC mutation (vaccinated group), followed by challenge with a fully virulent L. monocytogenes strain 15 days postvaccination to probe potentially immunizing effects of exposure to L. monocytogenes with inlA PMSC mutations. Vaccinated guinea pigs showed reduced bacterial loads in internal organs and improved weight gain postchallenge, indicating reduced severity of infections in guinea pigs exposed to natural strains with inlA PMSC mutations. Our data support that (i) inlA PMSC mutations are causally associated with attenuated virulence in mammalian hosts and (ii) naturally occurring virulence-attenuated L. monocytogenes strains commonly found in food confer protective immunity.
* Corresponding author. Mailing address: Department of Animal Sciences, Colorado State University, 108B Animal Sciences Building, Fort Collins, CO 80523-1171. Phone: (970) 491-1156. Fax: (970) 491-5326. E-mail: kendra.nightingale{at}colostate.edu
Published ahead of print on 12 September 2008.
Supplemental material for this article may be found at http://aem.asm.org/.
Applied and Environmental Microbiology, November 2008, p. 6570-6583, Vol. 74, No. 21
0099-2240/08/$08.00+0 doi:10.1128/AEM.00997-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.
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