Submitted to: Applied and Environmental Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/20/2003
Publication Date: 4/3/2004
Citation: WONDERLING, L.D., BAYLES, D.O., WILKINSON, B.J. THE HTRA (DEGP) GENE OF LISTERIA MONOCYTOGENES 10403S, A SEROTYPE 1/2A ISOLATE, IS ESSENTIAL FOR OPTIMAL GROWTH UNDER STRESS CONDITIONS. APPLIED AND ENVIRONMENTAL MICROBIOLOGY. 2004. Vol. 4T7. pg. 1935-1943 Interpretive Summary: The food-borne pathogen Listeria monocytogenes is responsible for sporadic and outbreak cases of listeriosis in the United States and abroad. Ready- to-eat foods continue to be of particular concern since there have been outbreaks of listeriosis directly traced to consumption of these products. In addition to public health concerns, there are economic losses for food producers when ready-to-eat foods test positive for Listeria or are recalled due to contamination with L. monocytogenes. L. monocytogenes is a difficult pathogen to control in foods since the organism can survive and sometimes grow in foods containing added NaCl and at refrigeration temperature, conditions commonly associated with ready-to-eat foods. This study identified a gene necessary for optimal L. monocytogenes growth in high salt environments. L. monocytogenes strains that did not contain the functional gene were characterized by increased sensitivity to salt, elevated temperature, and oxidative stress, conditions that are frequently encountered in food, food processing, and during infection. The gene identified encodes a serine protease enzyme that has been shown to be necessary for some bacteria to grow in unfavorable environments. Compounds able to inhibit the functioning of this enzyme may be useful in controlling or eliminating L. monocytogenes in food environments and may possibly reduce the organism's ability to cause infection.
Technical Abstract: This study describes a mutant of Listeria monocytogenes strain 10403S (serotype 1/2a) with a defective response to conditions of high osmolarity, an environment this food-borne pathogen is exposed to in some ready-to-eat foods. A library of Tn917 mutants of Listeria monocytogenes was generated and scored for sensitivity to 4% NaCl to identify genes responsible for growth or survival in elevated NaCl environments. One L. monocytogenes Tn917 mutant, designated strain OSM1, was selected and subsequently tested for its responses to elevated NaCl, elevated temperature, and H202 challenge. L. monocytogenes OSM1 showed a 19% decreased growth rate compared to the parent strain when grown in complex media containing 1 M NaCl. In defined media with 0.5 or 0.7 M NaCl, growth of the parent strain was reduced 48 or 38%, respectively, compared to the no salt condition, but the OSM1 mutant was unable to grow at either NaCl concentration. In addition, challenge with 300 or 600 ug of hydrogen peroxide caused 121 and 125% larger zones of inhibition, respectively, for OSM1 compared to the parent strain, and OSM1 grew at a 50%-reduced rate compared to the parent strain when challenged at 44 deg C. The gene interrupted by Tn917 in OSM1 was completely sequenced. A BLAST search of the translated amino acid sequence indicated the gene product was a homolog of HtrA (DegP), a serine protease identified in several gram-positive and gram-negative bacteria. The OSM1 stress response phenotype is similar to that observed for other HtrA- organisms, which suggests the L. monocytogenes HtrA degrades misfolded proteins that accumulate due to salt, temperature, and oxidative stress. The L. monocytogenes HtrA protein may also contribute to viability of the organism in food environments and during infection.