Involvement of Bacterial Quorum-Sensing Signals in Spoilage of Bean Sprouts

Abstract

Bacterial communication signals, acylated homoserine lactones (AHLs), were extracted from samples of commercial bean sprouts undergoing soft-rot spoilage. Bean sprouts produced in the laboratory did not undergo soft-rot spoilage and did not contain AHLs or AHL-producing bacteria, although the bacterial population reached levels similar to those in the commercial sprouts, 10 8 to 10 9 CFU/g. AHL-producing bacteria (Enterobacteriaceae and pseudomonads) were isolated from commercial sprouts, and strains that were both proteolytic and pectinolytic were capable of causing soft-rot spoilage in bean sprouts. Thin-layer chromatography and liquid chromatography-high-resolution mass spectrometry revealed the presence of N-3-oxohexanoyl-L-homoserine lactone in spoiled bean sprouts and in extracts from pure cultures of bacteria. During normal spoilage, the pH of the sprouts increased due to proteolytic activity, and the higher pH probably facilitated the activity of pectate lyase. The AHL synthetase gene (I gene) from a spoilage Pectobacterium was cloned, sequenced, and inactivated in the parent strain. The predicted amino acid sequence showed 97% homology to HslI and CarI in Erwinia carotovora. Spoilage of laboratory bean sprouts inoculated with the AHL-negative mutant was delayed compared to sprouts inoculated with the wild type, and the AHL-negative mutant did not cause the pH to rise. Compared to the wild-type strain, the AHL-negative mutant had significantly reduced protease and pectinase activities and was negative in an iron chelation (siderophore) assay. This is the first study demonstrating AHL regulation of iron chelation in Enterobacteriaceae. The present study clearly demonstrates that the bacterial spoilage of some food products is influenced by quorum-sensingregulated phenotypes, and understanding these processes may be useful in the development of novel food preservation additives that specifically block the quorum-sensing systems.Many gram-negative bacteria use chemical signal molecules, acylated homoserine lactones (AHLs), to regulate the expression of particular phenotypes as a function of cell density. This phenomenon, known as quorum sensing (QS), is involved in the expression of virulence factors in several pathogenic bacteria and in the expression of factors important for the bacterial colonization of higher organisms (35,52,59,62,63). The growth and activity of gram-negative bacteria are also important in food microbiology, where spoilage of fresh foods, such as fish, meat, milk, and vegetables, is mostly a consequence of the degradative activity of gram-negative bacteria growing to high densities (10 8 to 10 9 CFU per gram). Food spoilage is a major economic and societal problem, and large amounts of foods are lost due to bacterial growth and spoilage (9). Understanding of the microbial processes leading to spoilage would facilitate development of novel preservation techniques and reduce loss of food. We hypothesized that bacterial fooddegradative processes could be regulated by QS, and if QS systems...

Curcumin from Curcuma longa affects the virulence of Pectobacterium wasabiae and P. carotovorum subsp. carotovorum via quorum sensing regulation

“…It has also been reported that the secretion of PCWDEs, motility, butanediol fermentation as well as type III secretion system are being regulated by AHL dependent QS mechanism in Pectobacterium spp. (Pirhonen et al 1993;Rasch et al 2005). Since, the production of these enzymes is regulated by the secretion of QS signal molecules; the blockage of such mechanism would eventually pave the way to reduce the pathogenicity of Pectobacterium spp.…”

A review of microbial biofilms of produce: Future challenge to food safety

“…AHL (3-oxo-C6-HSL) controls protease and pectinase production, as well as the siderophore production, of Enterobacteriaceae and Pseudomonas spp. on beans sprouts (50).…”

Quorum Sensing in the Context of Food Microbiology

“…In particular, this QSI exerts a strong antagonistic effect on LUXR-based QS. The antimicrobial action of these plant extracts has been widely used in the food and flavor industry, whereas their ability to produce AHL-degrading bacterial enzymes, which is known in vitro, remains to be evaluated in situ so that they can be used as QS inhibitors (5,14,50,56,82,83,99,100,101,116,117,124). Inhibitory substances or compounds that may mask the QS effect have been reported in foods of animal origin (5,116,117).…”