The enteric pathogen Salmonella enterica serovar Typhimurium uses autoinducer-2 (AI-2) as a signaling molecule. AI-2 requires the luxS gene for its synthesis. The regulation of global gene expression in Salmonella Typhimurium by luxS=AI-2 is currently not known; therefore, the focus of this study was to elucidate the global gene expression patterns in Salmonella Typhimurium as regulated by luxS=AI-2. The genes controlled by luxS=AI-2 were identified using microarrays with RNA samples from wild-type (WT) Salmonella Typhimurium and its isogenic DluxS mutant, in two growth conditions (presence and absence of glucose) at mid-log and early stationary phases. The results indicate that luxS=AI-2 has very different effects in Salmonella Typhimurium depending on the stage of cell growth and the levels of glucose. Genes with p 0.05 were considered to be significantly expressed differentially between WT and DluxS mutant. In the mid-log phase of growth, AI-2 activity was higher (1500-fold) in the presence of glucose than in its absence (450-fold). There was differential gene expression of 13 genes between the WT and its isogenic DluxS mutant in the presence of glucose and 547 genes in its absence. In early stationary phase, AI-2 activity was higher (650-fold) in the presence of glucose than in its absence (1.5-fold). In the presence of glucose, 16 genes were differentially expressed, and in its absence, 60 genes were differentially expressed. Our microarray study indicates that both luxS and AI-2 could play a vital role in several cellular processes including metabolism, biofilm formation, transcription, translation, transport, and binding proteins, signal transduction, and regulatory functions in addition to previously identified functions. Phenotypic analysis of DluxS mutant confirmed the microarray results and revealed that luxS did not influence growth but played a role in the biofilm formation and motility.

Glass impingers (AGI-30) were used at a commercial sludge application site to determine the levels of airborne bacteria and pathogen indicators. Even though heterotrophic bacteria averaged 10 5 CFU/m 3 , none of the sites showed the presence of Salmonella spp. or indicators such as fecal coliforms or coliphages. Indicators such as H 2 S producers and pathogenic clostridia were present in locations having significant physical agitation of the sludge material. PCR-based ribotyping using the 16S-23S interspacer region is a promising method to identify the genetic relatedness and origins of airborne clostridia.

Autoinducer-2 (AI-2) is a compound that plays a key role in bacterial cell-to-cell communication (quorum sensing). Previous research has shown certain food matrices inhibit this signaling compound. Using the reporter strain, Vibrio harveyi BB170, quorum-sensing inhibitors contained in poultry meat wash (PMW) samples were characterized by molecular weight and hydrophobic properties using liquid chromatography systems. Most fractions that demonstrated AI-2 inhibition were 13.7 kDa or less, and had hydrophobic properties. Hexane was used to extract inhibitory compounds from a PMW preparation and the extract was further separated by gas chromatography (GC). Several fatty acids were identified and quantified. Linoleic acid, oleic acid, palmitic acid, and stearic acid were each tested for inhibition at 0.1, 1, and 10 mM concentrations. All samples expressed AI-2 inhibition (ranging from approximately 25% to 99%). Fatty acids, combined in concentrations equivalent to those determined by GC analysis, expressed inhibition at 59.5%, but higher combined concentrations (10- and 100-fold) had inhibition at 84.4% and 69.5%, respectively. The combined fatty acids (100-fold) did not demonstrate a substantial decrease in colony plate counts, despite presenting high AI-2 inhibition. These fatty acids, through modulating quorum sensing by inhibition, may offer a unique means to control foodborne pathogens and reduce microbial spoilage.

Autoinducer-2 (AI-2) molecules are used by several microorganisms to modulate various processes, including bioluminescence, biofilm formation, and virulence expression. Certain food matrices, including ground beef extracts, possess compounds capable of inhibiting AI-2 activity. In the present study, we identified and characterized these AI-2 inhibitors from ground beef extract using hexane solvent extraction and gas chromatography. Gas chromatographic analysis revealed the presence of several fatty acids such as palmitic acid (C16:0), stearic acid (C18:0), oleic acid (C18:omega9), and linoleic acid (C18:omega6) that were capable of inhibiting AI-2 activity. These fatty acids were tested (using Vibrio harveyi BB170 and MM32 reporter strains) at different concentrations (1, 5, and 10 mM) to identify differences in the level of AI-2 activity inhibition. AI-2 inhibition ranged from 25 to 90%. A mixture of these fatty acids (prepared at concentrations equivalent to those present in the ground beef extract) produced 52 to 65% inhibition of AI-2 activity. The fatty acid mixture also negatively influenced Escherichia coli K-12 biofilm formation. These results demonstrate that both medium- and long-chain fatty acids in ground beef have the ability to interfere with AI-2-based cell signaling.

Since biosolids could harbor a variety of potentially infectious microorganisms, there are concerns of human exposure to these microbial pathogens in population centers surrounding sites where biosolids are land applied. A field study was conducted at a large commercial biosolid application site to determine if thermotolerant clostridia could be employed as a microbial indicator in determining the presence of such fecal contamination in aerosols. Even though the applied biosolids harbored as much as 107 MPN fecal and total coliforms per gram wet weight, these traditional indicators were undetectable at locations having the greatest potential for aerosolization. Thermotolerant clostridia and bacteriophages, however, were detectable in 73% and 53% of the samples, respectively. Hydrogen sulfide producing bacteria as a group, were detected in 93% of the samples. Even at sites directly in the vicinity of the biosolid application, thermotolerant clostridia were detected in 26% of the samples, as compared with the fecal and total coliforms which were detected in only 1 out of 15 (6.6%) samples. Since municipal sewage sludges usually undergo anaerobic digestion before land disposal, it would favor the selection of thermotolerant clostridia within these biosolids. Moreover, clostridla also could be ribotyped using the 16S–23S interspacer region length polymorphism to identify the origins or sources of aerosol contamination. Thermotolerant clostridia could thus serve as a reliable indicator to determine the presence of microbial pathogens and biosolid derived microbial populations in aerosols being generated from biosolid application programs.