The effects of reduced susceptibility of Salmonella enterica to DTAC on some indicators of pathogenicity

Herson, Diane S.The reduced susceptibility of microorganisms to biocides due to repeated or continuous exposure is an ongoing area of study and may be a potential public health concern. The reason for concern is from the possibility of a decrease in susceptibility to antibiotics accompanying reduced susceptibility to biocides, especially for pathogenic microorganisms. This study compares a parental strain of Salmonella enterica with four strains derived from it, which have Reduced Susceptibility (SRS) to the biocide dodecyltrimethylammonium chloride (DTAC). Quaternary Ammonium Compounds (QACs) such as DTAC are commonly used in a wide range of cleaning products. The minimal inhibitory concentrations (MICs) of the SRS strains were above the concentration of DTAC recommended for cleaning products. A microarray gene expression study of one of the SRS strains indicated that expression differed in one third of the genome between the SRS strain and the parental strain. A few indicators of pathogenicity were chosen for further comparison between the parental and SRS strains; these were invasion, fimbriae, gene expression, biofilm formation and growth patterns. All four SRS strains showed 100 fold fewer invasive cells in a Caco-2 cell culture model. The SRS strains had lower levels of transcription of the fimbrial genes fimA and csgG measured by real time reverse transcriptase PCR and produced fewer fimbriae as observed by transmission electron microscopy. The transcription levels of the virulence regulator spvR which regulates the growth rate of Salmonella post internalization was lower in three of the four SRS strains than in the parental strain. The efflux pump gene associated with reduced susceptibility, acrB, was upregulated in all four of the SRS strains. The SRS strains produced biofilm both in the presence and absence of DTAC,whereas the parental strain was less affected by the presence of DTAC. Growth in liquid culture was similar for the SRS and parental strains. At a level of 150 percent of their respective MICs the parental and the SRS strains were killed by DTAC. This finding shows that periodic sanitation using high concentrations of DTAC could effectively control the growth and spread of these SRS strains. Though the individual indicators of pathogenicity studied here the SRS strains appear to be less pathogenic than the parental strain, this notion still must be confirmed with animal studies given the magnitude of genes with different expression levels as seen in the microarray.University of Delaware, Department of Biological SciencesM.S