Walton, Julie (University of Wolverhampton, 2012-07)
This study investigated the effect of detergent treatment on susceptibility of attached Escherichia coli and Listeria monocytogenes to subsequent disinfectant treatment, in relation to food industry cleaning procedures. E. coli attached to stainless steel surfaces became significantly more susceptible to benzalkonium chloride (BAC) after treatment with sodium alkyl sulphate (SAS) by 0.51 Log10 cfu ml-1 and fatty alcohol ethoxylate (FAE) by 0.96 Log10 cfu ml-1. No change in susceptibility was observed with sodium dodecyl sulphate (SDS), sodium lauryl ethyl sulphate (SLES) or polyethoxylated alcohol (PEA). L. monocytogenes became significantly less susceptible to BAC after treatment with anionic detergents SAS by 0.79 Log10 cfu ml-1, SDS by 0.33 Log10 cfu ml-1 and SLES by 0.22 Log10 cfu ml-1, yet no change in susceptibility was observed with FAE. Following treatment with all detergents both organisms became significantly more susceptible to sodium dichloroisocyanurate (NaDCC) demonstrating that the effect of the disinfectant was independent of detergent type. Flow cytometry using the fluorochrome propidium iodide (PI) revealed significant increases in cell membrane permeability of both organisms by all detergents except sodium dodecyl sulphate (SDS) and the effect was much greater in E. coli. Increasing above the in-use concentration of SAS and FAE had no further effect on cell membrane permeability, or susceptibility to BAC. Hydrophobic interaction chromatography (HIC) showed that E. coli became less hydrophobic following treatment with SAS, SDS, FAE and L. monocytogenes became less hydrophobic following treatment with SAS and SDS but no effect was seen with FAE. Investigations into carbon chain length of detergent revealed that SAS and the C18 standard increased susceptibility of E. coli to BAC which, with permeability results, suggests a link between increase in susceptibility to BAC and increase in membrane permeability. Efflux experiments with L. monocytogenes showed that efflux of ethidium bromide (EtBr) was greater from cells treated with SAS than with FAE suggesting that the anionic charge on the detergent molecule influences an efflux mechanism that reduces susceptibility to BAC. Overall the results demonstrate that detergent type can influence the sensitivity of persistent food borne microorganisms to BAC and NaDCC and the significance of the findings may impact on the choice of agents used in cleaning procedures in the food industry.
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