Factors affecting survival of bacteria on abiotic surfaces

Dudman, Angélique (2013) Factors affecting survival of bacteria on abiotic surfaces. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

The ability of pathogenic bacteria to be retained on the surfaces of processing equipment constitutes a potential health problem in the food industry. Stainless steel is commonly used in the food industry, but there is an increasing demand for surfaces with enhanced hygienic properties. One way to combat microbial surface fouling is the use of novel antimicrobial alloys. Although many metal ions in solution demonstrate a significant antimicrobial effect, in the relative absence of moisture, surface release rates and the efficacy of the antimicrobial agent are altered. The aim of this study is to define conditions that minimise survival of bacteria on stainless steel and stainless steel coated with titanium nitride, alloyed with silver, a putative antimicrobial surface. Characterisation of the test substrata revealed smooth, thus hygienic surfaces with no leaching of silver observed. A method was developed to assess survival of Escherichia coli and Listeria monocytogenes on substrata under different equilibrium relative humidities (ERH): 11%, 52% and 86%ERH. Any cells remaining on the surfaces post-swabbing were detected by epifluorescence microscopy, or by metabolic dye. The survival of both microorganisms on surfaces was recorded via live/dead staining. The effect of surface re-use was also assessed. Gram positive L. monocytogenes survived better than E. coli in the highest 86%ERH and silver concentration TiN/Ag 120W, presumably due to its thicker cell wall. The increase of humidity did not affect any antimicrobial effect, but increased concentration of silver in the surface coating reduced the viability of bacteria. Re-use of the surfaces showed similar results to first time use in all experiments. The presence of meat extract increased the time of survival of E. coli from 4 hours to 16 hours on the stainless steel but the presence of silver decreased cell numbers when compared to other coatings. Finally, a differential staining method was developed to detect live/dead E. coli cells as well as meat extract to simulate contamination in situ. This novel method may be used in future studies to investigate the survival of microorganisms in food soil. Surfaces show potential being able to retain antimicrobial activity post use, low wear and no release of silver ions. Future experiments may include the use of the silver containing coating surfaces in situ, in different food industries, to evaluate their potential for reducing outbreaks.