Survival of dried Escherichia coli O157:H7 in dried droplets
Abstract
The survival of Escherichia coli O157:H7 deposited from droplets of cultures containing partially digested proteins were studied to help advance our fundamental understanding of pathogen surface-inoculation methods and potential transmission processes. Specifically, cultures containing between 0.8 and 6% digested protein were spotted in liquid droplet onto microscope slides and subsequently dried and stored at RH ranging from 68 ± 7% and 20 ± 7% and the number of survivors were determine with storage time. The drying process initially formed reproducible but different types of deposition patterns that included areas with: (i) exposed cell agglomerations typically in a ring pattern, (ii) exposed deposited single or small clusters of cells, and/or (iii) cells encapsulated within the dry digested protein matrix. Results from epi-fluorescence microscopy performed with an E. coli O157:H7 strain constructed to express green fluorescent protein showed location of cells and when nutrient agar plugs were overlaid over the samples cell-grown patterns showed the location of culturable and non-culturable zones. Results showed that the exposure of cells to more humid conditions resulted in overall lower survival rates regardless of their location within patterns or the concentration of solutes than those obtained at low humidity. Specifically at the lower humidity, viable bacteria were recovered for more than 3-weeks and epi-fluorescence microscopic studies demonstrated that the cells survived in regions where both protein and cells were deposited together. Trehalose, a known cell membrane protectant against dehydration and rehydration processes, was also used as a control and demonstrated that in areas where cell and trehalose overlap, survival was similar to rates observed with the digested protein. In contrast, bacteria dried with bovine serum albumin, a large globular protein, and cells desiccated with no accompanying solutes survived for less than 24 hours. The results demonstrate that the presence of partially digested proteins in water prior can extend the survival rate of E. coli O157:H7 during drying and desiccation. These findings can shed light on potential transmission processes along the food chain.
Degree
Ph.D.
Advisors
Nivens, Purdue University.
Subject Area
Food Science|Microbiology
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