Attachment of Listeria Monocytogenes to Austenitic Stainless Steel

Abstract

The objectives of this research were: 1. Investigate the attachment of Listeria monocytogenes, which were grown in nutrient-rich medium (brain heart infusion, BHI) or starved in minimal medium, to austenitic stainless steel No.4 satin finish at 4C, 20C, 30C, 37C, or 42C. 2. Determine the attachment of L. monocytogenes on the three zones of weld including the weld zone, the heat affected zone, and the base metal before and after exposure to a corrosive environment. 3. Determine the effect of surface finish including No. 2B finish, No. 4 satin finish, and No. 8 mirror finish on bacterial attachment.

For objective 1, a droplet (10 µl) containing ~10^7 CFU/ml of L. monocytogenes suspended in BHI or minimal medium was placed on the stainless steel surface with No.4 satin finish. After holding in saturated humidity for 3 h at the desired temperature the coupon was washed and treated with osmium tetroxide. Samples were gold coated and examined using scanning electron microscopy (SEM) to determine the number of cells of L. monocytogenes attached on the surface. For each nutrition and temperature treatment, six coupons were used, and 60 fields of view were used in determining bacterial counts. The number of attached cells which were grown in rich medium of BHI or starved in minimal medium was significantly affected by attachment temperatures in which the maximum attachment was observed at the temperatures of 30C and 37C. The attachment of L. monocytogenes to stainless steel surface was greater when cultivated in rich medium of BHI vs starved in the minimal medium. For objective 2, austenitic stainless steel 304 (304 SS) sheet and fillers with the same composition as 304 SS were subjected to four different welding settings based on heat inputs and travel speeds. Welding was performed using tungsten inert gas equipment. Welds were then exposed to surface finishing and corrosive media. For the bacterial attachment, a drop of 10 µl BHI containing 10^7 CFU/ml of L. monocytogenes was placed on each test surface. After 3 h at 23C, the surfaces were prepared for SEM. Using SEM, attachment of L. monocytogenes was determined by counting cells remaining on each test surface. All data (bacterial counts) were normalized to account for differences in the surface area of the inoculum. For each surface treatment, 6 coupons were tested, and 60 fields of view were used in determining bacterial counts. Polished stainless steel welds do not lead to differences in bacterial attachment. However, corrosion of the different weld zones leads to differential attachment of L. monocytogenes to stainless steel. The attachment of L. monocytogenes was greater on the corroded surfaces than on the uncorroded surfaces. For objective 3, a drop of 10 µl BHI containing 10^7 CFU/ml of L. monocytogenes was placed on each test surface of No. 2B finish, No. 4 satin finish, and No. 8 mirror finish. After 3 h at 23C, the surfaces were prepared for SEM. For each surface treatment, six coupons were tested, and 60 fields of view were used in determining bacterial counts. The results of both the normalized number of bacteria and non-normalized number of bacteria indicated that polishing a surface to certain smoothness may give rise to more adhesion of bacteria, and the No. 2B finish is the best choice among the other two for food contact surface in limiting the initial attachment of L. monocytogenes.