Food Protection Trends - March 2010 - 170

TABLE 1. Mean log CFU attached L. monocytogenes and P. putida per cm2 polyethylene cutting board formulated with or without antimicrobial silver ions (n = 15 per treatment) L. monocytogenes Cutting board Control Silver ion 1 a P. putida 24 h dry time 6.46a,y 6.53a,y 0 h dry time1 6.86a,x 6.83a,y 24 h dry time 3.97a,z 3.95a,z time at 25°C between rinsing away unattached cells and sampling surface with cotton-tipped applicator values within columns are not significantly different by t test (P > 0.05) values within row with different superscripts are significantly different by t test (P < 0.01) x,y,z applicator was moistened by dipping into PBS and used to rub one flat surface of each square. The swab was rubbed over the entire square, the square was rotated 90 degrees and re-sampled with the same swab, the square was again rotated 90 degrees and the same swab was used a third time to rub over one flat surface. Each swab was then again placed into 9 ml PBS. The number of L. monocytogenes cells was estimated by plating 0.1 ml of serial dilutions onto the surface of duplicate modified oxford (MOX) agar (Remel, Lenexa, KS 66215). All MOX plates were incubated at 35°C for 24 h before small black colonies characteristic of L. monocytogenes were counted. P. putida cells were enumerated by direct plating 0.1 ml from serial dilutions onto the surface of duplicate BHI agar plate incubated at 35°C for 24 h. Experimental design and statistical analysis Three replications of each experiment were conducted for each organism, with five duplicate cutting board squares per replicate (n = 15). The mean number of colonies per ml was converted to log CFU/cm2 and geometric means were calculated. Differences were determined by Students t test, with significance assigned at P < 0.01. RESULTS AND DISCUSSION Bacterial count data on the surface of cutting boards are presented in Table 1. L. monocytogenes attached to the surface of the plastic cutting boards and grew to a density of more than 106 cells per cm2 regardless of the presence of silver ions. When attached, L. monocyto- genes cells were left on the cutting board for 24 h, the numbers were substantially lower, less than 104/cm2, than when sampling occurred immediately after the growth phase. This was observed on cutting boards with or without silver ions. It was assumed that P. putida, an efficient biofilm producer, would perform at least as well as L. monocytogenes; therefore, no testing was done immediately after the growth phase. Following twenty-four hours of dry incubation after unattached cells had been rinsed away, more than 106 cells P. putida could be found per cm2 plastic cutting board. As was the case for L. monocytogenes, the silver ion treatment did not affect the numbers of P. putida detected attached to the surface. The current data provide conclusions that differ from those of other published work. MacKeen et al. (8) found that silver-containing fiber filters placed in liquid medium significantly lowered the number of surviving Escherichia coli and Pseudomonas aeruginosa. This decline in numbers was found to be significant at 26°C and faster at 37°C. Our studies did not employ ideal bacterial growth temperatures, because it is assumed that surfaces in a poultry further processing plant would be kept cool; as such, our treatment temperature of 25°C represents a worse-case scenario. In previous research, silver-containing polystyrene caused a dramatic decrease in bacterial numbers within 24 h at 35°C and within 72 h at 5°C (6). The same researchers examined refrigerator lining material formulated with silver ions and reported that the treated refrigerators and food stored therein had lower levels of bacterial contamination than levels for untreated control refrigerators. These conclusions, however, were based on visual observation of growth media and not quantitative data subjected to statistical analysis (6). Gram positive species were proposed to be less sensitive to silver ion treatment than Gram negative species (6). However, in the current work, we found no difference between the effects of silver on a Gram positive (L. monocytogenes) and a Gram negative (P. putida) bacteria. The presence of silver ions in a plastic cutting board did not affect the attachment, growth or survival of either organism on the surface. Our data suggest that L. monocytogenes cells attached to plastic cutting boards are more susceptible to drying than are cells of P. putida. It may be that L. monocytogenes becomes uncultureable (especially on the selective medium used in the current study) because of dry stress, while P. putida is more resistant to drying. However, another explanation is that L. monocytogenes cells in a dry biofilm are less well attached, making them prone to removal during pre-sample rinsing. This possibility is supported by an earlier report showing that an L. monocytogenes biofilm is more likely to cause transfer to a secondary contact surface when the biofilm has been subjected to drying (10). CONCLUSIONS/ RECOMMENDATIONS Although silver ion surface treatment has been reported to have antimicrobial utility in other circumstances and 170 FOOD PROTECTION TRENDS | MARCH 2010

Food Protection Trends - March 2010

Table of Contents for the Digital Edition of Food Protection Trends - March 2010