The objective of this study was to evaluate the antibacterial activity of copper surfaces on two highly prevalent enteropathogens, S. enterica and C. jejuni. The results showed that the copper surfaces do have an important antibacterial activity on these two pathogens, tests were conducted at 10°C and 25°C, conditions that resemble those utilized during food processing and after manipulation (during the night) respectively. The control surfaces, stainless steel and a synthetic polymer, which are materials commonly used by the food industry, showed no antibacterial activity. The inhibitory effect of copper was dependent on the time of exposure of bacterial suspensions. In most cases, antibacterial activity was already evident within two to four hours of exposure, and increased thereafter. Temperature was also an important factor, since the antibacterial activity was always more effective at 25°C. Figure (25°C) shows that at 2 hours, the decrease in CFU/ml was about 2 log, enough to reduce the initial inoculum of S. enterica below the infective dose for this pathogen. At 4 hours, the inhibitory effect of copper was remarkable, showing a decrease of 4 log and 6 log with respect to the initial inoculum (t = 0) and to the controls respectively (Figure ). The antibacterial activity on S. enterica was also present at 10°C (Figure ), the decrease in the viable bacterial concentration was about 2 log at 8 hours with respect to the initial inoculum.
Copper surfaces also had an important inhibitory effect on C. jejuni at 25°C, the decrease in the CFU/ml was about 4 log with 4 hours of exposure to this metal; with 8 hours of exposure the reduction was about 5 log, reaching a CFU/ml below the infective dose (Figure ). At 10°C the inhibitory effect was less intense (Figure ), with a decrease in the viable C. jejuni cells of about 2 log at 8 hours of exposure to the copper sheets. Absence of the two hour point in the plot was due to some technical difficulties in the assays, but the available data showed almost no inhibitory effect at this point. Thus, the antibacterial effect was similar for S. enterica and C. jejuni at 10°C and was slightly higher for S. enterica at 25°C. The differences observed in the inhibitory effect with the temperature may be due to an increase in the release of Cu(II) ions, or to an increase in the copper intake by the bacterial cell, with a higher temperature.
Taking together, these findings suggest that metallic copper may have a potential application in decreasing the bacterial load in some areas with a high exposure to bacterial contamination like that usually found in poultry slaughterhouses. Thus, the use of copper sheets would be particularly useful to decrease the cross contamination caused by bacterial pathogens like S. enterica and C. jejuni.
As we were carrying out the assays, the appearance of the copper surfaces changed, the metallic aspect of new copper sheets turned a dark brown color. In addition, we realized that the bacterial suspensions were acquiring a pale blue color, indicative of the release of Cu++ ions, which was deeper with time of exposure. To see the effect of copper ions release in exposed food we tested the content of this metal in meat samples. The analysis of residual copper showed that effectively, there was an increase in the copper content of meat pieces with the time, reaching 2 mg/100 g after a 50 minute exposure (Figure ). Copper acquisition was faster in the first 30 minutes, being slower afterwards. Assays done for 2 or more hours of exposure indicated only a small increase compared to the 50 minutes point (data not shown). These results shows that the copper adsorption by meat due to a continuous exposure to a copper surface tends to reach a maximum value of about 2.5 mg/100 g.
The Recommended Dietary Intake (RDI) for copper is 0.9 mg for adults (19–50 years), while the upper limit of intake for this metal is 10 mg for the same population. An excessive copper release is therefore a limiting factor to take into account. Under normal processing circumstances the exposure of food items to the surfaces is normally short, not more than a few minutes, so copper acquisition would remain very low. It should be pointed out that the use of copper surfaces would not be intended to be the first line of defense against pathogenic bacterial contamination, or to replace the normal hygiene procedures in a food processing plant. The main application of copper surfaces would be decreasing the bacterial load that may remain on it after the daily work, which could survive the normal cleaning procedures. Copper sheets may decrease the capacity of harmful bacteria to remain viable or to form biofilms after some hours on the surface, and may be useful to reduce cross contamination or to act as auto-disinfectant surfaces in areas where a continuous bacterial load is produced.
Future studies will include the utilization of different copper alloys that diminish the copper liberation while maintaining an antimicrobial activity.