What is in this article?:
- Copper kills â€” deadly to Salmonella bacteria
- Pure copper?
- New research may have uncovered a way to reduce the number of food poisoning cases due to Salmonella and possibly other bacteria: prepare food on surfaces made with materials that contain some amount of the element copper, known as copper alloys.
Each year a tiny, rod-shaped species of bacteria with a fondness for proliferating on human food causes numerous cases of food poisoning around the world, sometimes leading to severe illness and even death.
The culprit, Salmonella enterica, is a leading cause of diarrheal illness worldwide, said Sadhana Ravishankar, an assistant professor in the University of Arizona department of veterinary science and microbiology.
But Ravishankar’s lab may have discovered a way to reduce the number of food poisoning cases due to Salmonella and possibly other bacteria: prepare food on surfaces made with materials that contain some amount of the element copper, known as copper alloys.
Ravishankar’s lab collaborated with Chris Rensing, formerly an associate professor in the UA department of soil, water and environmental sciences and now at Research Triangle Institute International, for the study, which was published recently in the journal Food Microbiology.
“Chris Rensing had already done some research with copper, and he knew that copper surfaces have antimicrobial activity,” said Ravishankar.
The International Copper Association donated six samples of copper alloys for the study, including samples of copper mixed with metals such as nickel, iron, chromium, phosphorous and tin that varied in their copper concentration from 60 to 99.9 percent.
Copper is harmful to bacteria because it reacts with oxygen in the atmosphere over time in a process called oxidation, which produces a residue that is toxic to some bacteria. Oxidation is what makes pure copper change in color over time from a rusty gold to a watery green.
“We decided to see the antimicrobial effect of all these copper alloy surfaces on Salmonella,” said Ravishankar.Salmonella was selected as the microbial guinea pig for the study because of its prevalence and the significant harm it causes worldwide because of diarrheal disease.
“Salmonella has caused outbreaks from eating a broad range of different types of foods, including meats and poultry, dairy products, peanut products, ice creams and even chocolate,” said Ravishankar.
Ravishankar found that because of oxidation, food contact surfaces made of materials containing copper are far less habitable for bacteria than stainless steel, which showed no antimicrobial properties at all.
“Right now, food industries use stainless steel,” said Ravishankar, “and stainless steel does not seem to have any antimicrobial activity.” If there are bacteria on a stainless steel surface, she said: “They will survive for a long time.”
One test by Ravishankar’s lab manager, Libin Zhu, showed that Salmonella can survive for longer than two weeks on stainless steel surfaces. By contrast, the bacteria showed significant reductions on copper alloys.
In most cases, Salmonella on steel surfaces dropped in concentration from 10 million cells to 1 million cells, said Zhu. However, on copper alloys, the concentrations of bacteria dropped by a far greater number, to 100 cells or less.
“We tested three copper-resistant strains and one copper-sensitive strain,” said Zhu. Copper-resistant strains are lineages of bacteria that have been exposed to copper for several generations, long enough for the cells to develop genetic resistance to its antimicrobial effects. Copper-sensitive strains, by contrast, have never been exposed to copper and are much more susceptible to the toxicity of oxidation.