Nano-scale additives fight food pathogens

CHICAGO. Nano products are all the rage, even in food science. Here, at the Institute of Food Technologists’ annual meeting, on July 18, scientists described dramatic success in fighting food-poisoning bacteria by doctoring foods or their packaging with microbe-killing nanoparticles – sometimes along with natural anti-bacterial agents.

The nano of interest: Zinc oxide. When particles of this compound are very small, they exhibit properties not seen with bigger ones. Specifically, they appear to damage cell membranes so that a bacterium’s innards can leak out. The nanoparticles radically deformed bacteria as well – offering further, visual evidence that the bugs had sustained serious injury.

E. coli O157:H7 is a particularly nasty germ. It’s the one that was responsible four years ago for a national outbreak associated with commercially packaged spinach that killed several people and sickened hundreds. It also served as the trigger for the Leafy Greens initiative, the imposition of a large number of ostensibly voluntary – and costly –  food-safety measures by the fresh-produce industry. It was the first of many efforts aimed at heading off future costly and reputation-damaging outbreaks.

Zinc oxide shows great promise in thwarting this particular bug, reports Azlin Mustapha of the University of Missouri-Columbia. She and her colleagues treated colonies of this strain of E. coli with solutions seeded with nano zinc oxide – tiny particles less than 70 nanometers in diameter. Exposed bacteria quickly began exhibiting marked distress. Compared to untreated colonies of these bugs, just 60 percent as many E. coli survived exposure to a 3 millimolar concentration of zinc oxide. Increase the concentration to 6 mM and the survival fell a bit. Double the concentration to 12 mM and all of bad bugs died.

Mustapha’s photos of the treated microbes graphically illustrate zinc oxide’s impact. Bacteria that were normally rod-shaped now sported kinks or had blobbed into short, fat balls. Some developed constrictions that appeared to give them a head. And material inside the cell sometimes congregated at one end of the bacterium (it should have been evenly distributed) or disappeared altogether – probably because it leaked out.

Further probing, she said, now indicates that some of the bacterium’s structural fatty elements have been compromised and some proteins appear to have been destroyed.

When her team added those nanoparticles to a broth spiked with Salmonella, another food pathogen, they had considerably more trouble killing the bugs, although the researchers could slow the bacterium’s growth.

Germ-fighting bottles and jars
Food technologist Z.T. (Tony) Jin offered data suggesting one way to get around this problem of fighting germs that taint a liquid. His team at the U.S. Department of Agriculture’s Eastern Regional Research Center in Wyndmoor, Pa., mixed super-tiny zinc-oxide particles (about 7 nanometers in diameter) into a food-grade polymer (polylactic acid), then applied a thin coating of the mix onto the inside of a glass bottle or jar. After it dried, the researchers added a liquid tainted with Listeria, Salmonella or Campylobacter  – all major food-poisoning microbes.

In most cases, the bacteria began dying within hours or days. But the best results occurred when the USDA scientists augmented their polymer with a booster: nisin, a natural toxin produced by certain bacteria known for their production of lactic acid. These bugs make nisin as a poison to thwart competing bacteria. Nisin is among a host of such bacterial-derived antibacterial agents, known as bacteriocins, that have been federally approved to battle the growth of pathogens in foods.

Containers lined with the polymer containing both nano zinc oxide and nisin killed germs more quickly than when their protective coating contained zinc oxide alone, Jin noted. In some cases, where the nanoparticles weren’t able to fully eradicate the germs, zinc oxide’s pairing with nisin kept cell numbers low for a longer period than when the particles were used alone. For instance, the nisin-particle pairing kept liquid eggs safe from experimentally added germs for at least 55 days, Jin reported.

But not indefinitely. The zinc oxide and nisin slowly diffuse out of the polymer. So as the coating releases the germ-fighting duo, it will lose some of its antibacterial potency.

The same nanoparticle-nisin duo could be added to the plastic wrap used on refrigerated foods in grocery stores or other packaging, Jin volunteered. Indeed, he described plans to investigate the incorporation of zinc oxide and nisin in beverage bottles extruded out of polylactic acid.

As good as these treatments might ultimately become, none is designed to substitute for good food handling practices. But they offer clever back-up strategies to combat a leading source of gastrointestinal disease.