Japanese bento boxes have come with anti-bacterial silver ions for some time, like these ones. This helps to prevent your food from going bad, especially in hot weather.
Now Lock n Lock’s EZ-Lock range also some with Ag+ ions. After you remove the cardboard packaging, the Ag+ boxes can be distinguished by their lids of a slightly lighter shade of blue compared to standard EZ-Lock. View the latest 2009-2010 Lock n Lock catalogue here (only works with Internet Explorer).
Here are some suitable sizes for bento.