So that was what wikipedia was referring to when it said there has been a study done on shopping carts, thanks nxakt for the additional info.
In 2004 shopping carts were identified as a source of germs and became a major public health concern. This was primarily because of the media spotlight on a Japanese research study indicating a large amount of bacteria was found on shopping carts. This was confirmed in 2007 when the University of Arizona released a study called, “Research Report on Shopping Cart Bacterial Contamination - Dr. Charles P. Gerba”. Now sanitary wipes, for cleaning hands and shopping carts, are seen near the entrances of most retailers.