That or carry around some anti-septic wipes (the types you can get for cleaning public toilet seats before you let your toddler use them) and wipe the shopping trolley down with in the places that a toddler is likely to touch it.  I.e. handle and sides of the infant chair.  If I have taken my babies shopping when they were smaller, I have plonked the car seat inside of the shopping trolley, so it was not an issue.