Potable water
Our airline (not a major by the way but a charter) carries out regular potable water checks. If any contamination is found a sterilisation program is initiated with repeat checks until the contaminant is erradicated.
The cleaners are also taught to use different rags for galleys and loos.
The mess in the forward hold where we store the return catering supplies however, that is disgusting! (not spelling dogma!) It can be very difficult to get the cleaners to clean out the hold on a quick TR