To be fair, they live in a renovated sewers and regularly swim through/traverse the older tunnels.
I think Salmonella is the least of their problems
Though I would like to think that as half human hybrids they are either less at risk of that, or at least are aware that its a problem that could affect themselves personally and would take appropriate action to mitigate it.
(Not from kissing before anyone asks - minds out of the sewer people! I mean from the bacteria that naturally would live on normal turtles skin and whether that would affect a half human nervous/digestive system if ingested.)