Have a look at the "fascinating (and beautiful) images produced by students at the University of Surrey when they imprinted their smartphone onto a bacterial growth medium."
gizmag.com/smartphone-bacte...
Brings a new meaning to the phrase "Personalising your smartphone"!
(Note that most of these bacteria are benign, although as the article states: "The (student) exercise does occasionally show the presence of disease-causing bacterial pathogens.")
Neil