I think we have to be very concerned about introducing bacteria into the worlds that we study. I can think of nothing worse than if we introduced life into a place where there was none and we later discovered life (coincidentally like the life we earlier introduced), or if we introduced life where there was a pure Jovian lifeform, and we damaged it by introducing our version of life. It's best to come as observers and not contaminate the landscape. Much effort must be made to do so.