Two fixed patterns account for every line here. 겠거니 하다 means to assume something and leave it there: living on the assumption that there is probably some bacteria, figuring it was somebody else's problem, taking it that there was a reason. And 주거니 받거니, which appears four times, is a set phrase for two people passing something back and forth, whether that is drinks, jokes or affection. Outside these frames you will rarely meet it.
Register: neutral; both patterns are common in speech