What are the chances of contracting a bacterial infection from sharing a straw with another person?

Maybe none, if...: If the other person doesn't have a bacterial infection to transmit, then there is no chance of "contracting a bacterial infection" from the other person. One cannot "catch" something that the other person does not have. Beyond that, it is not possible to predict how easily infections, such as strep throat, will pass via the straw in each specific case (there are too many other factors involved).