PBS is a buffered solution. It does not really make much of a difference. I need to know more about the experiment to comment whether it would have an effect or not. It can be used in case of normal saline.

PBS is saline and some phosphate salts which act as a buffer. PBS can be used in various pH ranges and hence its advantages over normal saline. It will help in protecting the bacteria from pH shock. If the bacteria are to be transported over long distances or kept as an inoculum, buffered media are prefereable and hence the use of PBS instead of normal saline. So most bacteria are happier in PBS than saline, except lactic acid bacteria.