Hi, thanks for your questions I'm happy to help :)

Probiotics in my opinion are rather controversial, not their effectiveness but their marketing. I wouldn't take them in the form of pills unless I had symptoms that required their use. In that case I check which one to buy at examine.com It's an awesome website that tests supplements.
What you can do all year round is to eat foods that feed the good bacteria. They are called pre-biotics. I don't have any solid data, but I think that would be even more effective than taking probiotics ocasionally. There are limits to this, for example if you weren't breastfed or you had a course of antibiotics then I would take them in form of pills. Make sure you take at least 40-50 bilion CFU-s a day.

As for the bacterial diarrhoea. It's rather rare condition in developed countries and far more serious. It can come in form of food poisoning and then it's very mild or it doesn't happen at all in my experience. It's over withing 48 hours as opposed to viral that lasts up to week in most cases. Others forms of diarrhoea caused by baceria are not really of concern unless you had an aggressive antiobiotic treatment (C. diff. overgrowth) or you caught cholera (which is not likely to happen).

Thanks for the feedback :)