Probiotics Or make your own by fermenting food, a jar full of salted(Sea salt) water and the vegetable of your choice, leave it 3 days in the dark area and enjoy.
But it is not enough, the balance of your gut bacteria thrives on your diet, so you should reduce your carbohydrate intake to a minimum and focus on vegetables , fruit (with low glycemic index like kiwi ) , seeds , nuts and meat.
I recently started taking Now Foods Probiotic. If you have the money VSL-3 is probably the best one. I felt a euphoric effect and a generalized sense of well being after taking it.