Yogurt is often suggested as a source of healthy bacteria. However, the probiotics used to ferment milk (L. bulgaricus and S. thermophilus) are not believed to survive the trip through the gastrointestinal tract. Acid-resistant probiotics (e.g., L. acidophilus) added to yogurt can make the product an effective probiotic source.
http://www.sciencebasedmedicine.org/ive-been-prescribed-an-antibiotic-should-i-take-a-probiotic/
http://umm.edu/health/medical/altmed/supplement/lactobacillus-acidophilus
http://www.nlm.nih.gov/medlineplus/druginfo/natural/790.html