Since antibiotics kill the beneficial as well as the harmful gastrointestinal bacteria, we often want to replace the friendly organisms, particularly Lactobacillus acidophilus. Consequently, we purchase yogurt or other products claiming to contain it. Also, Lactobacillus acidophilus  predominates in the normal vagina and women with vaginal infections are lacking in Lactobacilli. Since they were wondering about possible recolonization of the vagina, researchers tested the actual content of a number of non-prescription Lactobacillus  products, most of which claimed to contain L. acidophilus. * 

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Most products tested contained no Lactobacillus acidophilus and many were contaminated.

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For the Lactobacillus to be effective in killing harmful organisms, it needs to be a strain that produces hydrogen peroxide (H2O2). Most products tested contained no Lactobacillus acidophilus and many were contaminated. "Thus, there was no yogurt, acidophilus powder, capsule, tablet, or milk that contained H2O2-producing L acidophilus in pure form."

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*Hughes, Vicki L., and Sharon L. Hiller, "Microbiological Characteristics of Lactobacillus Products Used for Colonization of the Vagina," Obstetrics & Gynecology, 75(2): 244-8, February 1990.