The right resident strains - quite a few probiotics on the market contain things like active yeasts or soil based organisms. While these ingredients may be helpful to some, if you want to optimize your gut health, you need to look for strains that are resident to the human gut like those from the Lactobacillus, Bifidobacterium, and Streptococcus families.
Dr. Michelle Schoffro Cook says there’s no single best strain of bacteria, though some strains, like L. Acidophilus or B. Bacterium, have wide-reaching effects. They often act as a starting point from which to add other strains with more specific impacts. Already have probiotics on hand or want to arm yourself with raw knowledge before you shop? The table below shows the research- and expert-backed strains we looked for in each use case:

Back to the question of what are probiotics: There are five main types, each in its own classification called a genus. Within each genus there are multiple species, and within those species there are multiple strains. For example, Lactobacillus is a very common genus of probiotics. Within that genus are numerous species, such as rhamnosus. The genus and species of a microbe are always italicized, making it easy to know that a microbe is involved. In a species such as Lactobacillus rhamnosus (often abbreviated L. rhamnosus), there are many strains. An example of the name of a strain is L. rhamnosus GR-1.
While probiotics have been around as long as bacteria have, they were first officially identified for their health benefits in the early 20th century by Russian-born biologist Élie Metchnikoff. Metchnikoff believed that “good bacteria” like the microbes that produce lactic acid could prolong life and stave off senility, and actually recommended drinking sour milk daily for overall health. While Metchnikoff’s theories were pooh-poohed by many of his contemporaries, the first commercial probiotic, Yakult, hit the market in 1935 and is still on the shelves today.
Yeast Infections and Bacterial Vaginosis (BV) – If you’ve ever experienced the discomfort of a yeast infection or BV, then you are probably aware that an overgrowth of bacteria causes these unfortunate episodes. While your reproductive system is supposed to foster a healthy environment for yeast, sometimes these levels can increase causing the pain and discomfort of a yeast infection or bacterial infection. By taking a probiotic that contains prebiotics and feminine health probiotics, you can treat and prevent recurrent symptoms!
Uh, no. “There have been studies that suggest a theoretical link between the microbiome and even changing the microbiome, and changes in the brain,” Lebwohl tells me. He specifically points to this study, in which healthy women with no gastrointestinal issues had their brains imaged before and after a month of consuming yogurt products containing probiotics twice daily.
We have to give a nod to the most famous probiotic food: yogurt. Whether you love Greek or regular, low-fat or full-fat, look for the phrase "live active cultures" on the label. And although choosing a plain yogurt has less added sugar than the flavored kinds, Tallmadge gives you the okay to choose a fruit-infused flavor if that's the only way you'll eat it. Just be sure to aim for fewer than 15 grams of the sweet stuff per serving; sugar can feed the bad bugs in your gut.
Amy Shapiro, MS, RD, CDN, founder and director of Real Nutrition based in NYC, discusses the effect probiotics have on immune system issues. "Probiotics compete with pathogenic microorganisms and produce chemicals that inactivate or kill pathogens," says Shapiro. "They help prevent immune-mediated diseases by improving the gut mucosal immune system. Overall, probiotics protect the body from infections and allow the body to maintain homeostasis."
Renew Life Ultimate Flora offers 30 billion CFUs with 10 scientifically studied strains of Bifidobacterium and Lactobacillus, which according to current scientific research, are the two most prevalent good bacteria found in a healthy gut. In other words, you might choose these if you’re seeking a careful balance designed to re-establish proper immune health and digestive equilibrium.
Jo A. Panyko is a Master Nutrition Therapist and author of two books about probiotics. She is a professional member of the National Association of Nutrition Professionals (NANP) and works as a functional medicine nutrition therapist (nutritionist) at her company, Chrysalis Nutrition and Health. She writes about how to be healthy with probiotics on her website, powerofprobiotics.com.
Our favorite probiotics right now are the Smart Belly line of “Daily” products.  This brand is a little unique in that they make separate products for men, women and children.  Overall, the results we experienced were some of the best we’ve ever seen (and the quickest)!  This brand has pretty much everything we like to see in a top-of-the-line probiotic… a great mix of strains and number of CFUs, they’re very allergy-friendly, and use a patented delivery system to make sure their probiotics survive stomach acid.  When it’s all said and done, we think these are the best probiotics currently available and give them our highest rating!   Read the full review here… or click here to go buy it now!
Taking relatively high doses of these probiotic strains before, during, and after antibiotic treatment can help your microbiome get back on its feet: B. Lactis, B. Infantis, L. Acidophilus, L. Casei, L. Bulgaricus, L. Paracasei, L. Rhamnosus GG, and S. Boulardii. Our antibiotic combatant MegaFood MegaFlora has six of these strains, as well as eight others. One serving contains 20 billion CFUs to replenish the good bacteria.
Inside your body bacterial and other microbial cells live in close contact with your own cells, and in healthy conditions the thing that separates them from you is a layer of mucus. Between that layer of mucus and the inside of you is a layer of skin-like cells one-cell thick. One thing you have to understand about your gastrointestinal (GI) tract is that although it resides within you, it is really connected to the outside world, from your lips to your anus, and the things that protect you from the outside world are the mucus; the skin-like layer of cells with immune, nervous, and endocrine cells below it; and the beneficial microbes. At least 70 percent of your immune system is in your GI tract! This is why taking probiotics benefits the immune system greatly.
It is well known that people with lactose intolerance can often consume yogurt with few symptoms. This is because the probiotics in yogurt help digest the lactose in the small intestine, before it reaches the colon. In addition, the yogurt starter cultures L. bulgaricus and Streptococcus thermophilus help to break down the lactose. Because of its probiotics, yogurt is a good way for people with lactose intolerance to consume the recommended servings of dairy without experiencing uncomfortable symptoms they may get from other dairy products.

A popular dish in Japan consisting of fermented soybeans, natto contains the extremely powerful probiotic bacillus subtilis, which has been proven to bolster your immune system, support cardiovascular health and enhance digestion of vitamin K2. Natto can also contain vitamin B12, which is usually lacking in vegan diets. It’s also one of the highest plant-based proteins. 
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