This is another numbers game. A strain is the individual bacterium within a species. For example, most people think that acidophilus is one bacterium, or strain…but it’s a species, like human beings are. So, there are many different strains of acidophilus – some great, some less so. The strains should be listed with their full name on the label, such as Lactobacillus acidophilus CL1285. You’ll often see the very lazy argument that since your own gut flora is super diverse, a single strain couldn’t possibly work…and that more strains means a more diverse product that will work better. But nope. There are single strain products that are excellent, and multi-strain products that are garbage. Just give the good bacteria in a high enough dosage, and they’ll do their job. Another potential issue with multi-strain probiotics is that since bacteria compete with each other, if the probiotic hasn’t been properly tested, you could be buying a ten strain product that in reality, has one or two strains that outcompete all the rest…and you’re really not getting what you paid for. Instead, focus on ensuring that the probiotic actually works, which you can verify by using the criteria I just talked about above.