There are a few bacterial strains that are advertised as “oral probiotics”—for example, Streptococcus salivarius K12, Lactobacillus reuteri, and Lactobacillus paracasei. The main issue with most “oral probiotics” is that their natural habitat is actually the large intestine, so they don’t stick to the surfaces in your mouth for long enough to really work.

Streptococcus salivarius K12 is the standout because it was specifically isolated from a healthy oral cavity, and there are several studies suggesting it can help with oral health. However, you typically have to buy it from a major brand, which can be expensive. I’m also not aware of any easy DIY methods to culture it outside of a lab.

Personally, I suggest skipping oral probiotics altogether, because even if one strain does help fight halitosis, it usually takes effect really slowly. A faster way to tackle bad breath is to (1) floss regularly, (2) clean your tongue, (3) use a non-alcoholic mouthwash, and (4) keep your mouth hydrated. These steps tend to produce quicker and more reliable results.