A paper in Sexually Transmitted Infections details a cheap way to kill off gonorrhea in the mouth - alcohol-containing mouthwash. 

Gonorrhea is caused by bacteria and  curb the growth of the bacteria responsible.  Gonorrhea of the mouth has become more common among primarily gay men as fear of AIDS has declined. That disease is quite treatable today and so fear of it has declined, meaning a decline in condom use.

Gonorrhea is also treatable, with antibiotics, but those heighten the risk of the emergence of antibiotic resistant strains of Neisseria gonnorheae, the bacteria responsible for the infection, so concern is high.

If gay men won't use condoms, maybe they'll use mouthwash

The paper notes that Listerine has been considered a cure for gonorrhea 1879, well before the age of antibiotics. The in vitro study used different dilutions (up to 1:32) of Listerine Cool Mint and Total Care, both of which contain 21.6% alcohol, were applied to cultures of N. gonorrheae. By way of a comparison, a salt water (saline) solution was similarly applied to an identical set of cultures. Listerine at dilutions of up to 1 in 4, applied for 1 minute, significantly reduced the number of N. gonorrheae on the culture plates, whereas the saline solution did not.

For the randomized clinical trial, 196 gay men were recruited at one sexual health clinic in Melbourne, Australia, between May 2015 and February 2016. Of those, 58 (30%) were culture positive before using the solution. After gargling the allocated solution, men in the Listerine group were significantly less likely to be culture positive on the pharyngeal surface (52%) compared with men in the saline group (84%) (p=0.013). 

Their data suggest Listerine significantly reduces the amount of N. gonorrheae on the pharyngeal surface, though the monitoring time was short, so it is unknown if the effects of the mouthwash might be short-lived.