outcompetes in artificial urine medium through secretion of ammonia and other volatile compounds.

and form mixed biofilms in catheter-associated urinary tract infections. However, co-inoculation of with in artificial urine medium (AUM) resulted in a drastic reduction of cells in both biofilm and planktonic growth. Here, the mechanism behind this competitive interaction was studied. Both pH and aqueous ammonia (NH) increased in mixed cultures (to 9.3 and 150 mM, respectively), while viable cells dramatically diminished over time (>6-log reduction, < 0.05). Mixed cultures developed in either 2-(-morpholino) ethanesulfonic acid (MES)-buffered AUM (pH 6.5) or AUM without urea did not show bacterial competition, evidencing that the increase in pH and/or NH concentration play a role in the competitive interaction. Viability of single-species cultures decreased 1.5-log in alkaline AUM containing 150 mM NH after 24 h inoculation, suggesting that ammonia is involved in this inter-species competition. Besides NH, additional antimicrobials should be present to get the whole competitive effect. Supernatants from -containing cultures significantly diminished viability in planktonic cultures and affected biofilm biomass ( < 0.05). When subjected to evaporation, these supernatants lost their antimicrobial activity suggesting the volatile nature of the antimicrobial compounds. Exposure of to volatile compounds released by significantly decreased cell viability in both planktonic and biofilm cultures ( < 0.05). The current investigation also evidenced a similar bactericidal effect of volatiles over and . Altogether, these results evidence the secretion of ammonia and other volatile compounds by , with antimicrobial activity against gram-negative uropathogens including . This investigation provides novel insight into competitive inter-species interactions that are mediated by production of volatile molecules.