Haloalkaliphilic chemolithoautotrophic sulfur-oxidizing bacteria belonging to the genus Thioalkalivibrio ar e highl y a bundant in micr obial communities found in soda lakes and dominant in full-scale bioreactors removing sulfide from industrial waste gases. Despite certain soda lakes being remote and unaffected by anthropogenic activities, haloalkaliphilic microorganisms, including Thioalkalivibrio strains, possess various antibiotic resistance genes. In this study, we investigated the impact of the antibiotic ampicillin on a co-culture of two Thioalkalivibrio species, Tv. thiocyanoxidans ARh2 T and Tv. versutus AL2 T, both experimentally and through in silico analysis of antibiotic resistance. Cell gro wth d ynamics were monitored over time at increasing ampicillin concentrations using repand qPCR. Within ten days after the addition of ampicillin, the co-culture transitioned from a Tv. thioc y anoxidans ARh2 T -dominated to a stable Tv. ver sutus AL2 T -dominated cultur e. This shift w as attributed to Tv. versutus AL2 T displaying a lower susceptibility to ampicillin, making it more competitive. These results emphasize the potential implications of antibiotic pressure on microbial comm unities, w Here a resistant species can outcompete a stable co-culture. This study presents the first evidence of such dynamics in haloalkaliphilic chemolithoautotrophs. By understanding the antibiotic resistance and the competitive dynamics of haloalkaliphilic bacteria like T hioalkalivibrio, we can gain insights into their behaviour and stress response. [ABSTRACT FROM AUTHOR]