Thymidylate Limitation Potentiates Cephalosporin Activity toward Enterococci via an Exopolysaccharide-Based Mechanism

Multidiugrresistant enterococci are major causes of nosocomial infections. Prior therapy with cephalosporins increases the risk of developing an enterococcal infection due to. the the intrinsic resistance of enterococci to these antibiotics. While progress has been made toward understanding the :genetic and biochemical mechanisims of cephalosporin,,resistance, available data indicate that as -yet -unidentified resistance,factors must exist. Here, we describe results,of a screen to identify small molecules capable of sensitizing enterococci to broad-spectrum cephalosporins:: We found that both Enterococcus faecalis and Enterococcus faecum were sensitized to broad and expanded-spectrum ceplialosporins when thymidylate production, Was impaired, whether by direct inhibition of thymidylate synthase, or by limiting production of cofactors required for its activity. Cephalosporin potentiatiori is the result of altered exopolysaccharide.-production due to-reduced dTDP-glucose synthesis. Hence, exopolysaccharideproduction is a previously undescribed contributor to the intrinsic cephalosporin resistance of enterococci and serves as a new target for antienterocotcal therapeutics.