Staphylococcus aureuscounters organic acid anion-mediated inhibition of peptidoglycan cross-linking through robust alanine racemase activity

Abstract

Weak organic acids are commonly found in host niches colonized by bacteria, and they can inhibit bacterial growth as the environment becomes acidic. This inhibition is often attributed to the toxicity resulting from the accumulation of high concentrations of organic anions in the cytosol, which disrupts cellular homeostasis. However, the precise cellular targets that organic anions poison and the mechanisms used to counter organic anion intoxication in bacteria have not been elucidated. Here, we utilize acetic acid, a weak organic acid abundantly found in the gut to investigate its impact on the growth ofStaphylococcus aureus. We demonstrate that acetate anions bind to and inhibit D-alanyl-D-alanine ligase (Ddl) activity inS. aureus. Ddl inhibition reduces intracellular D-alanyl-D-alanine (D-Ala-D-Ala) levels, compromising staphylococcal peptidoglycan cross-linking and cell wall integrity. To overcome the effects of acetate-mediated Ddl inhibition,S. aureusmaintains a high intracellular D-Ala pool through alanine racemase (Alr1) activity and additionally limits the flux of D-Ala to D-glutamate by controlling D-alanine aminotransferase (Dat) activity. Surprisingly, themodus operandiof acetate intoxication inS. aureusis common to multiple biologically relevant weak organic acids indicating that Ddl is a conserved target of small organic anions. These findings suggest thatS. aureusmay have evolved to maintain high intracellular D-Ala concentrations, partly to counter organic anion intoxication.