CTH and infection: The fact that endogenous H2S, produced from the CSE enzyme, exerts a protective mechanism against MDR P. aeruginosa infection but not against infection elicited by other gram-negative bacteria, such as K. pneumoniae, indicates that host-derived H2S interacts with a unique system that is crucial for the pathogenesis of infections by P. aeruginosa. Quorum sensing (QS) is such a unique system, which controls the virulence of P. aeruginosa [24,31]. Modulation of H2S levels, by the addition of an H2S donor or an H2S biosynthesis inhibitor, did not affect in vitro bacterial growth (Fig 5A).