(A) The presence of dense heat-killed bacteria is not sufficient to protect dilute, live Lp cells from HGA inhibition. (B) The susceptibility of wild type Lp is nearly identical to that of Lp with a deletion of lqsR, the proposed quorum sensing response regulator in L. pneumophila. (C) Schematic illustrating part of the stringent response pathway in L. pneumophila. In low nutrient conditions, RelA and SpoT generate the alarmone ppGpp, which activates a variety of downstream stress responses via the LetA/LetS two component system and the RpoS sigma factor. (D) L. pneumophila stringent response mutants show similar, density-dependent susceptibility to HGA as wild type. Complementation of relAspoT, rpoS, and letA mutants by plasmid expression (with pSpoT, pRpoS, and pLetA respectively) similarly had little effect on HGA-mediated inhibition.