Heat shock-related protease A is an essential fitness gene under four different experimental conditions (abscess formation; epithelial colonization; tobacco smoke survival; and black pigmentation), as determined using TnSeq technology [12, 14, 28]. Interestingly, HtrA has been shown to be important to several virulence-related functions of P. gingivalis, including (A) quorum sensing [42]; (B) a functional T9SS and gingipain transport [38, 41]; (C) murine abscess formation [40]; (D) oxidative stress protection [39, 40]; (E) murine lethality [40]; and (F) temperature stability [39].