Table 1.

Two-component receptor systems (TCS) sorted by signal type. TCS represented by the histidine kinase (HK) and response regulator (RR) pairs. Result of TCS may be due to either repression or activation post-signal response. Systems marked with * are highly conserved TCSs across genera.

	Signal	HK-RR Pair	Result	Genus
Metal Ions	Zn2+	ColS-ColR	LPS modification	Pseudomonas [5]
	Cu+/Ag+	CusS-CusR	Tripartite efflux pumps	Escherichia [6], Klebsiella [7]
	Mg2+	CsrR-CsrS	Activation of virulence repertoire	Streptococcus [8]
	Mg2+, Ca2+	PhoQ-PhoP*	LPS modification, Low Mg2+ adaptation, Antimicrobial resistance, PmrAB cross talk	Escherichia [7], Salmonella [9], Yersinia [10], Stenotrophomonas [11]
	Mg2+, Fe3+, Zn2+	PmrB-PmrA* BasS-BasR	Stress response, Antimicrobial resistance, PhoPQ cross talk	Escherichia [7], Pseudomonas [12], Salmonella [13]
Amino Acids	Glutamate	GluK-GluR	Antibiotic synthesis	Streptomyces [14]
	Glutamate, Aspartate	AauS-AauR	Swarming, Motility, Biofilm formation	Pseudomonas [15,16]
Amino Acid Derived Molecules	Serotonin (Via Tryptophan)	CpxA-CpxR	Type 3 Secretion System	Citrobacter [17], Escherichia [17]
	Epinephrine/Norepinephrine (Via Tyrosine)	QseC-QseB	Biofilm formation, Flagellar motility	Escherichia [18], Haemophilus [19], Salmonella [20]
	Epinephrine (Via Tyrosine)	QseE-QseF	Type 3 Secretion System	Escherichia [21]
	Nitrate/Nitrite (Via Arginine)	NarX-NarL NarQ-NarP	Motility, Biofilm formation, Nitrate Sensing and Reduction	Burkholderia [22], Escherichia [23], Pseudomonas [24], Salmonella [25]
Environment & Other	Acidic pH	EvgS-EvgA	Multidrug Transporter	Escherichia [26]
	Acidic pH	TrxR-TrxS	Biofilm formation	Streptococcus [27]
	Acidic pH	HK11-RR11	Biofilm formation	Streptococcus [28]
	Osmolarity	EnvZ-OmpR	Intracellular iron regulation	Escherichia [29], Klebsiella [7], Pseudomonas [7], Yersinia [100]