Table 2.

Two-component systems targeted on antibiotic resistance in Salmonella.

TCS System	System Activation Signals	Components of System		Antibiotic Resistance	Function	Reference
		HK	RR
PhoPQ	low Mg2+; acidic pH; cationic antimicrobial peptide (CAMP); hyperosmotic stress	PhoQ	PhoP	polymyxin	LPS modifications (by adding 4-aminoarabinose to lipid A); downregulation of the expression of genes located on SPI-1; increasing resistance to both conventional antibiotics and antimicrobial peptides	[116,117,118]
CpxAR	acidic pH	CpxA	CpxR	β-lactam	resistance to β-lactam antibiotics in the absence of AcrB efflux pump; downregulation of the MDR-related genes expression; decreasing porin expression	[119]
BaeSR	spheroplasting and exposure to indole, tannin, zinc, or cooper	BaeS	BaeR	ceftriaxone	upregulation of MDR efflux pumps; drug resistance by regulating the gene expression encoding drug transporters	[120]
EnvZ/OmpR	acidic pH	EnvZ	OmpR	β-lactam	activation of the ssrA transcription and production of SsrA and SsrB; decreasing porin expression, upregulation of MDR efflux pumps	[8,121]
PmrAB	acidic pH; high Fe3+/Al3+; activation by the PhoPQ system	PmrA	PmrB	polymyxin	LPS modifications (modification of lipid A)	[122]

HK—histidine kinase; RR—response regulator.