Table 3.

Role of different two-component systems involved in mediating antibiotic resistance in response to acid stress. All abbreviations are listed at the end of the manuscript.

Treatment under Acid Stress	Two-Component Systems Involved	Acquired Antibiotic Resistance/Tolerance	Phenotypic Expression 1	Reference
ΔtatC, over-expressed nlpE	CpxRA	Cationic antimicrobial peptides (CAPs)	Increased tolerance	[145]
ΔrcsF, ΔrcsB, ΔcpxR	RcsCB, CpxRA	Mecillinam and cefsulodin	Increased tolerance	[139]
ΔcpxR	CpxRA	Cephalexin	Increased tolerance	[156]
W3110 tolC732::kan, W3110 acrB747::kan, W3110 mdtB774::kan, W3110 mdtF769::kan, W3110 emrY776::kan, W3110 emrB767::kan, W3110 marR751::kan	MarRAB, AcrAB, EmrKY, MdtABC and TolC	Extended-spectrum β-lactamases (ESBLs)	Increased tolerance	[152]
Δmar	MarRAB, AcrAB and TolC	Beta-lactamase, rifampicin, spectinomycin, streptomycin, tetracycline, nalidixic acid	Increased resistance	[157]
ΔrcsF and ΔrcsB	RcsBC	Cefsulodin	Increased tolerance	[139]
	RcsBC, CpxRA, BaeSR	Mecillinam and cefsulodin
baeR cloned pTrc99A plasmid	BaeRS, MdtABC, ArcAB	Ceftriaxone,	8 fold increased resistance	[158,159]
		novobiocin,
		deoxycholate
pH stress only	ArcAB, MarRAB	Ceftriaxone,	Presence ofhyper-resistantcolonies	[60]
		amikacin,
		nalidixic acid
	ArcAB, MarRAB, MdtABC	Multidrugresistance		[144,160]
	RcsCB	Cationic antimicrobial peptides (CAPs)	Intrinsic resistance	[161]
		Aztreonam		[162]
		Beta-lactams		[161]
		Daptomycin		[96,163,164]
ΔdpiA, ΔcpxR	RcsBC, CpxRA	Ampicillin	Increased tolerance	[165]
ΔpmrA, ΔpmrB	PmrAB, arn operon	PolymyxinB	Increased tolerance	[137]
ΔacrB	BaeSR, RcsBC, CpxRA, EvgAS, ArcAB	Multidrug resistance	16- to 32-fold increased resistance	[166]
ΔmarR	MarRAB	Norfloxacin	Increased tolerance	[44].
ΔacrBΔevgAS, ΔacrBΔemrKY, ΔacrBΔyhiUVΔemrKY, ΔacrBΔyhiUVΔemrKY/pUCevgA	ArcAB, EvgAS, EmrKY	Multidrug resistance	4 fold increased resistance	[167,168]
Overexpression of baeR, evgA, rcsB	BaeSR, RcsBC, CpxRA, EvgAS, ArcAB	Multidrug resistance	16- to 32-fold increased resistance	[166]

¹ Resistance is an increase in MIC above the breakpoint; tolerance is the loss of killing with no change in MIC.