Efficacy of combinations of colistin with other antimicrobials involves membrane fluidity and efflux machinery

Figures & data

Table 1 Bacterial susceptibility to antimicrobials administered either alone or in combination with colistin at ½ its MIC

Acinetobacter baumannii	COL	CHL	CHL+COL	TET	TET+COL	LZD	LZD+COL	VAN	VAN+COL
ABAU 137	1	64	≤0.125	>128	1	>256	4	>256	1
ABAU 80	1	64	0.5	>128	1	>256	8	256	2
ABAU 226	1	128	0.5	4	1	>256	16	>256	8
ATCC 17978	1	64	0.5	2	0.25	>256	16	256	4
ABAU 8	1	64	≤0.125	>128	0.5	>256	16	256	1
ABAU RUH-875	1	>128	0.25	32	0.5	>256	8	128	4
ABAU 176	1	128	0.5	>128	1	>256	16	256	8
ABAU 34	0.5	64	0.5	>128	2	>256	16	>256	2
Pseudomonas aeruginosa
PA 362	0.25	64	4	4	2	>256	64	256	2
PA 350	0.5	8	4	8	4	256	32	128	2
PA 056	0.5	64	2	16	4	>256	32	>256	8
PA 846	0.25	8	4	4	0.5	128	16	128	8
PA O1	0.5	16	2	8	4	>256	32	128	16
PA 023	0.5	8	0.25	16	2	>256	64	>256	8
PA 666	1	128	2	16	2	>256	32	128	16
PA 086	0.5	8	0.5	8	1	>256	64	256	16
ATCC 27853	0.5	64	4	128	1	>256	64	256	8
Escherichia coli.
MDR 211453	0.5	4	4	1	1	256	32	256	8
MDR 39255	0.5	4	1	128	64	256	32	256	8
MDR 208691	0.5	>128	2	>128	32	256	32	256	8
MDR 205119	0.5	64	1	>128	16	128	16	128	16
MDR 246415	1	>128	16	>128	2	256	32	128	8
MDR 239910	1	16	2	128	16	256	16	256	8
MDR 238192	0.5	128	32	>128	32	256	16	256	16
MDR 215482	1	>128	64	>128	16	>256	8	>256	4
MDR 213873	1	4	4	128	16	128	64	64	16
ATCC 25922	0.5	1	1	1	0,25	128	16	128	16

Note: MICs are reported in µg/ml.

Abbreviations: COL, colistin; CHL, chloramphenicol; TET, tetracycline; LZD, linezolid; VAN, vancomycin.

Figure 1 Average acridine orange (AO) accumulation in the presence of the efflux inhibitor PaβN and sub-inhibitory concentrations of colistin. (A–C) colistin-sensitive strains; (D) a species intrinsically resistant to colistin.

Figure 2 Confocal light microscopy imaging after Live/Dead staining of Pseudomonas aeruginosa. (A) Untreated cells; (B) cells treated with colistin at 0.25 μg/mL and (C) cells treated with colistin at bactericidal concentration. Similar images were obtained for Acinetobacter baumanii and E. coli (not shown). TEM of thin sections of S. marcescens after treatment with colistin. Blebs formed by the outer membrane are easily visualized.

Table 2 Changes in membrane fluidity (anisotropy) in the presence of colistin

	Colistin (μg/ml)	Anisotropy (r)
		DPH	TMA-DPH
E. coli	0	0.2787±0.0219	0.267±0.039
	0.25	0.26986±0.020	0.265±0.028
	2.00	0.2488±0.009	0.2681±0.0212
P. aeruginosa	0	0.3139±0.034	0.284±0.0247
	0.25	0.3105±0.0359	0.274±0.0199
	2.00	0.2877±0.0213	0.273±0.0212
A. baumannii	0	0.314±0.027	0.255±0.005
	0.25	0.306±0.029	0.260±0.0081
	2.00	0.307±0.0025	0.260±0.006