2.6 Changes at membrane level in persister cells
The next section of this review which deals with therapeutics, will show that the membrane is considered to be a main target for anti-persister cell agent development [115].
However, few studies have investigated the lipidome of persister cells (Table 1). In Mycobacterium tuberculosis dormant cells, an accumulation of triacylglycerol (TG) linked to the membrane or as inclusion bodies [116], [117] was highlighted [118]. TG could be involved in energy storage used during hibernation [119], [120] and used to reduce the toxic burden of free fatty acids causing a reductive stress [121]. It could also contribute to antibiotic tolerance by redirecting cellular carbon fluxes away from the tricarboxylic acid cycle [122]. In addition, genes encoding TG biosynthetic enzymes were found predominantly in virulent M. tuberculosis strains, suggesting a potential role of these genes in the infection [118]. In A. baumannii , the lipidome of persister cells under ciprofloxacin treatment (100 × MIC) was analyzed using LC-MS (Table 1) [123]. It was demonstrated that membranes of persister cells displayed specific lipids like lyso-1-phosphatidyl-2-acyl-glycerol-3-phosphoethanolamines (LPAGPE) and 1-phosphatidyl-2-acyl-glycerol-3-phosphoethanolamines (PAGPE). These specific lipids may compensate a repulsive effect of the phosphate negative charges due to an increase in cardiolipin abundance. Moreover, the acyl chains of their lipids were modified by hydroxylation or di-unsaturations. The presence of di-unsaturation and hydroxylation could contribute to a change in membrane fluidity. In persister cells, a rigidification of the membrane could allow bacteria to modify the permeability of their membranes in order to survive, and may accompany to the bacterial growth arrest. Finally, the persister cells were shown to accumulate particular wax esters (WE) composed of two fatty acids and a fatty diol. These uncommon WE could be a specific metabolic key forA. baumannii to store energy during growth arrest and used for rapid regrowth? [123].
Concerning membrane proteins, genes coding for OmpA and OmpW were shown overexpressed by a factor of 5.5 and 10.5 respectively, in 15 × MIC meropenem persisters of the Acb-1 strain (Table 1) [124]. In agreement with this, virulence assays on Galleria mellonella indicated that these persisters were more virulent than control cells [124]. On the contrary, ompW was under-expressed in tobramycin persisters. [63] Its expression in persisters may thus be drug-dependent. The authors suggest that over-expression can helpA. baumannii survive exposure to meropenem by increasing drug efflux.
Finally, it was already shown that A. baumannii persister cells underwent morphological changes. Indeed, a study on persisters generated by 100 × MIC meropenem showed that cells had a spherical shape with a reduced permeability (Table 1) [125]. These morphological changes were also observed for the ATCC 17978 strain under a 6 μg/mL meropenem treatment [126]. Furthermore, images of persister cells under tobramycin (10 mg/L) or meropenem (15 mg/L) treatment by transmission and scanning electron microscopies showed the presence of a division septa at the cell midpoint [35]. The presence of this septum was intriguing. It was suggested that the persister cells could still be in a multiplying state to maintain a balance between multiplication and cell death, as also suggested in Mycobacterium smegmatis [127].
Table 1 : Molecular mechanisms involved inA. baumannii persister cells physiology .