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 .