2.3 Second Messengers
The second messengers, (p)ppGpp and cyclic dimeric diguanosine
monophosphate (c -di-GMP), are main signal molecules, the level of
which determines the entry in persister state [69], [70],
[71].
(p)ppGpp : Indeed, in E. coli , (p)ppGpp is
clearly a major player in induction of persister state [72].
Increased levels of (p)ppGpp induce transcription of RpoS and RpoE
[73]. ppGpp inhibits deoxyribonucleic acid (DNA) primase[74],
and ribosomal ribonucleic acid (rRNA) transcription [75],
deregulates an early step in lipopolysaccharide (LPS) formation
[76], and binds to ribonucleic acid (RNA) polymerase [77]. ppGpp
could directly inhibit the negative DNA supercoiling [78], and
indirectly induces transcription of ribosomal modulation factor (Rmf)
and hibernation promotion factor (Hpf), which plays a role in ribosome
dimerization (90S and 100S) [79]. In A. baumannii it was
shown that the formation of (p)ppGpp, is mediated by RelA (ABUW_3302)
(Table1) [80]. RelA is a monofunctional protein only contributing to
pppGpp synthesis. Moreover, the reduction of pppGpp into ppGpp is
mediated by SpoT, a monofunctional (p)ppGpp hydrolase [81]. TherelA deletion resulted in the formation of 4-fold less persister
cells than in WT AB5075 strain under 50 × MIC colistin or 80 × MIC
rifampin treatments. The mutant relA underwent also variations of
its motility, morphology or colony morphology [80]. In other strains
like ATCC 17978, it was shown that a mutant deficient in (p)ppGpp
production (ΔA1S _0579 ) is 2- to 4-fold more susceptible
to antibiotics, i.e. MICs are reduced by a factor of 1 to 4
compared to wide type (WT) or complemented strain [82]. The authors
suggest this susceptibility could be due to the under-expression of
efflux transporters (AdeI, AdeJ and AdeK ) and, less
obviously, to the over-expression of certain proteins belonging to the
transporter systems, such as AetA, AdeB and AdeM [82], [83].
c-di-GMP : It was already shown that this second
messenger molecule increases the proportion of persister cells inE. coli [84]. It also influences virulence factors in many
bacteria [85], [86], [87]. In P. aeruginosa , HigB
decreases the intracellular level of c -di-GMP, which is
responsible for the increased expression of the T3SS genes and the
repression of biofilm formation [88]. It is worth noting that the
enzymes, diguanylate cyclases, involved in c -di-GMP synthesis
contain a catalytic site with a GGDEF/EAL motif [89]. In A.
baumannii ATCC 17978 strain, several diguanylate cyclases were
identified and characterized but their involvement in persister cells
has never been studied [90].