2.4 SOS response in persister cells
The SOS pathway is an inducible DNA damage repair system that is pivotal
for bacterial adaptation, pathogenesis, and diversification of
population. The SOS response activates systems to carry out adaptation
processes to face hostile or even toxic environments. These systems can
contribute to growth arrest, DNA repair, repair of replication errors or
mobilization of mobile genetic elements (transposons and insertion
sequences) [91], [92], [93]. Activation of the SOS response
in persister cells could explain the appearance of adaptative mutations,
especially in the presence of antibiotics inducing reactive oxygen
species (ROS) production leading therefore to DNA damages [94].
Persister could thus be a key stage in the course of genetic resistance
development [95]. The SOS response seems to be a key player in
persister physiology, for both maintaining DNA integrity and remodeling
it [96]. However, DNA damage response is a multifactorial response
that appear to be species-specific and may have a major influence on the
development of new capabilities of the species [97].
The DNA damages induced by ROS can be produced by antibacterial
molecules and leads to an SOS response [98]. This response is
mediated by RecA, which binds to single-stranded DNA and protects it
from degradation. In many bacteria, LexA is the transcriptional
repressor of RecA and of all SOS response genes. It is cleaved by RecA
if the latter is bound to a single strand of DNA activating the
transcription of SOS genes. In Acinetobacter , the mechanism seems
different, as the genome lacks the lexA gene [99], [100].
Indeed, the error-prone reserve polymerase UmuD (polymerase V) is
recruited during SOS response. UmuD is repressed by UmuDAb,
itself repressed by DdrR. DdrR overexpression represses the DNA
repair genes [101].
It was shown that, after 1h tobramycin treatment at 10 × MIC,umuD and ddrR of ATCC 17978 strain were over-expressed
[102]. The transcriptomics of ATCC 17978 persisters selected by
treatment of 50 × MIC of ciprofloxacin, showed overexpression of
different systems, like the DNA repair system, the phenylacetic acid
degradation pathway, the leucine catabolism, HicAB toxin-antitoxin
system and ROS response (Table1) [63]. Several genes displayed
significant changes like pare (A1S_3359); recA (A1S_1962); umuC (A1S_2008); umuD (A1S_1174,
A1S_1389); ddrR (A1S_1388); ruvB (A1S_2588);recB (A1S_0356) and repA (A1S_0663). The construction of
deletion mutants of recA (A1S_1962) and umuD homolog
(A1S_1389) was performed. The umuD mutant formed fewer persister
cells than the WT strain, when the recA mutant did not form any
persister cells after 3 hour (h) of antibiotic treatment [63]. These
results suggested that these proteins (umuD and recA ) are
involved in persister formation. It should be also noticed that RecA is
involved in resistance to some β-lactam antibiotics and contributes to
survival after ultraviolet (UV) treatment, and to virulence in
macrophages in vitro and in mice in vivo [103],
[104].
Other transcriptomic analyses on ATCC 19606 and two clinical strains
treated by 10 mg/L of ciprofloxacin and 10 mg/L of imipenem for 6 h were
also performed [105]. They showed similarly that the three strains
overexpressed dam, recC, umuD, phoU , and glpD . The
methyltransferase Dam, when subcloned under the control of an inducible
promoter, positively regulated recC , umuD , phoU ,
and glpD genes. Consequently, the percentage of persister cells
was more abundant in the Dam-overexpressing mutant. The RecC
overexpressing-mutant presented also an increased rate of persister cell
formation (Table 1) [105].
Antibiotic shock treatment frequently leads to the generation of ROS
[106], [107], [108], [109]. As seen previously, ROS may
damage DNA leading to the SOS response [96]. However, the bacteria
may also activate some other mechanisms to face the ROS stress and
maintain the cell integrity. These mechanisms are essential for a rapid
adaptation or the bacterial survival. This response is indirectly
involved in the state of persister cells [110]. As an example,
pyocyanin generates ROS in A. baumannii . It was shown ROS
production leads to catalase and Sod increased expression in order to
eliminate ROS [111]. Clearance of A. baumannii polymyxin B
persister by Rifampicin was shown to be related to a significant down
regulation of sodB and therefore to an enhanced generation of ROS
(Table 1) [112]. Additionally, it was shown that a SodB A.
baumannii mutant was more sensitive to antibiotics and generated less
persister cells [112].