# DKsA Modulates Antimicrobial Susceptibility of _Acinetobacter baumanii_
## 3 Discussion  
The (p)ppGpp-deficient and _\(\Delta\)dksA_ mutants of _E. coli_ exhibit increased susceptibility to antimicrobial agents [17; 18], implying that (p)ppGpp and DksA contribute to antimicrobial resistance in _E. coli_. The (p)ppGpp-deficient mutant of _A. baumannii_ ATCC 17978 also exhibited increased susceptibility to antimicrobial agents [20]. However, in the present study, _\(\Delta\)dksA_ mutant of _A. baumannii_ ATCC 17978 exhibited decreased susceptibility to quinolones and tetracyclines, whereas _\(\Delta\)dksA_ mutants of _A. baumannii_ ATCC 17978 and 1656-2 exhibited increased susceptibility to aminoglycos.  
The deletion of _dksA_ upregulated the expression of _adeB_, _adel_, _adeB_/ and /or _tetA_ in _A. baumannii_ ATCC 17978 and the clinical 1656-2 strain. _A. baumannii_ ATCC 17978 was susceptible to quinolones and tetracyclines, whereas the clinical 1656-2 strain was resistant to quinolones and tetracycline and susceptible to Giocycline [21]. In the 1656-2 strain, resistance to quinolones was mediated by the mutations in the quinolone-resistance determining region of _gyrA_, and resistance to tetracyclines was potentially mediated by several efflux pump genes [22]. Furthermore, multidrug-resistant _A. baumannii_ strains decrease cell envelope permeability against antimicrobial agents [23]. Therefore, the upregulation of efflux pump genes directly contributed to increased MICs of quinolones and tetracyclines in the \(\Delta\)dksA mutant of ATCC 17978, although efflux pump gene upregulation could not change the MICs of quinolones and tetracyclines in \(\Delta\)dksA mutant of 1656-2. Both \(\Delta\)dksA mutants of ATCC 17978 and 1656-2 were more susceptible to aminoglycosodes than the WT strains. In a previous study, the (p)ppGpp-deficient strain of _A. baumannii_ ATCC 17978 was more susceptible to aminoglycosidase than the WT strain [20]. Because (p)ppGpp and DksA inhibit the transcription of genes involved in the synthesis of translational machinery during the stringent response or stressful conditions [15; 24], (p)ppGpp-deficient and \(\Delta\)dksA mutants cannot inhibit the transcription of ribosomal genes, potentially increasing susceptibility to aminoglycos. Combined with the previous results, the present study demonstrates that (p)ppGpp and DksA play an opposing role in the regulation of genes  
Figure 2: A morphological difference between _A. baumannii_ 1656-2 and its \(\Delta\)dksA mutant NY0298D strains. Bacteria were cultured in LB with shaking to reach the indicated OD\({}_{600}\) and stained with Gram’s reagents. Bacterial morphology was observed using a light microscope. Magnification, 1000\(\times\).  
associated with efflux pumps. Further studies would be required to understand the regulatory mechanisms of multiple genes linked with intrinsic resistance by DksA and (p)ppGpp in _A. baumaniii_.  
The present study demonstrated that \(\Delta\)_dksA_ mutants of clinical 1656-2 exhibited more morphological heterogeneity than the WT strain. Previous studies have reported that \(\Delta\)_dksA_ mutant and (p)ppGpp-deficient mutant strains exhibited more morphological heterogeneity than the WT _A. baumaniii_ ATCC 17978 strain [20,25]. The (p)ppGpp-deficient and \(\Delta\)_dksA_ mutants in _E. coli_ also exhibited a more filamentous morphology than the WT strain [16]. These results indicate that (p)ppGpp and DksA coordinately regulate genes associated with cellular morphology or cell division.  
This study demonstrates that _dksA_ deletion upregulates efflux pump gene expression in _A. baumaniii_ strains. However, (p)ppGpp deficiency downregulates the expression of efflux pump genes in _A. baumaniii_[16]. Overall, RNAP-binding global regulators (p)ppGpp and DksA can modulate antimicrobial susceptibility in _A. baumaniii_, but they play opposite roles in antimicrobial resistance through regulating the efflux pump genes.