**3.7** *Klebsiella pneumoniae*

ever produced [34]. Such multiple antibiotics resistance has developed by acquiring MGE through HGT. This results in mutations that alter drug binding sites on molecular targets leading to an increase in the expression of endogenous efflux pumps. These resistant strains fight antibiotics by deactivating β-lactam binding proteins [11]. Due to its increasing antibiotic resistance to penicillin and methicillin, the bacteria remain a growing pandemic through mechanisms including HGT and antibiotic alterations [35]. Moreover, *S. aureus* is not far from the Gram-negative bacteria which are resistant to antibiotics mediated by TCS. Thus, the TCS VanRASA regulates the necessary mechanism of resistance in vancomycin resistance *S. aureus* (VRSA) [36]. Again, the EPs from *S. aureus* have been categorized recently in six different diverse groups. They were found to be either chromosomal or extrachromosomal except *qac*A/B and *smr* which were found only on the studied plasmid

*P. aeruginosa*, the Gram-negative nosocomial pathogen, is considered as an epit-

ome of AMR due to its major involvement in causing chronic and nosocomial diseases. This high rate of resistance is directly related to their various inherent resistance mechanisms expressed, including the down-regulation of porin manufacturing system (carbapenems and cefepime), overexpression of efflux pumps (carbapenems) or production of other beta-lactamases besides the high production of AmpC beta-lactamase. The most frequently administered

antipseudomonal antibiotics are aminoglycosides, fluoroquinolones and β-lactams that are susceptible to the known resistance mechanisms in *P. aeruginosa*. Its mutants, with upregulated EPs, have been reported that makes it difficult to find an effective antibiotic [38]. Moreover, only inhibition of the EP, in the recent clinical MDR isolates, has almost no effect in increasing susceptibility toward the tested antibiotics [39]. However, inhibition of histidine kinases (HKs), a part of TCS, using benzothiazole-based HK inhibitors, resulted in a reduced production of mol-

Tuberculosis (TB) poses serious global health crisis as an important chronic infectious disease caused by strains of *M. tuberculosis* (MTB). It is an extremely dangerous human pathogen that infects one-third of the world's population and causes almost two million fatalities each year [41]. Besides that, the total number of cases have been still increasing, due to strains of MTB being resistant to first-line drug therapy [41]. This involves resistance to the two most powerful anti-TB drugs, rifampicin and isoniazid, thereby evoking the title of multidrug resistance TB (MDR-TB). The existence of even more resistant MTB strains has been described as extreme drug-resistant (XDR)-TB, which shows resistance against the injectable second line drugs such as kanamycin, amikacin or capreomycin [42]. A more alarming situation has arisen with the depiction of MTB strains showing resistance to all antibiotics available for testing, with the species being termed as total drug resistant (TDR)-TB [43]. Therefore, the early onset of detection and prevention of MDR-TB, will enable the therapeutic treatment to reduce the spread of infection. Thus, a better understanding of the mechanisms of action of anti-TB drugs will facilitate the development of new drug targets aimed at improving outcomes from diseased patients [44]. In fact, it has been found that MprA, part of TCS MprAB.

ecules which are linked to quorum-sensing and redox-balance. It also showed reduced motility and attachment ability, rendering it to be less

samples [37].

virulence [40].

**8**

**3.6** *Mycobacterium tuberculosis*

**3.5** *Pseudomonas aeruginosa*

*Antimicrobial Resistance - A One Health Perspective*

*K. pneumoniae* is a Gram-negative hospital-acquired pathogen causing nosocomial pneumonia and urinary tract infections. The increased incidence of carbapenemase-producing and thus, carbapenem resistant *K. pneumoniae* (CRKP), has posed a major threat to global human health. Diseases caused by CRKP were treated successfully in combination therapies of antimicrobial agents [47]. It has been reported that tigecycline and the polymyxins (polymyxin B or colistin) showed variable susceptibilities to treat infections caused by CRKP [4]. This has led to the emergence of CRKP, against which there are very few antibiotics in development that can treat the infections [11]. Incidentally, the minimum inhibitory concentration (MIC) of eravacycline has been increased as a consequence of increased expression of two EP complexes OqxAB and MacAB in *K. pneumoniae* [48]*,* which suggests their contribution to resistance against this antibiotic.

#### **3.8** *Enterococcus faecium*

Generally associated with HAI in immunocompromised patients, *E. faecium* is a Gram-positive bacterium, often showing resistance to β-lactam antibiotics, including penicillin and other antibiotics of last resort. Reportedly, there has also been an increase in vancomycin-resistant enterococci (VRE) strains, exhibiting resistance to vancomycin-A [11]. These VRE strains show an ability to produce and share their resistance through HGT, as well as code for virulence factors that regulate phenotypes. These virulence phenotypes differ from the wild types in producing thicker biofilms for development in a variety of environments, including medical devices such as urinary catheters and cardiovascular prosthetic valves [14]. The thicker biofilms function as a "mechanical and biochemical shield" that protects the bacteria from antibiotics and is the most efficient protective system against bacterial treatment [13]. In fact, the intensive use of antibiotics in animal rearing resulted in the development of resistance in *E. faecium* [49]. Moreover, recently, a study identified few new antibiotics resistance genes related to EP, namely, *optrA* and *poxtA* besides the new gene *cfr*-like variant in *E. faecium* [50]. Earlier, the expression of the EP proteins, EfrAB, has been shown to be increased upon halving the MIC of gentamicin and got lowered upon the addition of 3 mM EDTA [51]. Furthermore, TCS like ChtSR has been found to be responsible for chlorhexidine tolerance in MDR *E. faecium*, upon testing by targeted deletion mutation of *cht*R and *cht*S genes [52]. Again, the TCS CroRS was reported to be crucial in resistance to cell wall antibiotics in *E. faecium* [53].

## **3.9** *Enterobacter*

*Enterobacter* are Gram-negative bacterial species which trigger UTI and blood diseases. They show resistance against different drug therapies, thus, requiring the development of new and efficient antibiotic treatments [54]. In fact, colistin and tigecycline are, only two of the antibiotics, presently being used as medication while no other feasible antibiotics are apparently being developed. Other most commonly reported antimicrobials in *Enterobacter* infections are aminoglycosides, cephalosporins, carbapenems and fluoroquinolones. Moreover, in some species, a 5- to 300 fold rise in the MIC was reported when subjected to several gradually increasing

benzalkonium chloride concentrations [55]. In fact, the EP protein, SugE, in *Enterobacter cloacae,* which is a member of small multidrug resistance (SMR) protein family, has been found to be responsible for resistance against toxic compounds such as cetylpyridinium chloride and benzalkonium chloride [56]. Another EP protein of the resistance nodulation cell division (RND) category, AcrB, has also been found to be very essential in the pathogenicity and antibiotics resistance of *E. cloacae* [57, 58].

alterations in the *dnaK* gene reduce the growth of bacteria within the host [69]. In fact, during infection, bacteria activate their heat shock genes like DnaK to protect their cellular machinery from the consequently activated host immune system for defense mechanisms and thereby strengthen their virulence strategy [69]. This phenomenon, thus, provides an insight into structural mechanism of DnaK, leading to misfolding and its role in controlling protein activity contributing to the pathogenicity of multidrug-resistant bacteria, such as the opportunistic human pathogen *A. baumannii* [70]. In fact, DnaK mutants showed decreased viability and improved susceptibility under strained circumstances during systemic infection as reported for *dnaK* mutants of *S. aureus* with increased sensitivity to oxacillin and methicillin

Antibiotic resistance can be triggered, in MDR bacteria, by four discrete mechanisms viz. target modification, reduced permeability and improved efflux, drug inactivation and drug extrusion by the multidrug efflux pumps (EP) [73]. Due to their poly-substrate specificity, besides having the potential to expel a broad variety of antibiotics, these EP also manage the development of other resistance mechanisms by decreasing intracellular antibiotics concentration and stimulating mutation accumulation [73]. Consequently, over-expression of multidrug EP is involved with clinically related antibiotic resistance. Thus, there has been increasing evidence of EP having biochemical functions in bacteria along with their appearance under strict regulations in response to some physiological and environmental signals [73]. Hence, a systematic knowledge of EP is important for the development of

EP are present in almost all bacterial species involved in AMR. They can be located on plasmids or chromosomes that encode this class of proteins. The five families of bacterial EP, found to be involved in MDR, are the major facilitator superfamily (MFS), the multidrug and toxic compound extrusion (MATE) family, the ATP-binding cassette (ABC) superfamily, the small multidrug resistance (SMR) family, and the resistance-nodulation-division (RND) family, based on their composition, energy sources and substrates used [73]. Importantly, only RND superfamily is found in Gram-negative bacteria due to its structure containing tripartite complex and the efflux systems of the other four families are widely distributed in both Gram-positive and -negative bacteria. These EP can be either single or multiple-component transporters depending on their specific classes. They comprise both an inner and an outer membrane transporter, like the RND type. It has been found that RND family pumps are frequently associated with therapeutically important bacterial resistance such as AcrB in *S.* Typhimurium and *E. coli* and MexB in *P. aeruginosa* owing to their tripartite complex, enabling various drugs to be immediately extruded from cytoplasm to outside the bacterial cells [74].

In fact, antibiotics such as fluoroquinolone, tetracycline, rifampin, novobiocin,

chloramphenicol and B-lactams were used to analyze the substrate profile of housekeeping efflux system AcrAB-TolC in *E. coli* [74]. Similarly, the *S.* Typhimurium AcrAB-TolC efflux system was also capable of expelling various antibacterial agents such as tetracycline, quinolones and chloramphenicol [75, 76]. The two RND efflux pumps, MexAB-OprM and MexXY-OprM, homolog to AcrAB-TolC system in *E. coli*, are also expressed in *P. aeruginosa*. Thus, these systems can actively export chloramphenicol, tetracycline and fluoroquinolones. In addition to these substrates, MexAB-OprM export B-lactams and novobiocin whereas MexXY

[71] and *dnaK/dnaJ* mutants of *E. coli* having increased sensitivity to

*Strategic Role Players of Important Antimicrobial-Resistant Pathogens*

*DOI: http://dx.doi.org/10.5772/intechopen.92742*

EP inhibitors as promising AMR intervention strategies.

system exports aminoglycosides (**Table 1**) [77].

**11**

fluoroquinolones [72].

**4.2 Efflux pumps**
