*4.1.1.2. Hypermutators*

During a prolonged non-lethal antibiotic selective pressure a small bacterial population en‐ ters a transient state of a high mutation rate which is called hypermutable state. Hypermuta‐ tors are found in many bacteria species such as E. coli, S. enterica, Neisseria meningitides (N. meningitides),H. influenzae, S. aureus, Helicobacter pylori (H. pylori), Streptococcus pneumoniae (S. pneumoniae),and P. aeruginosa [85]. Various studies suggested that hyper‐ mutations play an important role in acquisition of antibiotic resistance in pathogens [31; 32; 33].

#### *4.1.1.3. Adaptive mutagenesis*

Adaptive mutations arise in non-dividing or slowly dividing cells during the presence of a non-lethal selective pressure that favours them. A great number of antibiotic resistant mu‐ tants may come from this mutation process under bacterial natural conditions [85].

#### *4.1.1.4. Horizontal gene transfer*

Horizontal transfer of genetic material between bacteria is the most commonly used way to spread antibiotic resistance. In general, this exchange is accomplished mainly through the processes of transduction (via bacteriophages), conjugation (via plasmids and conjugative transposons), and transformation (via incorporation into the chromosome of chromosomal DNA, plasmids, and other DNAs) [34]. This type of genetic transfer not only occurred be‐ tween closely related bacteria but can also occur between phylogenetically distant bacterial genera, in particular between gram-positive and gram-negative bacteria [35]. Plasmid-en‐ coded antibiotic resistance encompasses most classes of antibiotics in practice, such as ami‐ noglycosides, cephalosporins and fluoroquinolones [36]. Transposons spread quicker than genes in chromosomes and are transferred by conjugation, transformation, or transduction [23; 24]. Integrons acquire and exchange exogenous DNA, known as gene cassettes, by a site-specific recombination mechanism. They can integrate stably into other DNAs where they deliver multiple antibacterial resistant genes in a single exchange. Resistance gene cas‐ settes encoding the metallo-β- lactamases IMP and VIM confer resistance to the potent car‐ bapenem β-lactams imipenem and meropenem [36].

*4.1.2.2. Target modification*

these enzymes [45].

*4.1.2.3. Efflux pumps and outer membrane (OM) permeability*

namides, tetracycline and trimethoprim, dyes and detergents [50; 51].

Another important resistance mechanism is the modification of antibiotic targets which makes the antibiotic unable to bind the targets properly. β-lactams target the bacterial en‐ zymes of cell wall biosynthesis (the so-called penicillin-binding proteins, PBPs). Alterations in PBPs can reduce affinity for β-lactams, possibly causing β-lactam resistance in many bac‐ teria strains, such as H. influenzae, N. gonorrhoeae, N. meningitidis, anaerobes, S. dysenter‐ iae [40]. For instance, the mecA resistance gene which encodes PBP2a, a new penicillin binding protein with decreased affinity for oxacillin and most other β-lactam drugs, induces resistance to methicillin and oxacillin in S. aureus [41]. The resistance to antibiotics that in‐ terfere with protein synthesis or transcription is achieved by modification of the specific tar‐ get. rRNAmethylases encoded by a number of genes modificate the 16S rRNA molecule at specific positions critical for aminoglycosides binding [42]. Modification in the 23S rRNA component of the 50S ribosomal subunit also leads to resistance to the macrolide, lincosa‐ mide and streptogramin B group of antibiotics in many pathogen strains [43; 44]. Mutations of topoisomerase IV and gyrase genes can sufficiently alter affinity of fluoroquinolones to

The Antibacterial Drug Discovery http://dx.doi.org/10.5772/52510 295

*Efflux pumps* Membrane proteins that export antibiotics from the cell and maintain their low intracellular concentrations are called efflux pumps. Drug efflux pumps play a key role in drug resistance not just because they can produce multidrug resistance but also because they can elevate level of other resistance mechanisms [46; 47]. Bacterial drug efflux trans‐ porters are currently classified into five families: the ATP-binding cassette (ABC) superfami‐ ly, the major facilitator superfamily (MFS), the multidrug and toxic compound extrusion (MATE) family, the small multidrug resistance (SMR) family, and the resistance-nodulation‐ division (RND) superfamily [47]. Efflux transporters can be further classified into single or multicomponent pumps. Tetracycline and macrolide transporters are single component ef‐ flux systems that have narrow substrate profiles, while the RND family members have broader substrates and can pump out multiple structurally unrelated compounds [24; 46]. Efflux pumps exist in both Gram-positive and Gram-negative bacteria [48; 49]. MexAB-OprM efflux pumps in Pseudomonas aeruginosa, which belong to RND family, result in higher inhibitory concentration of a large number of antibiotics, such as penicillins, broad‐ spectrumcephalosporins, chloramphenicol, fluoroquinolones, macrolides, novobiocin, sulfo‐

*OM permeability* The OM is an asymmetric bilayer: the phospholipid form the inner leaflet and the lipopolysaccharides (LPS) form the outer leaflet. OM of Gram-negative bacteria pro‐ vides a formidable barrier that must be overcome by drugs. Drug molecules pass the OM by diffusion through porins or the bilayer, or by self-promoted uptake [85]. Small hydrophilic drugs (e.g., β-lactams), enter to the intracellular through the pore-forming porins, while macrolides and other hydrophobic drugs diffuse during their entry [52]. Some resistant clin‐ ical strains of Neisseria meningitidis, K. pneumoniae and Enterobacteraerogenes exhibit a noticeable porin variability resulting in decrease of antibacterial uptake [53]. Reduction of
