**2.4 Inhibition of microbial DNA synthesis**

*Pseudomonas aeruginosa* - Biofilm Formation, Infections and Treatments

polymixin B uptake ("self-promoted" uptake) [31].

**2.3 Inhibition of microbial protein synthesis**

certain stages of the cycle.

The mode of action of certain antimicrobial agents may be due to the ability of such medicines to increase membrane permeability, making it easier for them and other compounds to penetrate. Antibacterial cationic agents, increased permeability of the outer membrane to the lysozyme and hydrophobic compounds has been identified, such as polymyxin B. The initial function of these antimicrobial agents is to interrupt the structure of the outer membrane, allowing the cell to join itself and other compounds and inhibit unique metabolic processes [30]. There are several cell-damaging properties of Polymixin B: (i) the surface charge, lipid composition and membrane structure are disturbed; (ii) the K+ gradient on the cytoplasmic membrane is dissipated; and (iii) the cytoplasmic membrane is depolarized. One of the key factors regulating bacterial exposure to polymixin B is the permeability of the external membrane to lipophilic compounds. Since polymixin B is bulkier than its displacement of inorganic divalent cations, in the presence of polymixin B, the packing order of lipopolysaccharides (LPS) is changed. This results in increased permeability of a variety of molecules to the outer membrane and also promotes

Microbial protein synthesis inhibition a range of groups of antimicrobial agents work by inhibiting the synthesis of bacterial proteins (ribosome function). That include aminoglycosides, macrolides, tetracyclines, ketolides, lincosamides, streptogramins, chloramphenicol and oxazolidinones [26, 32]. The synthesis of microbial proteins is led by ribosomes in conjunction with cytoplasmic factors which, during the initiation phase, elongation phase and termination phases, bind transiently to particles. Microbial ribosomes contain 70S particles consisting of two 50S and 30S subunits, which join at the initiation stage of the synthesis of proteins and split at the termination stage. In bacterial protein synthesis, antimicrobial agents block various steps by interfering with the work of either the cytoplasmic factors or the ribosomes. Inhibitors which bind to the ribosomal subunit of 30S primarily interfere with initiation, although some interfere with the pairing of the AA- tRNA anticodon with the mRNA codon, elongation is thus impaired. The steps involved in the elongation process interact with inhibitors that bind to the 50S ribosomal subunit or to elongation factors that are transiently connected to ribosomes at

Through binding to particular ribosomal subunits [33], aminoglycosides function. By inducing the development of aberrant, non-functional complexes as well as causing misreading, aminoglycoside-type drugs may combine with other binding sites on 30S ribosomes and destroy bacteria. Spectinomycin is an antimicrobial agent that is closely linked to the aminoglycosides of aminocylitol. It binds and is bacteriostatic but not bactericidal to a particular protein in the ribosome. Tetracyclines are other agents which bind to 30S ribosomes. These agents tend to inhibit aminoacyl tRNA binding to the A site of the bacterial ribosome. Tetracycline binding is temporary, so it's bacteriostatic for these agents. Nevertheless, a wide range of bacteria, *chlamydias* and *mycoplasmas* are inhibited and highly helpful agents [29]. There are three major groups of medicines that inhibit the ribosomal subunit of 50S. A bacteriostatic agent that inhibits both gram-positive and gram-negative bacteria is chloramphenicol. By binding to a peptidyltransferase enzyme on the 50S ribosome, it prevents peptide bond formation. Macrolides are large compounds of the lactone ring that bind to 50S ribosomes and tend to impair the reaction or translocation of peptidyltransferase, or both. Erythromycin, which inhibits gram-positive species and a few gram-negative species, such as *haemophilus, mycoplasma, chlamydia* and

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The modulation of chromosomal supercoiling by topoisomerase-catalyzed strand breakage and rejoining reactions is needed for DNA synthesis, mRNA transcription and cell division [34]. Depending on whether they catalyze reactions involving transient breakage of one (type I) or both (type II) strands of DNA, DNA topoisomerase enzymes are classified into two groups, I and II [35]. The topological state of DNA inside cells is regulated by topoisomerases and is important for the vital processes of protein translation and cell replication. The enzyme that negatively super-coils DNA in the presence of ATP is DNA gyrase, a type II DNA topoisomerase [36]. Moreover, in the absence of ATP, this enzyme plays a role in the catenation and decatenation reaction of a double-stranded DNA circle, resolves knots in DNA, and also relaxes supercoiled DNA negatively. As a result, for almost all cellular procedures involving duplex DNA, including replication, recombination and transcription, the enzyme is vital. It is unique to the prokaryotic kingdom and is essential to the organism's survival. Thus, for antibacterial drugs, DNA gyrase remains an ideal and attractive target. The most effective DNA gyrase-targeted antimicrobial agents are quinolones. Nalidixic acid, a naphthyridone inadvertently discovered as a by-product during chloroquine synthesis, was the source of the compounds [37].

Quinolones are unique DNA-gyrase inhibitors. DNA gyrase reactions such as supercoiling and relaxation involving DNA breakage and reunion are inhibited by quinolones, specifically interfering with the DNA gyrase breakage-reunion reaction by interacting with subunit A (GyrA) [38]. Relatively poor antimicrobial activity is found in first-generation quinolones, nalidixic acid and oxolinic acid. However, the synthesis and improvement over many generations of fluoroquinolones, such as norfloxacin and ciprofloxacin (second generation), levofloxacin (third generation), and moxifloxacin and gemifloxacin (fourth generation), has resulted in a variety of potent antimicrobial agents [38]. Most bacterial pathogens possess an additional essential topoisomerase, topoisomerase I (Topo I), in addition to the type II topoisomerases. Topo I is architecturally and mechanistically distinct from gyrase and topoisomerase IV, and is an attractive candidate for new antibacterial chemotypes to be discovered as such [36].
