Target Modification. In general, antibiotics have specific targets to which they bind to inhibit cellular processes essential for bacterial growth or survival. Mutants in target proteins that modify the antibiotic binding site or sterically hinder antibiotic binding can confer resistance to these toxic compounds. Importantly, these mutations must disrupt the antibiotic binding site without drastically affecting function of the target protein, or growth or fitness of the bacterium may suffer. Mutations that effectively confer resistance without a significant growth or fitness cost may persist in a bacterial population in the absence of antibiotic pressure. Resistance mutations that significantly impact the activity of the target protein and slow growth or diminish general fitness may be lost from the population in the absence of antibiotic selection, or may lead to the development of compensatory mutations that restore growth rate and fitness (discussed in Chapter 2). Mutations in the PBPs of N. gonorrhoeae have been shown to contribute to penicillin resistance. The xxxX xxxx, which encodes PBP2, may contain an addition of an aspartic acid residue at position 345, which is just downstream of the penicillin acylation site, and 4 to 8 substitutions in the carboxyl terminal region of the protein, resulting in a slower acylation rate by penicillin (30). This xxxX variant arose from the acquisition of segments of xxxX from commensal Neisseria species with PBP2s demonstrating decreased acylation rates by penicillin (29, 233). The mosaic xxxX allele decreases susceptibility of gonococci harboring this mutation (231). Additionally, the xxxX xxxx, which encodes PBP1, may contain a T to C transition mutation that results in the substitution of proline for leucine at position 421, lowering the affinity for penicillin and decreasing the acylation rate (208). Neither the xxxX nor ponA mutations are sufficient to confer clinically significant levels of penicillin resistance alone, and require the presence of penB and mtrR mutations to impart an altered resistance phenotype on gonococcal cells (78, 208). Macrolide antibiotics, such as erythromycin, inhibit protein synthesis by binding to the 23S rRNA of the 50S subunit of the ribosome causing the release of incomplete peptides. Bacteria can resist macrolides by methylation of an adenosine in the peptidyl transferase domain of the 23S rRNA, which interferes with macrolide binding to the target site. Methylation is carried out by enzymes known ...
Target Modification. Antibiotics act to either kill or inhibit the growth of bacteria by targeting and disrupting a specific cellular function or process. Bacteria utilize target modification as a tool to combat the action of an antibiotic. Mutations that arise and result in subtle changes to specific cellular targets of the antibiotic can render the bacteria resistant to an antimicrobial agent. For example as mentioned earlier, β-lactams such as penicillin disrupt cell wall synthesis by covalently binding to penicillin-binding proteins (PBPs), which results in irregularities in peptidoglycan synthesis (208). However, a T to C transition mutation in ponA, encoding PBP1, leads to a substitution of proline for leucine at position 421 thus lowering the affinity to penicillin (162). Additionally, the xxxX xxxx encodes altered forms of PBP2 that contain an insertion of an additional aspartic acid residue at position 345, resulting in acylation by penicillin that is reduced by 5-10 fold (185). Moreover, a mosaic-like xxxX xxxx isolated from male patients with urethritis conferred reduced susceptibility to cefixime. The mosaic-like xxxX xxxx harbors a transpeptidase-encoding domain similar to that of xxxX genes in Neisseria perflava (N. sicca), Neisseria cinerea, Neisseria flavescens, and Neisseria meningitidis. Thus the exchange of genetic material between Neisseria gonorrhoeae and other Neisserial species can occur resulting in the acquisition of mosaic-like genes, such as xxxX, that can contribute to changes in susceptibility to antibiotics used to treat gonorrhea (6, 110, 116). The antibiotic tetracycline acts to kill a bacterial cell by inhibiting protein synthesis. Specifically, the drug inhibits the binding of amino-acyl tRNA to the mRNA- ribosome complex. In the gonococcus, tetracycline resistance can be acquired from the plasmid-mediated tetM determinant whose product functions by mimicking translation elongation factors thus preventing the antibiotic from binding to the ribosome or promoting release of the bound antibiotic (29, 138, 160, 169). High-level chromosomal tetracycline resistance can occur due to a Val-57-Met point mutation in the rpsJ gene that encodes the ribosomal protein S10, thus altering the affinity of tetracycline for the ribosome (87). Fluoroquinolones function by disrupting DNA replication in the gonococcus. Mutations in gyrA and parC, both of which are responsible for DNA maintenance, result in high levels of resistance to the fluoroquinolone ciprofloxa...
