Bacteria use several mechanisms to become resistant to antibiotics. They can also transfer resistance genes to other bacteria through plasmids and transposons. This spread contributes to the high prevalence of multidrug resistance, which is a major clinical problem. Antibiotic resistance can be high level or low level.
High level resistance: Antibiotic resistance that can’t be overcome by increasing the antibiotic dose. For example, Neisseria gonorrhoeae can become penicillin-resistant by producing penicillinases. In this situation, you need to treat with an antibiotic from a different class.
Low level resistance: Antibiotic resistance that can be overcome by increasing the antibiotic dose. For example, strains of Neisseria gonorrhoeae can become resistant to penicillin by altering the structure of penicillin-binding proteins. This may be overcome by increasing the dose of penicillin.
Following are the mechanisms of antibiotic resistance:
Drugs can enter the bacterial cell by diffusion through porins, diffusion through the bilayer, and by self uptake.
Porins are channels in the outer membrane of gram-negative bacteria. Beta-lactams and quinolones use porins to enter the bacterial cell. Bacteria can reduce the number of porins, which decreases antibiotic entry and leads to resistance.
Pseudomonas aeruginosa can lower the permeability of its outer membrane and acquire resistance to antibiotics.
Efflux pumps are located in the cytoplasmic membrane. They pump antibiotics out of the cell, lowering the intracellular drug concentration. Most efflux pumps are multidrug transporters, meaning they can pump out many different antibiotics and are a major mechanism of multidrug resistance. Antibiotics of all classes except polymyxin are susceptible to efflux pumps.
If the target site changes, the antibiotic can’t bind effectively. This is often acquired through spontaneous mutation of a bacterial gene on a chromosome. Examples include:
Alteration in 30S or 50S ribosome subunit: Antibiotics that bind to ribosomes can’t bind to their targets because of structural changes in the subunit. This is seen with drugs acting on the 30S and 50S subunits, such as aminoglycosides, macrolides, streptogramins, lincosamides, chloramphenicol, etc.
Alteration in penicillin binding proteins: This is seen in resistance to beta-lactam antibiotics. In S. aureus, the gene mec A codes for an altered PBP called PBP 2a. Beta-lactam antibiotics can’t bind to PBP 2a, and the bacteria becomes resistant to beta-lactams.
Altered cell wall precursors: This plays a role in resistance to vancomycin and teicoplanin. Normally, these drugs bind to D-Alanine-D-Alanine. Gram-positive bacteria can change this to D-Alanine-D-lactate, which prevents antibiotic binding and confers resistance.
Alterations in DNA gyrase/Topoisomerase IV: This is a mechanism of fluoroquinolone resistance because DNA gyrase/topoisomerase IV is the normal binding site. Mutations in gyr A and gyr B lead to alterations in DNA gyrase, while mutations in par C and par E lead to altered topoisomerase IV.
Tetracycline resistance: This can occur by protecting the tetracycline binding site on the ribosome using specific cytoplasmic proteins, and by modifying the 16S rRNA at the tetracycline binding site. Tet M and Tet O are important genes involved in resistance.
Rifampin resistance: This is acquired by a missense mutation in a region encoding the rifampicin binding site on the beta subunit of RNA polymerase (rpoB).
The table below summarizes the resistance mechanisms acquired by alteration of the target sites.
| Altered target | Characteristics of Resistance |
|---|---|
| Altered 30S and 50S rRNA | 50S: Chloramphenicol, Oxazolidinones, Macrolides , Lincosamides and Steptogramins. 30S: Aminoglycosides and Tetracyclines |
| Altered PBP | E.faecium to ampicillin; S pneumoniae to penicillin; MRSA |
| Altered cell wall precursors | Vancomycin and teicoplanin; Van A gene for high level resistance to vancomycin and teicoplanin by E. faecium and E. faecalis; Van B and Van C genes for resistance only to vancomycin. |
| Altered DNA gyrase or Topoisomerase IV | Fluoroquinolones; gyr A, gyr B, par C , par E genes. |
| Altered ribosomal binding site | Tetracyclines |
| Altered RNA Polymerase | Rifampin |
Bacteria can produce enzymes that chemically inactivate antibiotics.
Beta Lactamases: These enzymes hydrolyze the beta-lactam ring of penicillins, cephalosporins, monobactams, and carbapenems, making them inactive. A special class of beta-lactamases called ESBLs (extended spectrum beta lactamases) are produced by many enteric gram-negative bacteria such as E.coli, Klebsiella pneumoniae, Enterobacter, Proteus etc. This makes them resistant to all cephalosporins.
Beta lactamases are classified as follows.
| Class | Description |
|---|---|
| Class A (Penicillinases) | e.g. TEM-1, SHV-1, ESBL. They are inhibited by clavulanic acid, sulbactam, tazobactam. ESBLs are resistant to penicillins, third generation cephalosporins, aztreonam, cefoperazone but are sensitive to cephamycins and carbapenem. |
| Class B (metallo beta lactamases) | Require zinc, heavy metals. Not inhibited by clavulanic acid, sulbactam, aztreonam and carbapenems. |
| Class C (cephalosporinases) | e.g. Amp C. Produced by all gram negative bacteria except Salmonella and Klebsiella. Resistant to all beta lactams except carbapenems, not inhibited by clavulanic acid. |
| Class D (oxacillin hydrolyzing enzymes) | Resistant to Oxacillin, Cloxacillin, Penicillin and Methicillin. Only weakly inhibited by clavulanic acid. |
Aminoglycoside modifying enzymes: These include phosphoryl transferases, nucleotidyl transferases, and adenylyl transferases, which inhibit the action of aminoglycoside antibiotics. This is seen in E.faecalis, S.aureus, S. pneumoniae.
Chloramphenicol acetyl transferases: This enzyme acetylates chloramphenicol so that it can’t bind to the 50S ribosomal subunit. It is produced by a few gram-positive and gram-negative bacteria, including H. influenzae.
Minimum inhibitory concentrations are defined as the lowest concentration of an antimicrobial that will inhibit the visible growth of a microorganism after overnight incubation.
Minimum bactericidal concentrations are defined as the lowest concentration of antimicrobial that will prevent the growth of an organism after subculture on antibiotic-free media.
MICs are used by clinical laboratories to determine antibiotic resistance and for research purposes to study antimicrobials. MBC is mainly used to determine the correct bactericidal dose of antibiotics to effectively treat bacterial endocarditis.
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