Mechanism of antibacterial resistance in bacteria

Bacteria employ various mechanisms to acquire resistance to antibiotics. Not only do they themselves become resistant but they can transfer this characteristic to other species of bacteria via plasmids and transposons leading to high prevalence of multi drug resistance which is a menacing problem of this era. Antibacterial resistance can be “high level” or “low level”.

High Level Resistance: When antibiotic resistance cannot be overcome by increasing the dose of the antibiotic e.g. Neisseria gonorrhoeae becoming Penicillin resistant due to production of penicillinases. In such cases, another antibiotic from a different class has to be given to treat the infection.

Low Level Resistance: This type of antibiotic resistance can be overcome by increasing the dose of the antibiotic e.g. strains of Neisseria gonorrhoeae becoming resistant to penicillin by altering the structure of penicillin binding protein. That can be overcome by increasing the dose of penicillin.

Following are the mechanisms of antibiotic resistance:

Decreased uptake or efflux of antibiotic resulting in reduced intracellular concentrations of the antibiotic

1. Drugs can enter the bacterial cell by diffusion through porins, diffusion through the bilayer and by self uptake.

   Porins are channels located 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 decreasing the entry of these antibiotics and hence becoming resistant to them.

   Pseudomonas aeruginosa lowers the permeability of the outer membrane and acquires resistance to antibiotics.

2. Efflux pumps are located in the cytoplasmic membrane. They pump out antibiotics out of the cell. Most efflux pumps are multidrug transporters i.e. they can pump out many kinds of antibiotics and are a major mechanism of multidrug resistance. Antibiotics of all classes except polymyxin are susceptible to the action of efflux pumps.

Modification of target molecule

Alterations to the target site prevents binding of the antibiotic. This is acquired through spontaneous mutation of a bacterial gene on a chromosome. Below are the examples.

1. Alteration in 30S or 50S ribosome subunit: Antibiotics that bind to ribosomes cannot bind to their targets because of structural change in the subunit. Seen in drugs acting on 30S and 50S subunits like Aminoglycosides, Macrolides, Streptogramins, Lincosamides, Chloramphenicol etc.

2. 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 cannot bind to PBP 2a and the bacteria becomes resistant to beta lactams.

3. Altered cell wall precursors: This plays a role in bacterial resistance to vancomycin and teicoplanin. Normally these drugs bind to D-Alanine-D-Alanine. Gram positive bacteria can change that to D-Alanine-D-lactate to which antibiotics cannot bind hence conferring resistance.

4. Alterations in DNA gyrase/Topoisomerase IV: This is a mechanism of fluoroquinolone resistance as DNA gyrase/ Topoisomerase IV is the normal binding site for them. Mutations in gyr A and gyr B code for alterations in DNA gyrase while mutations in par C and par E code for altered topoisomerase IV.

5. Tetracycline resistance: This is acquired by protection of the tetracycline binding site on the ribosome by specific cytoplasmic proteins and by modifying the 16S rRNA at the tetracycline binding site. Tet M and Tet O are the important genes involved in resistance.

6. 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.

Enzymatic inactivation of antibiotic

Various enzymes are produced by bacteria to do this.

1. 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 like E.coli, Klebsiella pneumoniae, Enterobacter, Proteus etc which makes them resistant to all cephalosporins.

   Beta lactamases are classified as follows.

2. Aminoglycoside modifying enzymes: These are phosphoryl transferases, nucleotidyl transferases, adenylyl transferases which inhibit the action of aminoglycoside antibiotics. Seen in E.faecalis, S.aureus, S. pneumoniae.

3. Chloramphenicol acetyl transferases: It acetylates chloramphenicol so that it cannot bind to the 50S ribosomal subunit. Produced by a few gram positive and gram negative bacteria including H. influenzae.

What is MIC (Minimum Inhibitory Concentration)?

Minimum inhibitory concentrations are defined as the lowest concentration of an antimicrobial that will inhibit the visible growth of a microorganism after overnight incubation.

What is MBC (Minimum Bactericidal Concentration)?

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. Whereas MBC is mainly used to determine the correct bactericidal dose of antibiotics to effectively treat bacterial endocarditis.