Table of Contents
Clinically effective antimicrobial agents exhibit selective toxicity towards the microbe rather than the host, a feature which distinguishes them from the disinfectants. Selectivity is described in most cases by action on microbial processes or structures which differ from those of mammalian cells. For eg, some agents operate on synthesis of bacterial cell walls (an organelle that is not present in eukaryotes), and others on the bacterial ribosome of 70 S (but not the eukaryotic ribosome of 80 S). Such antimicrobials are basically nontoxic to the host, such as penicillin, until hypersensitivity occurs. For some, such as the aminoglycosides, the optimal therapeutic dose is fairly similar to the dangerous dose, resulting in much more reliable monitoring of dosage and blood pressure.
Antibiotics—Antimicrobials of microbial origin, the majority of which are formed by fungi or Streptomyces bacteria.
Antimicrobials—In the case of infectious diseases, this means that the medication is not an antibiotic in the strict sense of originating from a germ or virus, but it is often used in the treatment of pathogens.
Bactericidal—Antimicrobial action that kills bacteria as well as inhibits their development.
Bacteriostatic—Antimicrobial activity inhibits cell development but does not kill them. The mechanisms of host protection are primarily responsible for the infection’s eradication.
Minimal inhibitory concentration (MIC)—The lowest concentration (μg/mL) that can stop an in vitro microorganism from growing.
Resistant, nonsusceptible—When an antimicrobial agent does not destroy cells in clinically appropriate concentrations, this is referred to as resistance.
Sensitive, susceptible—Term applied to microorganisms implying that clinically achievable antimicrobial amounts will suppress them.
Spectrum—A list of the normally active microorganism classes against which an antimicrobial is active. Only a few species have action against a narrow-spectrum target. A broad-spectrum agent is effective against a wide variety of pathogens (e.g., Gram-positive and Gram-negative bacteria).
SELECTED ANTIBACTERIAL AGENTS
In clinical medicine, there are approximately 23 distinct classes and 18 subclasses of clinically effective antibiotics, totaling about 100 antibiotics. Though the classification scheme and number of antibiotics are complicated and continue to evolve as new classes emerge or existing classes are changed, their action mechanisms target bacterial cell wall biosynthesis, folate synthesis, DNA replication, RNA transcription, and mRNA translation. There are rational targets that are essential to the microorganism’s survival that are distinct enough from eukaryotic cells to allow for selective toxicity. Understanding the processes of antibiotic action, then, provides insight into the strategies used by microorganisms to avoid their harmful impact.
The good antimicrobial agent has differential toxicity, which means it may be harmful to pathogens but not causing harm to the host. Selective toxicity is often contextual rather than absolute, implying that a compound at a host-tolerable dose may damage an infecting microorganism. Selective toxicity may be the result of a certain receptor necessary for drug adhesion, or it may be the result of inhibiting biochemical events relevant to the pathogen but not to the host. The modes of action of antimicrobial drugs are divided into four categories:
- Inhibition of cell wall synthesis
- Inhibition of cell membrane function
- Inhibition of protein synthesis
- Inhibition of nucleic acid synthesis
INHIBITION OF CELL WALL SYNTHESIS
Most antibacterial agents function by targeting for synthesis of bacterial cell walls. Cell walls are not present in mammalian cells, which vary in composition between various types of bacteria. Cell wall synthesis thus offers a range of alternative therapeutic opportunities for anti-infective pharmaceutical products.
The cell wall includes a chemically distinct complex polymer “mucopeptide” (“peptidoglycan”) composed of polysaccharides and a polypeptide with high cross-linkage. Th e polysaccharides include the N-acetylglucosamine and acetylmuramic acid amino sugars daily. The first can only be found on bacteria. Tiny peptide chains are affixed to the amino sugars. The cell wall ‘s final rigidity is imparted by cross-linking the peptide chains (e.g., by pentaglycine bonds) as a result of several enzyme transpeptide reactions. The coating of peptidoglycan in the gram-positive cell wall is much thicker than that of gram-negative bacteria.
Cell wall biosynthesis inhibitors, β-lactams, and glycopeptides are the most widely used. β-Lactam antibiotics, such as penems, cephems, carbapenems and monobactams, act by binding penicillin-binding proteins (PBPs), which are bifunctional transpeptidase-transglycosylase enzymes that mediate cross-linking of peptidoglycans.
INHIBITION OF CELL MEMBRANE FUNCTION
The cytoplasms of all living cells are bounded by the cytoplasmic membrane which serves as a selective barrier to permeability and performs active transport functions and thus regulates the cell ‘s internal composition. When the cytoplasmic membrane ‘s functional integrity is compromised, macromolecules and ions evaporate from the cell , resulting in cell injury or death. The cytoplasmic membrane of bacteria and fungi has a distinct composition to that of animal cells and can be more quickly penetrated by other agents. The polymyxins consist of detergent-like cyclic peptides that are a major component of bacterial membrane, selectively destroying membranes containing phosphatidylethanolamine.
Daptomycin is a modern lipopeptide antibiotic, which is easily bactericidal after bound ion-dependent on the cell membrane, and allows bacterial membrane potential to be depolarised. This helps in intracellular production of potassium. Currently, this agent is approved for use in the treatment of S aureus bloodstream infections and skin and soft tissue infections caused by gram-positive bacteria , particularly organisms that are highly resistant to β-lactam agents and vancomycin. Others examples are amphotericin B, colistin and imidazoles and triazoles as agents acting by inhibition of the cell membrane structure.
INHIBITION OF PROTEIN SYNTHESIS
It is established that Macrolides, Lincosamides, Tetracyclines, Glycylcyclines, Aminoglycosides, and Chloramphenicol can inhibit protein synthesis in bacteria. These types of drugs have different modes of action.
There are 70S ribosomes in bacteria and 80S Ribosomes in mammalian cells. The sub-units of a ribosome type, its chemical composition and functional properties vary adequately so as to understand why antimicrobial drugs in bacterial ribosomes can inhibit protein synthesis without significant effect on mammalian ribosomes. In natural microbial protein synthesis, multiple ribosomes extending around the mRNA strand “read” the mRNA message at the same time.
Aminoglycosides are active in a wide range of bacteria, but only those organisms which can transport them through an oxidative phosphorylation mechanism into cells. Streptomycin, the original aminoglycoside, is bound to the 30S ribosomal subunit, but the younger and more involved aminoglycosides are associated at several locations, both 30S and 50S. This offers a wider range and less tolerance to younger agents because of the binding mutation in the cell.
The Tetracyclines block the attachment of amino-acyl-tRNA to the acceptor site on the mRNA ribosome complex by binding to the 30S ribosome subunit. Contrary to the aminoglycosides, they are not bacteriostatic but reversible.
The nitrobenzene structure of the Chloramphenicol ring can be mass produced through the production of chemical synthesis. The effect of this influence is to bind to the 50S ribosomal subunit and block peptidyl transferase action, which prevents peptide bonds which are essential for the extension of the peptide chain. In most vulnerable animals, the behaviors are reversible and thus bacteriostatic.
The macrolides; erythromycin, azithromycin, and clarithromycin, differ in their composition from a large ring structure with 14 or 15 members. They influence ribosomal protein synthesis by binding to the subunit 50S and blocking the translocation reaction. They mainly have a bacteriostatic influence.
INHIBITION OF NUCLEIC ACID SYNTHESIS
Examples of drugs acting by inhibition of nucleic acid synthesis are the quinolones, pyrimethamine, rifampin, sulfonamides, trimethoprim, and trimetrexate.
The DNA gyrase and topoisomerase IV are the targets of the Quinolones, the enzymes responsible for nicking, supercoiling, and sealing bacterial DNA during replication. Levofloxacin and ciprofloxacin are primarily excreted by the kidney, which results in high concentrations of drugs in the urine, making them suitable for the treatment of many urinary tract infections.
Sulfonamides are structural analogs of PABA and compete with it for the enzyme (dihydropteroate synthetase), which in the initial stage of folate synthesis incorporates PABA and pteridin. This blockage has multiple effects on the bacterial cells; the most significant of these is nucleic acid synthesis disruption.
Trimethoprim works on the pathway of folate synthesis but after sulfonamides after a point. It competitively inhibits bacterial dihydrofolate reductase activity which catalyzes folate conversion to its reduced active coenzyme form. When paired with sulfamethoxazole, a sulfonamide, trimethoprim contributes to the folate pathway being blocked in two stages, frequently resulting in synergistic bacteriostatic or bactericidal action.
Rifampin binds to the β-subunit of DNA-dependent polymerase RNA which prevents RNA synthesis from initiating. This agent is active against most Gram-positive bacteria and selected Gram-negative species, including, but not Enterobacteriaceae members, Neisseria and Haemophilus.
Antibiotic Targets and Pathways
|Drug Type||Drug Name||Species Range||Primary|
|Fluoroquinolones (DNA Synthesis inhibitor)||Nalidixic Acid, Ciprofloxacin, Ofloxacin, Levofloxacin, Moxifloxacin||Aerobic Gram-positive and gram-negative species, some anaerobic gram-negative species and Mycobacterium||Topoisomerase-II (DNA gyrase) Topoisomerase-IV||DNA replication, SOS response, cell division, ATP generation, TCA cycle, Fe-S cluster synthesis, ROS formation, and envelope and redox-responsive two-component systems|
|Trimethoprim-sulfamethoxazole (DNA synthesis Inhibitor)||Co-Trimoxazole (Combination Of Trimethoprim And Sulfamethoxazole In A 1:5 Ratio)||Aerobic gram-positive and gram-negative species||Tetrahydrofolic acid synthesis inhibitors||Nucleotide biosynthesis and DNA replication|
|Rifamycins (RNA synthesis inhibitors)||Rifamycins, Rifampin, Rifapentine||Gram-positive and gram-negative species and Mycobacteria||DNA-dependent RNA polymerase||RNA transcription, DNA replication and SOS response|
|Beta-Lactams (Cell Wall Synthesis)||Penicillins, Ampicillin, Cephalosporins, Carbapenems||Aerobic and anaerobic gram-positive and gram-negative species||Penicillin-binding proteins.||Cell wall synthesis, cell division, autolysin activity, SOS response, TCA cycle, Fe-S cluster synthesis, ROS formulation and envelope and redox-responsive two component systems|
|Glycopeptides and Glycolipopeptides (Cell wall synthesis inhibitors)||Vancomycin, Teicoplanin||Gram-positive species||Peptidoglycan units (terminal D-Ala-D-Ala dipeptide)||Cell wall synthesis, transglycosylation, transpeptidation, and autolysin activation (VncRS two-component system)|
|Lipopeptides (Cell wall synthesis Inhibitors)||Daptomycin, Polymixin B||Gram-positive species (daptomycin), gram-negative species (polymixins)||Cell membrane||Cell wall synthesis and envelope twocomponent systems|
|Aminoglycosides (Protein synthesis Inhibitors)||Gentamicin, Tobramycin, Streptomycin, Kanamycin||Aerobic gram-positive and gram-negative species, and M. tuberculosis||30S ribosome||Protein translation (mistranslation by tRNA mismatching), ETC, SOS response, TCA cycle, Fe-S cluster synthesis, ROS formation, and envelope and redox-responsive two-component systems|
|Tetracyclines (Protein synthesis inhibitors)||Tetracycline, Doxycycline||Aerobic gram-positive and gram-negative species||30S ribosome||Protein translation (through inhibition of aminoacyl-tRNA binding to ribosome)|
|Macrolides (Protein synthesis Inhibitors)||Erythromycin, Azythromycin||Aerobic and anaerobic gram-positive and gram-negative species||50S ribosome||Protein translation (through inhibition of elongation and translocation steps) and free tRNA depletion|
|Streptogramins (Protein synthesis Inhibitors)||Pristinamycin, Dalfopristin, Quinupristin||Aerobic and anaerobic gram-positive and gram-negative species†||50S ribosome||Protein translation (through inhibition of initiation, elongation, and translocation steps) and free tRNA depletion|
|Phenicols (Protein synthesis Inhibitor)||Chloramphenicol||Some gram-positive and gram-negative species, including B. fragilis, N. meningitidis, H. influenzae, and S. pneumoniae||50S ribosome||Protein translation (through inhibition of elongation step)|
- Textbook of Diagnostic Microbiology 5th Edition. Saunders Elsevier
- Jawetz, Melnick and Adelberg’s Medical Microbiology 26th Edition
- Sherris Medical Microbiology 6th Edition
- Bailey and Scott’s Diagnostic Microbiology 13th Edition