The antimicrobials produce their bactericidal or bacteriostatic effects by targeting at certain sites of the pathogens which are susceptible to the antibiotics. Generally, the mechanisms of all antibiotics can be divided into five basic types, or four targets toward which these antibiotics act, including: cell wall synthesis, protein synthesis, nucleic acid synthesis, folate biosynthesis, and cell membrane integrity.
Antibiotics Targeting at Cell Wall Synthesis
Antimicrobials that act on cell wall synthesis can be divided into β-lactams including penicillins, cephalosporins, carbapenems, and monobactams, and non-β-lactams including vancomycin, teicoplanin, telavancin, and bacitracin. The β-lactams antibacterial agents interfere with the transpeptidation reactions that seal the peptide cross links between glycan chains. They do so by interference with the action of the transpeptidase enzymes which carry out this cross-linking. These transpeptidase enzymes we call it in short penicillin-binding proteins (PBPs). Among different species, or even within one strain, the PBPs might have distinctions and vary in their avidity of binding to different β-lactam drugs.
The subtype of β-lactams are based on the chemical structure, the β-lactam ring, which is essential for antibacterial activity. Monobactams have a single β-lactam ring, penicillins and carbapenems have a β-lactam ring fused to a five-member thiazolidine penem ring, and cephalosporins have a β-lactam ring combined with a six-member dihydrothiazine cep hem ring. These differences on the structure affect the pharmacologic properties and spectrum of the specific drug. In general, β-lactam antimicrobials are highly bactericidal, but only to growing bacteria synthesizing new cell walls. One of the key factor that deterring the successful pharmacologic and physical effects of β-lactam is that the drug must penetrate or be transported across the outer membrane of the microbes to get in contact with its receptor, the PBPs.
For instance, penicillin G is active against certain Gram-positive organisms, Gram-negative cocci, and some spirochetes, but lack the activity against Gram-negative bacilli, as the outer membrane of these Gram-negative bacilli prevents passage of penicillin G to the site of action on cell wall synthesis.
For cephalosporins, a agent of a higher generation has a wider spectrum, and in some instances, more quantitative activity (lower MIC) against Gram-negative bacteria. Meanwhile as the Gram-negative spectrum increases, the drug typically loses some of their potency (higher MIC) against Gram-positive bacteria. Note that today there is a fifth-generation cephalosporins, ceftaroline, yes, those guys made it.
These inhibitors are capable of binding irreversibly to β-lactamase enzymes and, in the process, rendering them inactive. There are three β-lactamase inhibitors available, including clavulanic acid, sulbactam, and tazobactam. They also be referred to as suicide inhibitors since they must first be hydrolyzed by a β-lactamase before becoming effective inactivators of the enzyme. Therefore, for infections without β-lactamases, β-lactamase inhibitors are not able to enhance the bactericidal effect.
Another type of antimicrobials act against cell wall synthesis is glycopeptides. Each of these antimicrobials inhibits assembly of the linear peptidoglycan molecule by binding directly to the terminal amino acids of the peptide side chains (see the figure above). This effect is the same as with β-lactams: prevention of peptidoglycan cross-linking. Now there are three drug available, including vancomycin, teicoplanin, and telavancin (has additional theoretical advantage of cell membrane activity). Both of vancomycin and teicoplanin are bactericidal, but primarily only against Gram-positive bacteria. Because the both drugs are not absorbed by mouth, they could be used to treat CDI via oral route.
Inhibitors of Protein Synthesis
A variety of antibiotics are within this category of antimicrobials, including aminoglycosides, tetracyclines, chloramphenicol, macrolides, clindamycin, oxazolidinones, and streptogramins.
Aminoglycosides (30S subunit)
Amino glycosides are bactericidal drugs especially useful against many gram-negative rods. Certain ahminoglycosides are used against other organisms (e.g., streptomycin is used in the multi drug therapy of tuberculosis, and gentamicin is used in combination with penicillin G against enterococci). Amino glycosides are named for the amino sugar component of the molecule, which is connected by a glycosidic linkage to other sugar derivatives.
Both inhibition of the initiation complex and misreading of messenger RNA (mRNA) occur, the mechanisms for this class of drug to act, where the former is probably more important for the bactericidal activity of the drug. An initiation complex composed of a streptomycin-treated 30S subunit, a 50S subunit, and mRNA will not function – that is, no peptide bonds are formed, no polysomes are made, and a frozen “streptomycin monosome” results.
Misreading of the triplet codon of mRNA so that the wrong amino acid is inserted into the protein also occurs in streptomycin-treated bacteria. The site of action on the 30S subunit includes both a ribosomal proven and the ribosomal RNA (rRNA).
As a result of inhibition of initiation and misreading, membrane damage occurs and the bacterium dies.
Tetracyclines (30S subunit)
Tetracycline are a family of antibiotics with bacteriostatic activity against a variety of gram-positive and gram-negative bacteria, mycoplasmas, chlamydiae, and rickettsiae. They inhibit protein synthesis by binding to the 30S ribosomal subunit and by blocking the aminoacyl transfer RNA (tRNA) from entering the acceptor site on the ribosome. However, the selective action of tetracycline on bacteria is not at the level of ribosome, because tetracycline in vitro will inhibit protein synthesis equally well in purified ribosomes from both bacterial and human cells. Its selectivity is based on its greatly increased uptake into susceptible bacterial cells compared with human cells.
Chloramphenicol inhibits protein synthesis by binding to the 50S ribosomal subunit and blocking the action of peptidyltransferase; this prevents the synthesis of new peptide bonds. It inhibits bacterial protein synthesis selectively, because it binds to the catalytic site of the transferase in the 50S bacterial ribosomal subunit but not to the transferase in the 60S human ribosomal subunit.
Chloramphenicol inhibits protein synthesis in the mitochondria of human cells to some extent, since mitochondria have a 50S subunit (mitochondria are thought to have evolved from bacteria). This inhibition may be the cause of the dose-dependent toxicity of chloramphenicol to bone marrow.
Macrolide’s are a group of bacteriostatic drugs with a wide spectrum of activity. Macrolide’s inhibit bacterial protein synthesis by binding to the 50S ribosomal subunit and blocking translocation. They prevent the release of the uncharged tRNA after it has transferred its amino acid to the growing peptide chain. The donor site remains occupied, a new tRNA cannot attach, and protein synthesis stops.
The most useful clinical activity of this bacteriostatic drug is against anaerobes, both gram-positive bacteria such as Clostridium perfringens and gram-negative bacteria such as B. fragilis.
Clindamycin binds to the 50S subunit and blocks peptide bond formation by an undetermined mechanism. Its specificity for bacteria arises from its inability to bind to the 60S subunit of human ribosomes.
Linezolid is useful for the treatment of vancomycin-resistant enterococci, MRSA, and S. epidermis, and penicillin-resistant pneumococci. It is bacteriostatic against enterococci and staphylococci but bactericidal against pneumococci.
Linezolid binds to the 23S ribosomal RNA in the 50S subunit and inhibits protein synthesis, but the precise mechanism is unknown. It appears to block some early step (initiation) in ribosome formation.
Telithromycin (Ketek) is the first clinically useful member of the ketolide group of antibiotics. It is similar to the macrocodes in general structure and mode of action but is sufficiently different chemically such that organisms resistant to macrocodes may be sensitive to telithromycin.
Streptogramins cause premature release of the growing peptide chain from the 50S ribosomal subunit. The structure and mode of action of streptogramins is different from all other drug that inhibit protein synthesis, and there is no cross-resistance between streptogramins and these other drugs.
Raetapamulin is the first clinically available member of a new class of antibiotics called pleuromutilins. These drugs inhibit bacterial protein synthesis by binding to the 23S RNA of the 50S subunit and blocking attachment of the donor tRNA. Retapamulin is a topical antibiotic used in the treatment of skin infections, such as impetigo, caused by S. progenies and methicillin-sensitive S. aureus.
Inhibition of Nucleic Acid Synthesis
This class of drugs include sulfonamides, trimethoprim, fluoroquinolone, flu cytosine, and rifampin. Mechanisms for the inhibition of nucleic acid synthesis include inhibition of precursor synthesis, inhibition of DNA synthesis, and inhibition of mRNA synthesis.
Sulfonamides inhibits the precursor synthesis. The mode of action of sulfonamides is to block the synthesis of tetrahydrofolic acid, which is required as a methyl donor in the synthesis of the nucleic acid precursors adenine, guanine, and thymine. Sulfonamides are structural analogues of p-aminobenzoic acid (PABA), which condenses with a pteridine compound to form dihydropteroic acid, a precursor of tetrahydrofolic acid. Sulfonamides compete with PABA for the active site of the enzyme dihydropteroate synthetase. This competitive inhibition can be overcome by an excess of PABA.
The basis of the selective action of sulfonamides on bacteria is that many bacteria synthesize their folic acid from PABA-containing precursors, whereas human cells require preformed folic acid as an exogenous nutrient because they lack the enzymes to synthesize it. Human cells therefore bypass the step at which sulfonamides act, and similarly, bacteria that can use preformed folic acid are similarly resistant to sulfonamides.
Trimethoprim also inhibit the production of tetrahydrofolic, whose mechanism belongs to inhibition of precursors synthesis. However, trimethoprim has a different mode of action compared with sulfonamides that it inhibits the enzyme dihydrofolate reductase. Its specificity for bacteria is based on its much greater affinity for bacterial reductase than for the human enzyme.
Fluoroquinolone’s are bactericidal drugs that block bacterial DNA synthesis by inhibiting DNA gyrase (topoisomerase).
Flucytosine is an antifungal drug that inhibits DNA synthesis. It is a nucleoside analogue that is metabolized to fluorouracil, which inhibits thymidylate synthetase, thereby limiting the supply of thymidine.
Rifampin belongs to drugs that inhibits synthesis of mRNA. The selective mode of action of rifampin is based on blocking mRNA by bacterial RNA polymerase without affecting the RNA polymerase of human cells. Rifampin is red, and the urine, saliva, and sweat of patients taking rifampin often turn orange; this is disturbing but harmless.
Alteration of Cell Membrane Function
Two sub-categories of mechanisms are involved with alteration of cell membrane function, including: 1.Alteration of bacterial cell membranes; 2.Alteration of fungal cell membranes.
There are few antimicrobial compounds that act on the cell membrane because the structural and chemical similarities of bacterial and human cell membranes make it difficult to provide sufficient selective toxicity.
Polymyxins are a family of polypeptide antibiotics of which the clinical most useful compound is polymyxin E (colistin). Polymyxins are cyclic peptides composed of 10 amino acids, 6 of which are diaminobutyric acid. The positively charged free amino groups act like a cationic detergent to disrupt the phospholipid structure of the cell membrane.
Daptomycin is a cyclic lipopeptide that disrupts the cell membranes of gram-positive cocci. It is bactericidal for organisms such as S. aureus, S. epidermis, S. progenies, Enterococcus faecalis, and E. faecium, including methicillin-resistant strains of S. aureus and S. epidermis, vancomycin-resistant strains of E. faecalis and E. faecium.
Amphotericin B disrupts the cell membrane of fungi because of its affinity for ergosterol, a component of fungal membranes but not of bacterial or human cell membranes.
Nystatin is another polyene antifungal agent, which, because of its toxicity, is used topically for infections caused by the yeast Candida.
Terbinafine blocks ergosterol synthesis by inhibiting squalene epoxidase.
Azoles (Fluconzole, Ketoconazole, Voriconazole, etc.)
They act by inhibiting ergosterol synthesis. The block cytochrome P-450-dependent demethylation of lanosterol, the precursor of ergosterol.
Isoniazid (prodrug probably, see below)
Isoniazid is a bactericidal drug highly specific for Mycobacterium tuberculosis. Isoniazid inhibits mycolic acid synthesis, which explains why it is specific for mycobacteria and relatively nontoxic for humans. The drug inhibits a reductase required for the synthesis of the long-chain fatty acids called mycolic acids that are an essential constituent of mycobacterial cell walls. The active drug is probably a metabolite of isoniazid formed by the action of catalase peroxidase because deletion of the gene for these enzymes results in resistance to the drug.
Metronidazole is bactericidal against aerobic bacteria (also effective against certain protozoa such as Giardia and Trichomonas). Metronidazole is a prodrug that is activated to the active compound within anaerobic bacteria by ferredoxin-mediated reduction of its nitro group. This drug has two possible mechanisms of action, and it is unclear which is the more important. The first, which explains its specificity for anaerobes, is its ability to act as an electron sink. By accepting electrons, the drug deprives the organism of required reducing power. In addition, when electrons are acquired, the drug ring is cleaved and a toxic intermediate is formed that damages DNA. The precise nature of the intermediate and its action is unknown.
The second mode of action of metronidazole relates to its ability to inhibit DNA synthesis. The drug binds to DNA and causes strand breakage, which prevents its proper functioning as a template for DNA polymerase.
Ethambutol is a bacteriostatic drug active against M. tuberculosis and many of the atypical mycobacteria. It is thought to act by inhibiting the synthesis of arabinogalactan, which functions as a link between the mycolic acids and the peptidoglycan of the organism.
Griseofulvin is an antifungal drug that is useful in the treatment of hair and nail infections caused by dermatophytes. It binds to tubulin in microtubules and may act by preventing formation of the mitotic spindle.
Pentamidine is active against fungi and protozoa. It hinibits DNA synthesis by an unknown mechanism.