Pharmaceutical Microbiology Lecture 8 (2024/2025) PDF

Pharm PharmDDProgram Program (2024/2025) (2024/2025) Pharmaceutical Microbiology (PM 502) Lectu...

Pharm PharmDDProgram Program (2024/2025) (2024/2025) Pharmaceutical Microbiology (PM 502) Lecture No. (8) Antimicrobials-2 30 November 2024 www.su.edu.eg 1  Pharm PharmDDProgram Program (2024/2025) (2024/2025) Lecture’s Aim To give the student the chance to understand the modes of action of different antibiotics. 30 November 2024 www.su.edu.eg 2  Pharm PharmDDProgram Program (2024/2025) (2024/2025) Lecture’s Competencies To enable the student to discuss the mechanisms of action of different antibiotics and their implications on the effectiveness, spectrum and side effects of these antibiotics during treatment. 30 November 2024 www.su.edu.eg 3  Pharm PharmDDProgram Program (2024/2025) (2024/2025) Lecture’s Contents - Mode of action of antibiotics 2. Disruption of cell membrane 3. Inhibition of protein synthesis 30 November 2024 www.su.edu.eg 4  Pharm PharmDDProgram Program (2024/2025) (2024/2025) So why there is sometimes recurrence of infection after penicillin treatment has been terminated?? Bacterial cells that are not undergoing multiplication can survive in the presence of penicillin…why? because their peptidoglycan is unbroken and there is no reparative cross linking activity for the penicillin to block. 30 November 2024 www.su.edu.eg 5  Pharm PharmDDProgram Program (2024/2025) (2024/2025) 30 November 2024 11/30/2024 www.su.edu.eg 6 6 II- Disruption of cell membrane  The membrane active agents act by destroying the integrity of the membrane causing: - leakage of the cytoplasmic constituents, or - impairment of metabolic functions thus leading to the death of microbial cell. These antibiotics are poorly selective, acting against both microbial & human cell membranes. Consequently, they are usually too toxic to be given systematically and their use is often limited to topical applications.  Pharm PharmDDProgram Program (2024/2025) (2024/2025) 1) Polymyxin B and polymyxin E (colistin) They bind tightly to negatively charged phosphate groups on LPS in the cytoplasmic membrane & outer membrane of Gram-negative bacteria destroying the selective permeability barrier leading to the release of amino acids, purine, and pyrimidine from the cell. They have significant toxicity on human cell membranes. Polymyxins have bactericidal properties. 30 November 2024 11/30/2024 www.su.edu.eg 9 9  Pharm PharmDDProgram Program (2024/2025) (2024/2025) Polymyxins bind to the lipopolysaccharides within the outer membrane, causing structural changes to the membrane. This in turn causes a loss of membrane integrity, increasing its permeability. The polymyxins are now able reach the plasma membrane, disrupting it in a similar way. This causes the cell contents to leak out, eventually leading to cell death 30 November 2024 11/30/2024 www.su.edu.eg 10 10  Pharm PharmDDProgram Program (2024/2025) (2024/2025) 2) Daptomycin Daptomycin is a cell membrane inhibitor that is used to target Gram-positive bacteria. Daptomycin can insert itself into the plasma membrane forming pore-like structures that allow ions to leak out of the cell, eventually leading to cell death. Daptomycin is a relatively new antibiotic and is not in widespread use. 30 November 2024 11/30/2024 www.su.edu.eg 11 11  Pharm PharmDDProgram Program (2024/2025) (2024/2025) Daptomycin mechanism of action 30 November 2024 11/30/2024 www.su.edu.eg 12 12 3) Polyenes: Amphotericin B and nystatin They are antifungal agents possessing a strong affinity towards sterols, particularly ergosterol (in the fungal membrane) causing leakage of cytoplasmic constituents. However, they have low selectivity to cholesterol (found in membrane of human cells). The problem of kidney damage can be reduced but not eliminated by administration of amphotericin as a lipid complex or liposome.  Pharm PharmDDProgram Program (2024/2025) (2024/2025) 4) Imidazoles Act by inhibiting the synthesis of ergosterol of the fungal membrane causing rapid defects in fungal membrane integrity. Possess some action upon mammalian steroid metabolism, e.g. reduce testosterone synthesis. 30 November 2024 11/30/2024 www.su.edu.eg 14 14 III- Inhibition of protein synthesis There are 4 phases/stages of translation in the bacterial ribosome: a) Initiation The mRNA binds to the 30S ribosomal subunit to form a complex to which first the initiator tRNA and then the 50S ribosomal subunit bind. tRNA carries the initiator amino acid (N-formylmethionine). P-site P (Peptidyl) A-site A (Aminoacyl) b) Elongation Repeated cycles of aminoacyl tRNA binding, addition of the amino acid to the nascent peptide chain (peptide bond formation) and release of the deacylated tRNA. c) Translocation Movement of the ribosome along the mRNA to allow the addition of more amino acids (carried on tRNA). d) Termination Termination of the protein synthesis, release of the newly synthesized peptide and dissociation of the ribosome into its subunits.  The ribosome 70S 30S 50S Eukaryotic (human) ribosome is 80S (60S+40S). This is the basis of the selective toxicity of antibiotics to bacterial ribosome.  Pharm PharmDDProgram Program (2024/2025) (2024/2025) 30 November 2024 11/30/2024 www.su.edu.eg 18 18 1- Inhibitors of the 30S ribosomal subunits i) Aminoglycosides They disturb the shape of the A site of the 30S subunit & interfering with the positioning of the anti-codons of the aminoacyl tRNA causing misreading of mRNA. This results in the formation of toxic, nonfunctional proteins which have a lethal effect since the incorporation of these abnormal proteins into the cell membrane compromises its function and dramatically damages it (bactericidal).  Pharm PharmDDProgram Program (2024/2025) (2024/2025) Mechanism of action of aminoglycosides 30 November 2024 11/30/2024 www.su.edu.eg 20 20  Pharm PharmDDProgram Program (2024/2025) (2024/2025) ii) Tetracyclines Block the binding of the aminoacyl tRNA at the A- site of the 30S subunit consequently, inhibiting the protein synthesis (bacteriostatic). Should not be taken with -lactams which require the microbial cells to be growing to exert their action. 30 November 2024 11/30/2024 www.su.edu.eg 21 21  Pharm PharmDDProgram Program (2024/2025) (2024/2025) 30 November 2024 11/30/2024 www.su.edu.eg 22 22 iii) Oxazolidinones (linezolid) A new class of synthetic antibiotics first introduced in 1987. Act on the initiation step by blocking the formation of the initiation complex between the 30S subunit, mRNA and formylmethionine-tRNA. The oxazolidinones are the only drugs that inhibit this particular step. Due to this unique mechanism of action, cross-resistance between linezolid & other protein synthesis inhibitors is highly infrequent.  Pharm PharmDDProgram Program (2024/2025) (2024/2025) 30 November 2024 11/30/2024 www.su.edu.eg 24 24  Pharm PharmDDProgram Program (2024/2025) (2024/2025) 2- Inhibitors of 50S ribosomal subunits i) Chloramphenicol Inhibit the peptidyl transferase reaction consequently, it blocks peptide bond formation in the growing peptide. ii) Macrolides (erythromycin) Block the translocation & cause the release of incomplete polypeptides from the ribosome. 30 November 2024 11/30/2024 www.su.edu.eg 25 25  Pharm PharmDDProgram Program (2024/2025) (2024/2025) 30 November 2024 11/30/2024 www.su.edu.eg 26 26  Pharm PharmDDProgram Program (2024/2025) (2024/2025) 30 November 2024 11/30/2024 www.su.edu.eg 27 27 Inhibition of Protein Synthesis  Pharm PharmDDProgram Program (2024/2025) (2024/2025) Lecture’s References Pharmaceutical Microbiology. Edited by W.B.HUGO and A.D. Russell – Six Edition. 1998. 30 November 2024 www.su.edu.eg 29  Pharm PharmDDProgram Program (2024/2025) (2024/2025) 30 November 2024 11/30/2024 www.su.edu.eg 30 30