Inhibitors of Cell Wall Synthesis - Microbiology Notes PDF

Inhibiting cell wall synthesis Major Action Modes of Antibacterial Drugs Figure 20.2 Major Action Modes of Antibacterial Drugs.  Wall teichoic acid...

Inhibiting cell wall synthesis Major Action Modes of Antibacterial Drugs Figure 20.2 Major Action Modes of Antibacterial Drugs.  Wall teichoic acid Peptidoglycan Gram-Positive Cell Walls Lipoteichoic acid O polysaccharide Cell wall Core polysaccharide Granular layer Plasma Lipid A membrane Parts of the LPS Porin protein Protein Lipoprotein O polysaccharide Lipopolysaccharide Core polysaccharide Lipid A Outer membrane Phospholipid Cell wall Peptidoglycan Plasma membrane Gram-Negative Periplasm Protein Cell Walls Composition and Characteristics Peptidoglycan Polymer of a repeating disaccharide in rows: N-acetylglucosamine (NAG) N-acetylmuramic acid (NAM) Rows are linked by polypeptides Tetrapeptide side chain N-acetylglucosamine (NAG) Peptide cross-bridge N-acetylmuramic acid (NAM) Side-chain amino acid Cross-bridge amino acid NAM Peptide bond Carbohydrate "backbone" Structure. of peptidoglycan in gram-positive bacteria Peptidoglycan structure N-acetylglucosamine N-acetylmuramic acid b-1,4-glycosidic (NAG) (NAM) bond NAG NAM NAG NAM NAG NAM NAM NAG NAM NAG NAM NAG NAM 4 Side-chain Cross- amino acid bridge Cross-bridge Link amino acid NAG NAM NAG NAM NAG NAM NAG Inhibitors of Cell Wall Synthesis Peptidoglycan in bacterial cell wall is a unique feature. Great target for medications: high therapeutic index Three main groups: Beta lactams Glycopeptides Bacitracin Inhibitors of Cell Wall Synthesis β-Lactam antibiotics: Penicillins, Cephalosporins, Carbapenems & Monobactam All have the β-Lactam ring Differentiated by the chemical side chains attached to the ring Broken by the b-lactamases enzyme that are synthesized by many penicillin-resistant bacteria Penicillin G β-lactam ring Inhibitors of Cell Wall Synthesis Penicillin prevent the synthesis of peptidoglycan (major part in cell wall) thus inhibiting cell wall synthesis β-Lactam antibiotics -lactam ring Penicillin G (natural) Cephalothin (semisynthetic) Cephalosporin Methicillin (semisynthetic) Penicillins What is the shared Aztreonam (semisynthetic) Monobactam elements in all these antibiotics? Inhibitors of Cell Wall Synthesis β-Lactam antibiotics: Competitively inhibit enzymes that catalyze formation of peptide bridges between adjacent glycan strands; disrupt cell wall synthesis These enzymes are called penicillin- binding proteins (PBPs) since bind penicillin Interfere with peptidoglycan synthesis Weaken cell walls, leads to cell lysis Only effective against actively growing cells Inhibitors of Cell Wall Synthesis β-Lactam antibiotics: Differ in activity PBPs different in Gram-positives vs. Gram-negatives Peptidoglycan of Gram-positives exposed Outer membrane of Gram-negatives blocks Penicillin G β-lactam ring Inhibitors of Cell Wall Synthesis β-Lactam antibiotics: Some bacteria synthesize β-lactamase, which breaks critical β-lactam ring; many different types Penicillinase inactivates members of penicillin family Extended-spectrum β-lactamases (ESBLs) inactivate a wide variety of β-lactam antibiotics Gram-negatives produce a more extensive array of β- lactamases than Gram-positives ones. β-lactam ring Penicillinase Penicillin Penicilloic acid Mechanism of action Bacterial growth without antibiotics A bacterial cell wall is composed of a macromolecule of peptidoglycan composed of NAG-NAM chains that are cross-linked by peptide bridges between the NAM subunits. New NAG and NAM subunits are inserted into the wall by PBP enzymes, allowing the cell to grow. Other enzymes link new NAM subunits to old NAM subunits with peptide cross- links. NAG NAM NAG NAM NAG NAM NAM NAG NAM NAG NAM NAG NAM NAG NAM NAG NAM NAG NAM NAG Mechanism of action Bacterial growth with β-Lactam antibiotics ‘Penicillin’ Penicillin interferes with the linking enzymes (PBP), and NAM subunits remain unattached to their neighbors. Cells continues to grow and adds NAG and NAM subunits. Integrity of peptidoglycan is not maintained and the cell bursts from internal osmotic pressure NAG NAM NAG NAM NAG NAM Penicillin NAM NAG NAM NAG NAM NAG NAM Penicillin NAG NAM NAG NAM NAG NAM NAG Penicillin synthesis 1. Naturally occurring Produced from P. notatum (P. chrysogenum) fermentation. Benzyl penicillin (Penicillin G) Phenoxy methyl penicillin (Penicillin V). 2. Semisynthetic In 1959, scientists at Beecham Research Laboratories succeeded in isolating the penicillin ‘nucleus’, 6-amino penicillanic acid (6-APA). Penicillin synthesis Most penicillin’s are 6-aminopenicillanic acid derivatives and differ in side chain attached to amino group Penicillin synthesis 1. Fermentation from Penicillium mold into 6-APA Addition of phenylacetic (phenylethanoic) acid (C6H5.CH2.COOH) to the growth media (in which the Penicillium fungi is growing) result in the production of 6- APA. Penicillin synthesis 2. Removal of penicillin G side chain result in 6-APA Another way to produce 6-APA is by catalysing the removal of the side-chain from benzyl penicillin (penicillin G) by penicillin acylase penicillin acylase Penicillin synthesis Classification of Penicillin 1. Natural Penicillin’s Based on the original Penicillin G structure, include Penicillin G (injected), Procaine, & Penicillin V (oral). Narrow spectrum of activity against Gram-positives Effective against streptococci, staphylococci & some G-ve bacteria: Neisseria meningitidis (meningococcus). Susceptible to penicillinases (β-lactamases); S. aureus are penicillin resistant: produce penicillinase Common nucleus Penicillin G (injection) β-lactam ring Penicillin V (orally) Classification of Penicillin 2. Semisynthetic penicillin’s (Anti-staphylococcal) Contain chemically added side chains, making them resistant to penicillinases Methicillin, oxacillin, nafcillin, cloxacillin & dicloxacillin Targets penicillinase-producing staphylococci Methicillin resistant staphylococcus aureus (MRSA) is a serious source of hospital-acquired infections Common nucleus Oxacillin: Narrow spectrum, only gram-positives, but resistant to penicillinase β-lactam ring Classification of Penicillin 3. Semisynthetic penicillin’s (Broad Spectrum) Effective against gram-negatives and positive ones Spectrum similar to Penicillin G (G+ve strains of streptococci, staphylococci) plus G-ve bacteria. Aminopenicillins: ampicillin, amoxicillin 4. Anti-pseudomonal penicillins (Extended spectrum) Carbenicillin, ticarcillin, piperacillin, azlocillin, and mezlocillin Inactivated by many β-lactamases Common nucleus Ampicillin: Extended spectrum, many gram-negatives β-lactam ring Beta-Lactamase inhibitors Expression of Beta-lactamase enzymes is the most important mechanism through which organisms become resistant to b-lactams. The Beta-lactamase enzyme hydrolyse Beta-lactam ring (antibiotic) very efficiently, releasing fragments of the antibiotics rapidly. Clavulanic acid, sulbactam & tazobactam Penicillin G β-lactam ring are hydrolysed by the b- β-lactamase lactamases in the same manner as susceptible b- lactam antibiotics but with higher affinity Beta-Lactamase inhibitors Clavulanic acid Based on the penam structure, but differs in two ways: Replacement of sulphur in the penicillin thiazolidine ring with oxygen in the clavam oxazolidine ring Absence of the side chain at position 6. Isolated from streptomyces clavuligerus Potent inhibitor of staphylococcal b- lactamase Potent inhibitor of most types of b- lactamases produced by Gram-ve bacteria Especially those with a ‘penicillinase’ rather than a ‘cephalosporinase’ type of enzyme. Offer little protection against cephalosporinase Beta-Lactamase inhibitors Clavulanic acid Clavulanic acid has a poor antibacterial activities Used in combination to treat several infections: Amoxicillin + Clavulanic acid AUGMENTIN: given orally Can treat P. aeruginosa Ticarcillin + Clavulanic acid TIMENTIN: given parenterally IV, IM, SC Can treat Gram negative, particularly P. aeruginosa. It is also one of the few antibiotics can treat Stenotrophomonas maltophilia infections. Beta-Lactamase inhibitors Sulbactam The sulfur atom of the thiazolidine ring is 6 converted to a sulphone, and no side chain at position 6. Sulbactam is less potent against b- lactamases compared with clavulanic acid. Little protection against P. aeruginosa, Citrobacter, Enterobacter, & Serratia Sulbactam + cephalosporin = Cefoperazone Sulbactam + ampicillin = Sultamicillin Beta-Lactamase inhibitors Cefoperazone (Sulbactam + cephalosporin) Broad spectrum activity The two ingredients were separate entities Potential problem that their pharmacokinetics might not perfectly match, so that the two agents might not appear at infection site in the optimal concentration ratio at the same time. Used to target bacterial infection of the respiratory tract, urinary tract, skin, & female genital tract H. influenzae, S. aureus and S. pneumoniae Beta-Lactamase inhibitors Sultamicillin (Sulbactam + Ampicillin) Broad spectrum activity Sulbactam + ampicillin, are covalently linking and are well absorbed following oral administration They hydrolyzed to liberate equal proportions of the individual components (Trade name Unasyn) Treat acute and chronic sinusitis (URTI) caused by S. aureus, S. pneumoniae, H. influenzae & S. progenies. Treat LRTI - caused by S. pneumoniae, H. influenzae, S. aureus and S. progenies. Beta-Lactamase inhibitors Sultamicillin (Sulbactam + Ampicillin) Treat skin infections by S. aureus & S. pyogenes. Treat UTI - by E. coli, Proteus mirabilis, Klebsiella, Enterobacter & S. aureus. Treat Gynecological infections - by beta-lactamase producing strains of E. coli & Bacteroides sp. (including B. fragilis). Treat infections of the GIT- treatment of H. pylori infections  Cefoperazone 6 Sultamicillin Sulbactam Beta-Lactamase inhibitors Tazobactam Tazocin (Tazobactam + piperacillin) Piperacillin is a extended spectrum β-lactam antibiotic Given parenterally (IV - not orally) The combination has activity against many Gram+ve and Gram-ve bacteria including P. aeruginosa. Used to treat pelvic inflammatory disease, intra- abdominal infection, pneumonia, cellulitis, and sepsis. Inhibitors of Cell Wall Synthesis Natural penicillin’s Penicillinase-resistant penicillin’s Methicillin and oxacillin Broad spectrum penicillin’s Inactivated by many β-lactamases Extended-spectrum penicillins More resistant to inactivation by extended-spectrum β- lactamases (ESBLs) & active against Pseudomonas sp. Penicillins plus β-lactamase inhibitors Contain clavulanic acid, a noncompetitive inhibitor of penicillinase Inhibitors of Cell Wall Synthesis Cephalosporins Derived from fungus Acremonium cephalosporium Consist of a six- membered dihydrothiazine ring β-lactam Cephalosporin ring fused to β-lactam ring nucleus Structurally related to the penicillin. β-lactam ring differs from penicillin thus Penicillin ‘resistant to some β- nucleus lactamases’ Inhibitors of Cell Wall Synthesis In 1950s, a species of C. acremonium produced: 1. Acidic antibiotic, Cephalosporin P Subsequently found to have a steroid-like structure 2. Acidic antibiotic, Cephalosporin N Structure based on 6-APA (penicillin) 3. Cephalosporin C True cephalosporin (true cefem nucleus): 7-amino cephalosporanic acid (7-ACA) Discovered during purification of cephalosporin N 7-ACA: starting point for new cephalosporins Active against both G+ve & some G-ve bacteria e.g. Salmonella typhi. All cephalosporins possessing acetoxymethyl i.e. derivatives of 7-ACA are orally inactive. Inhibitors of Cell Wall Synthesis Cephalosporins Efforts are made to obtain a cephalosporin that has the following properties: 1. Orally stable 2. Broad spectrum activity against G+ve & Pseudomonas. 3. High resistance against b-lactamases (cephalosporinases). 4. High activity against Pseudomonas aeruginosa and MRSA. 5. Cross Blood Brain Barrier and to treat meningococcal infections. Cephalosporins are classified into different generations according to time of discovery & spectrum of activity. Inhibitors of Cell Wall Synthesis Cephalosporins generations Chemical modifications have led to 4 main generations Each newer generation has significantly greater gram negative antimicrobial properties than the preceding one Later generations more effective, resist β lactamases G+ve G+/-ve G-/+ve G+/-ve Cephalosporins generations Inhibitors of Cell Wall Synthesis Cephalosporins 1st generation Activity spectrum Effective against G+ve cocci Streptococcus & Staphylococcus (except MRSA). However, they are not the drug of choice for such infections. Modest activity against EKP 1. Cefadroxil Gram negative Bacteria: 2. Cephalexin Escherichia coli 3. Cephalothin Klebsiella 4. Cephapirin Proteus 5. Cefazolin (most Not active against P. aeruginosa common used) 6. Cephradine Inhibitors of Cell Wall Synthesis Cephalosporins 2nd generation Activity spectrum Increased activity against G-ve compared with 1st generation. Retain some activity against G+ve cocci. More resistant to b-lactamase. Useful agents for treating upper / lower 1. Cefaclor respiratory tract infections & sinusitis. 2. Cefoxitin Active against EKP (can treat UTI) 3. Cefprozil Not active against P. aeruginosa 4. Cefuroxime Inhibitors of Cell Wall Synthesis Cephalosporins 3rd generation Extended spectrum of activity against G-ve organisms. Resistant to B-lactamases. Many of them reach the central nervous system (CNS). Can treat P. aeruginosa infections. Ceftazidime: has best effectiveness against P. aeruginosa of all B-lactams Ceftriaxone & cefotaxime have excellent activity against most strains of Streptococcus pneumoniae, including the vast majority of those with intermediate and high level resistance to penicillin’s. With vancomycin, treat multidrug resistant pneumococcal infections In Jordan (cefixime = commercial name Suprax) Inhibitors of Cell Wall Synthesis Cephalosporins 4th generation Extended spectrum cephalosporins Similar activity against G+ve organisms (as 1st generation) but greater activities against G-ve bacteria Greater resistance to B-lactamases than 3rd generation Many can cross blood brain barrier & are effective in treatment of meningitis. 1. Cefepime 2. Cefluprenam 3. Cefozopran 4. Cefpirome 5. Cefquinome Inhibitors of Cell Wall Synthesis Cephalosporins 5th generation Broad-spectrum antibiotic Active against G+ve bacteria including MRSA. Have a broad-spectrum activity against G-ve bacteria, but its effectiveness is relatively much weaker Ceftaroline & Ceftobiprole : The only 5th generation cephalosporin to date