Antimicrobial Agents PDF

Antimicrobial agents used in treatment of infectious diseas e Definition of Antimicrobial agent Antimicrobial agent: A general term for drugs, chemicals, or other substances that either kill or slow the growth of microbes. Most microbiologist distinguish two groups of antimicrobial ag...

Antimicrobial agents used in treatment of infectious diseas e Definition of Antimicrobial agent Antimicrobial agent: A general term for drugs, chemicals, or other substances that either kill or slow the growth of microbes. Most microbiologist distinguish two groups of antimicrobial agents used in the treatment of infectious disease: Antibiotics, which are natural substances produced by certain groups of microorganisms, and Chemotherapeutic agents, which are chemically synthesized. Antibacterial spectrum:- broad-spectrum inhibit a variety of gram-positive and gram- negative bacteria, narrow-spectrum drug is active only against a limited variety of bacteria. Bacteriostatic : inhibits the growth of an organism.  The Criteria of the Ideal Antibiotic: Selectively toxic to microbe but nontoxic to host. Soluble in body- tissue , fluid Remains in body long enough to be effective - resists excretion and breakdown. Long Shelf life. Does not lead to resistance. Cost not excessive. Hypoallergenic. Microbiocidal rather than microbiostatic. Concernssuppression of normal flora - antibiotic  MECHANISMS OF ACTION OF ANTIBACTERIAL Mechanism of action include: DRUGS – Inhibition of cell wall synthesis – Inhibition of protein synthesis – Inhibition of nucleic acid synthesis – Inhibition of metabolic pathways – Interference with cell membrane integrity MECHANISMS OF ACTION Inhibition of Cell Wall Synthesis The major structural component of bacterial cell walls is the peptidoglycan (PG) layer. Cell wall (PG)consisting of alternating molecules of N- acetylglucosamine and N-acetylmuramic acid. The chains are then cross-linked with peptide bridges that create a rigid mesh coating for the bacteria. The building of the chains and cross-links is catalyzed by specific enzymes (e.g., transpeptidases, transglycosylases, carboxypeptidases) These regulatory enzymes are also called penicillin- binding proteins (PBPs) because they can be bound by  When growing bacteria are exposed to antibiotics, the antibiotic binds to specific PBPs in the bacterial cell wall and inhibits formation of cross-links between peptidoglycan chains. This in turn activates autolysins that degrade the cell wall, resulting in bacterial cell death. Thus the β-lactam antibiotics generally act as bactericidal agents. Antimicrobials which inhibits cell wall synthesis includes β lactam drugs Vancomycin Bacitracin  Penicillins and cephalosporins – Part of group of drugs called β – lactams Have shared chemical structure called β-lactam ring – inhibits function of penicillin- binding proteins Inhibits peptide bridge formation between glycan molecules This causes the cell wall to develop weak points at the growth sites and become fragile.  The weakness in the cell wall causes the cell to lyze. Penicillins and cephalosporins are considered bactericidal. Penicillins are more effective against Gram+ bacteria. This is because Gram + bacteria havemore penicillin binding proteins on their walls.  MECHANISMS OF ACTION OF ANTIBACTERIAL The cephalosporins DRUGS – Chemical structures make them resistant to inactivation by certain β-lactamases – Tend to have low affinity to penicillin-binding proteins of Gram + bacteria, therefore, are most effective against Gram – bacteria. – Chemically modified to produce family of related compounds First, second, third and fourth generation cephalosporins  Cephalospori ns There are 4 generations – First generation ( cephalothin, cefazolin ) effective against gram-positive cocci and few gram-negative – Second generation (cefaclor, cefonacid )more effective against gram-negative bacteria – Third generation ( cephalexin, ceftriaxone) – broad- spectrum activity against enteric bacteria with beta- lactamases – Fourth generation – cefepime – widest range; both gram- negative and gram-positive 1 2 Figure 12.8 1 3 Beta lactam antibiotics. They are the products of two groups of fungi, Penicillium and Cephalosporium molds, and are correspondingly represented by the penicillins and cephalosporins. Semi synthetic - Amoxycillin and Ampicillin have broadened spectra against Gram-negatives and are effective orally; - Methicillin is penicillinase-resistant. Vancomyci n – Inhibits formation of glycan chains – Inhibits formation of peptidoglycans/cell wall construction Does not cross lipid membrane of Gram –negative bac. (too large to cross outer membrane) Gram –ve organisms innately resistant – Important in treating infections caused by penicillin and oxacillin resistant Gram + organisms –Vancomycin interacts with the D-alanine-D-alanine termini of the pentapeptide side chains, which interferes with the formation of the bridges between the peptidoglycan chains. – some organisms are intrinsically resistant to vancomycin because the pentapeptide terminates in  Vancomycin: Mechanism of Action Vancomycin, the crucial “drug of last resort,” inhibits PG synth by binding directly to the D-Ala—D-Ala end of the peptide - forms a cap over the end of the chain; blocks cross- linking  Van Resistance: D-Ala-D- Lactate Vancomycin-resistant bacteria have peptidoglycan chains that end in D-Ala—D-Lactate, instead of the usual D-Ala—D- Ala D-Ala—D-Ala D-Ala—D- Lactate Bacitraci n –Bacitracin inhibits cell wall synthesis (by interfering with the recycling of the lipid carrier responsible for moving the peptidoglycan precursors through the cytoplasmic membrane to the cell wall) – used for the treatment of skin infections caused by gram-positive bacteria(creams, ointments, sprays) (used only for topical application) – Gram-negative bacteria are resistant to this agent – Resistance to the antibiotic is most likely caused by failure of the antibiotic to Inhibition of protein synthesis – Structure of prokaryotic ribosome acts as target for many antimicrobials of this class Differences in prokaryotic and eukaryotic ribosomes responsible for selective toxicity – Drugs of this class include Aminoglycosides Tetracyclins Macrolids Chloramphenicol Inhibition of protein synthesis Aminoglycosides Examples of aminoglycosides include Gentamicin, streptomycin tobramycin neomycin, kanamycin, Are Bactericidal The aminoglycoside antibiotics consist of amino sugars linked through glycosidic bonds to an aminocyclitol ring  – Irreversibly binds to 30S ribosomal subunit Causes distortion and malfunction of ribosome – Causes misreading of mRNA Not effective against anaerobes(Penetration through the cytoplasmic membrane is an aerobic, energy- dependent process) Enterococci and streptococci are resistant and to treat infection due to this organism use:- – synergistic combination with β-lactam drugs which allows aminoglycosides to enter cells that are often resistant Resistance to the antibacterial action of aminoglycosides can develop in one of four ways: (1)mutation of the ribosomal binding site, (2)decreased uptake of the antibiotic into the bacterial cell (3) increased expulsion of the antibiotic from the cell, or (4) enzymatic modification of the antibiotic Tetracycline – Reversibly bind 30S (Bacteriostatic) ribosomal subunit Blocks attachment of tRNA to ribosome – Prevents continuation of protein synthesis – Effective against certain Gram + and Gram -ve – Newer tetracyclines such as doxycycline have longer half- life Allows for less frequent dosing – Can cause discoloration of teeth if taken as young child Resistance to the tetracyclines can stem from a. Decreased penetration of the antibiotic into the bacterial cell b. active efflux of the antibiotic out of the cell c. alteration of the ribosomal target site, or d. enzymatic modification of the antibiotic Macrolid s – Reversibly binds to 50S ribosome Prevents continuation of protein synthesis – They have been used to treat pulmonary infections caused by Mycoplasma, Legionella, and Chlamydia species, as well as to treat infections caused by Campylobacter species and gram-positive bacteria in patients allergic to penicillin Macrolids include Erythromycin, clarithromycin and azithromycin The basic structure of this class of antibiotics is a macrocyclic lactone ring bound to two sugars, desosamine and cladinose Chloramphenicol – Binds to 50S ribosomal subunit (bacteriostatic) Prevents peptide bonds from forming and blocking proteins synthesis – Effective against a wide variety of organisms – Rare but lethal side effect is aplastic anemia – Resistance is mainly due to production of chloramphenicol acetyltransferase and alteration(mutation) the outer membrane porin proteins (This enzyme covalently attaches an acetyl group from acetyl-CoA to chloramphenicol, which prevents chloramphenicol from binding to ribosomes) Inhibition of nucleic acid – These include synthesis Quinolones/ Fluoroquinolones Rifamycins Metronidazole Quinolones and Fluoroquinolones All have Quinolone structure in common Fluorination of Quinolones - Fluoroquinolones inhibit bacterial DNA topoisomerase type II (gyrase) (found in Gram negative) or topoisomerase type IV (Gram positive bacteria), which are required for DNA replication,recombination, and repair.  Member Quinolones Fluoroquinolone Nalidixic acid s s Fluoroquinolone Lomefloxaci New n Generations s Levofloxacin Second First Generation Prulifoxacin ⚫Ciprofloxacin Sparfloxacin Third ⚫Norfloxacin Gatifloxacin ⚫Pefloxacin Gemifloxaci ⚫Ofloxacin n Moxifloxacin Fourth Trovafloxaci n Alatrofloxaci  CIPROFLOXAC IN spectrum Antibacterial Mainly effective against G – Enterobacteriac H. M. bacteria : ae influenzae catarrhalis Campylobacter Pseudomon N. Intracellular gonorrheae as M. pathogens Mycoplasm Chlamydi Tuberculosis a Brucella a Legionella ** Not effective against G+ and anaerobes Rifamycin s – Block prokaryotic RNA polymerase Block initiation of transcription – Effective against many Gram + and some Gram - as well as members of genus Mycobacterium – Primarily used to treat tuberculosis and Hansen’s disease as well as preventing meningitis after exposure to N. meningitidis – Resistance due to mutation coding RNA polymerase  Inhibition of metabolic pathways – Relatively few – Most useful are folate inhibitors Mode of actions to inhibit the production of folic acid – Antimicrobials in this class include Sulfonamides Trimethoprim  MECHANISMS OF ACTION OF ANTIBACTERIAL Interference with cell membrane DRUGS integrity – Few damage cell membrane Polymixn B most common – Binds membrane of bacterial cells Alters permeability – Leads to leakage of cell and cell death Also bind eukaryotic cells but to lesser extent – Limits use to topical application  RESISTANCE TO ANTIMICROBIAL DRUGS Mechanisms of resistance – Drug inactivating enzymes Some organisms produce enzymes that chemically modify drug – Penicillinase breaks β-lactam ring of penicillin antibiotics – Alteration of target molecule Minor structural changes in antibiotic target can prevent binding – Changes in ribosomal RNA prevent macrolids from binding to ribosomal subunits -Decreased uptake of the drug Alterations in porin proteins decrease permeability of cells – Prevents certain drugs from entering – Increased elimination of the drug Some organisms produce efflux pumps – Increases overall capacity of organism to eliminate drug » Enables organism to resist higher concentrations of drug E.g. Tetracycline resistance  REASONS FOR FAILURE OF CHEMOTHERAPY. 1-WRONG DIAGNOSIS 1 WRONG CHOICE Of DRUG 2 WRONG DOSE 3 DEVELOPMENT OF RESISTANCE 4INFECTIONS WITH MORE THAN ONE ORGANISM 5PRESENCE OF PUS ,BLOOD , NECROTIC TISSUES.  EFFECTS OF COMBINATIONS Combinations of OFantimicrobial DRUGS drugs should be used only for: 1. To prevent or minimize the emergence of resistant strains. 2. To take advantage of the synergistic effect. 3. To lessen the toxicity of individual drugs.  EFFECTS OF COMBINATIONS Sometimes the chemotherapeutic OF DRUGS effects of two drugs given simultaneously is greater than the effect of either given alone. This is called synergism. For example, penicillin and streptomycin in the treatment of bacterial endocarditis. Damage to bacterial cell walls by penicillin makes it easier for streptomycin  EFFECTS OF COMBINATIONS Other combinations OF ofDRUGS drugs can be For example, the simultaneous use of penicillin and tetracycline is often less effective than when drugs is used alone. Because Tetracycline will reduce the growth of bacteria(bacteriostatic) which will interferes the action of penicillin, which requires bacterial growth → antagonism  ANTIMICROBIAL SUSCEPTIBILITY Probably the most TESTING widely used testing method is the disk- diffusion method, also known as the Kirby- Bauer test.  SUSCEPTIBILITY OF BACTERIAL TO ANTIMICROBIAL DRUG Conventional disc diffusion method – Kirby-Bauer disc diffusion routinely used to qualitatively determine susceptibility – Standard concentration of – Clear zone of inhibition strain uniformly spread of standard media around – Discs impregnated with disc reflects susceptibility specific concentration of Based on size of zone antibiotic placed on plate and organism can be described incubated as susceptible or resistant Epsilo-meter test It’s a new technique for direct detection of MIC, a graduated increasing concentration of the antibiotic is fixed along a rectangular plastic test strip which is applied to the surface of an inoculated agar plate, after over night incubation a tear drop shaped inhibition zone is seen. The zone edge intersect the graded test strip at the MIC of the antimicrobial. E Test E Test