Medical Microbiology PDF

Mechanism of action of antimicrobial agents Antimicrobials kill or inhibit the growth of microorganisms by interfering with vital structures and processes. Antibiotics effective against prokaryotes which kill or inhibit a wide range of Gram-positive and Gram-negative bacteria are said to b...

Mechanism of action of antimicrobial agents Antimicrobials kill or inhibit the growth of microorganisms by interfering with vital structures and processes. Antibiotics effective against prokaryotes which kill or inhibit a wide range of Gram-positive and Gram-negative bacteria are said to be broad spectrum. If effective mainly against Gram- positive or Gram-negative bacteria, they are narrow spectrum. If effective against a single organism or disease, they are referred to as limited spectrum. A clinically-useful antibiotic should have as many of these characteristics as possible: - It should have a wide spectrum of activity with the ability to destroy or inhibit many different species of pathogenic organisms. - It should be nontoxic to the host and without undesirable side effects. - It should be non-allergenic to the host. - It should not eliminate the normal flora of the host. - It should be able to reach the part of the human body where the infection is occurring. - It should be inexpensive and easy to produce. - It should be chemically-stable (have a long shelf-life). - Microbial resistance is uncommon and unlikely to develop.  Antibacterials can be classified according to their site of action and their molecular structure. 1. Cell wall synthesis inhibitors: Cell wall synthesis inhibitors generally inhibit some steps in the synthesis of bacterial peptidoglycan. Generally they exert their selective toxicity against eubacteria because human cells lack rigid cell walls. Ex 1: Beta lactam antibiotics: Chemically, these antibiotics contain a 4-membered beta lactam ring. The structure of the rings attached to the b-lactam ring determines the class of antimicrobial. Cephalosporins have a six membered ring, whereas penicillins and carbapenems have five-membered rings. Penicillins bind to and inhibit the carboxypeptidase and transpeptidase enzymes that are required for the last step in peptidoglycan synthesis. Beta lactam antibiotics are normally bactericidal and require that cells be actively growing in order to exert their toxicity. Ex 2: Glycopeptides (ex: vancomycin): Block cell wall synthesis by binding to the end of the peptide chains of the peptidoglycan precursors; this prevents their incorporation into the cell wall. 2. Cell membrane inhibitors: These antibiotics disorganize the structure or inhibit the function of bacterial membranes. Ex: Polymyxin binds to membrane phospholipids and thereby interferes with membrane function. 3. Protein synthesis inhibitors: Most have an affinity or specificity for 70S (as opposed to 80S) ribosomes. Exs: the tetracyclines, chloramphenicol, the macrolides (e.g. erythromycin) and the aminoglycosides (e.g. streptomycin). Streptomycin binds to 30S subunit of the bacterial ribosome. The tetracyclines are broad-spectrum antibiotics with a wide range of activity against both Gram-positive and Gram-negative bacteria. Chloramphenicol has a broad spectrum of activity but it exerts a bacteriostatic effect. It inhibits the bacterial enzyme peptidyl transferase, thereby preventing the growth of the polypeptide chain during protein synthesis. Protein synthesis inhibitors 4. Effects on nucleic acids: Some antibiotics and chemotherapeutic agents affect the synthesis of DNA or RNA, or can bind to DNA or RNA so that their messages cannot be read. Ex: fluoroquinolone (ex: ciprofloxacin). 5. Competitive inhibitors: Many of the synthetic chemotherapeutic agents are competitive inhibitors of essential metabolites or growth factors that are needed in bacterial metabolism. The sulfonamides and trimethoprim are inhibitors of the bacterial enzymes required for the synthesis of folic acid. Sulfonamides are structurally similar to para aminobenzoic acid (PABA), the substrate for the first enzyme in the tetrahydrofolate (THF) pathway, and they competitively inhibit that step. Trimethoprim is structurally similar to dihydrofolate (DHF) and competitively inhibits the second step in THF synthesis mediated by the DHF reductase. Animal cells do not synthesize their own folic acid but obtain it in a preformed fashion as a vitamin. Since animals do not make folic acid, they are not affected by these drugs, which achieve their selective toxicity for bacteria on this basis.  Mechanism of Action INHIBITION OF DNA/RNA SYNTHESIS (cont’d)  p-aminobenzoic acid + Pteridine Pteridine synthetase Sulfonamides Dihydropteroic acid Dihydrofolate synthetase Dihydrofolic acid Dihydrofolate Trimethoprim reductase Tetrahydrofolic acid Thymidine Methionine Purines  Six main mechanisms by which bacteria become resistant to antimicrobial agents: 1. Alteration of the target site. 2. Destruction/inactivation of the antimicrobial. 3. Restricted transport of the agent into the cell. 4. Metabolic bypass. 5. Active removal of drug from the cell by activity of efflux pumps. 6. Protection of the target site by a bacterial protein. Alteration of target site: These may result in lower affinity for the antibiotic; active site of enzyme changes and that of ribosome changes, e.g. Erythormycin-resistant microorganisms have an altered receptor on the 50S subunit of the ribosome. Alteration of membrane permeability: transport protein changes, drug no longer enters; drug that does enter is actively pumped out. Enzymatic destruction of drug: e.g. penicillinases (beta- lactamases). “End around” inhibitor: bacteria learns to use new metabolic pathway, drug becomes no longer effective.  Mechanisms of drug resistance Genes responsible for resistance are chromosomal genes or carried on plasmids. Multiple drug resistance (MDR), multi-drug resistance or multi-resistance is a condition enabling disease-causing microorganism to resist one or more classes of antimicrobial agents. Examples: vancomycin-resistant Enterococci (VRE), methicillin-resistant Staphylococcus aureus (MRSA), extended-spectrum β-lactamase (ESBLs) producing Gram-negative bacteria. Methicillin-resistant Staphylococcus aureus (MRSA) is a type of staphylococcal organism resistant to traditional antibiotic therapy, including methicillin, oxacillin, amoxicillin, penicillin, and cephalosporins. To prevent the emergence of antimicrobial resistance: Use the appropriate antimicrobial for an infection. Identify the causative organism whenever possible. Select an antimicrobial which targets the specific organism, rather than relying on a broad- spectrum antimicrobial. Complete an appropriate duration of antimicrobial treatment (not too short and not too long). Use the correct dose for eradication. Beta- lactamase (penicillinase) is now wide-spread. Mechanism of Action CELL WALL SYNTHESIS INHIBITORS Resistance to β-Lactams – Gram-positive bacteria Mechanism of Action CELL WALL SYNTHESIS INHIBITORS Resistance to β-Lactams – Gram-negative bacteria  Chemical Methods of Microbial Control Effect of chemicals: Chemicals may prevent the growth of the bacteria, acting as a bacteriostatic agents, which can not eliminate pathogenic bacteria, others as bactericidal agents causing death of bacteria. Chemical substances that affect microorganisms may destroy the structural organization of the cell. Several critical factors play key roles in determining the effectiveness of an antimicrobial agent, including: Population size Types of organisms Concentration of the antimicrobial agent Duration of exposure Temperature pH Organic matter Biofilm formation 1. Phenol (carbolic acid): was first used by Lister as a disinfectant. Disrupts cell membranes and precipitates proteins, not sporicidal. Rarely used today because it is a skin irritant and has strong odor. Advantages: stable, persist for long times after applied, and remain active in the presence of organic compounds. 2. Alcohols: Kill most bacteria, fungi, but not endospores. Dissolve membrane lipids and coagulate proteins. Used to mechanically wipe microbes off skin before injections or blood drawing. Used in disinfecting surfaces as thermometers. 3. Halogens: Effective alone or in compounds, denature proteins. A. Iodine: Tincture of iodine. B. Chlorine: Used to disinfect drinking water, pools and sewage. When mixed in water, forms hypochlorous acid: Cl2 + H2O ------> H+ + Cl- + HOCl Hypochlorous acid 4. Heavy Metals: The property of heavy metal to exert biocidal effect is called oligodynamic action. They precipitate proteins. Silver: 1% silver nitrate used to protect infants against gonorrheal eye infections. Copper: copper sulfate is used to kill algae in pools and fish tanks. Zinc: zinc chloride is used in mouthwashes. 5. Oxidizing agents: Produce highly active hydroxyl-free radicals and damage proteins and DNA molecules. Ozone: Used to disinfect water. More effective than chlorine but less stable and more expensive. Hydrogen peroxide: Used as an antiseptic and effective in disinfection of inanimate objects. 6. Quaternary ammonium compounds: are cationic detergents. They are surfactants that alter cell permeability. Used as disinfectants and skin antiseptics. they are not sporicidal. 7. Aldehydes: Formaldehyde and gluteraldehyde react chemically with nucleic acid and protein, inactivating them. Aqueous solutions can be used as disinfectants. 8. Organic acids: Inhibit microbial metabolism, ex: benzoic acid. Control molds and bacteria in food. 9. Gaseous sterilization: Ethylene oxide (vapor form) requires a special EtO sterilizer to carefully control sterilization conditions. Inactivate nucleic acids and proteins. Used to sterilize heat-sensitive equipment and plastic ware, and to treat spices and dried foods.  Prophylaxis refers to the treatment given or action taken to prevent disease. Chemoprophylaxis refers to the administration of a medication for the purpose of preventing disease or infection. Alternative medicine refers to treatments or therapies that are outside accepted conventional medicine. Complementary medicine refers to the use of alternative therapies with or in addition to conventional treatment. Combination therapy or polytherapy is therapy that uses more than one medication. It is a suggested way to increase treatment efficacy, to prevent the development of drug resistance, and to reduce the duration of treatment. Pharmacology is the scientific study of the action and effects of drugs on living systems and the interaction of drugs with living systems. It is divided into pharmacodynamics and pharmacokinetics. Pharmacognosy is the study of plants or other natural sources as a possible source of drugs.  Pharmacokinetics is the study of what the body does to the drug, and Pharmacodynamics is the study of what the drug does to the body. Pharmacokinetics outline the timeline of the drug’s absorption, bioavailability, distribution, metabolism and how your body excretes it. Pharmacodynamics analyze the biochemical and physiological effects that the drug has on the patient. Characterizing the relationship between the pharmacokinetics (PK, concentration vs. time) and pharmacodynamics (PD, effect vs. time) is an important tool in the discovery and development of new drugs in the pharmaceutical industry.