Microbial Chemotherapy SYBSc PDF

ANTIMICROBIA L CHEMOTHERAP SYBSc 1-2 pm 19 & 26th Nov, 2024 th Y  Drugs: Contents Anti-mycobacterial Antifungal Antiviral...

ANTIMICROBIA L CHEMOTHERAP SYBSc 1-2 pm 19 & 26th Nov, 2024 th Y  Drugs: Contents Anti-mycobacterial Antifungal Antiviral Antiprotozoal History Principal groups of Selection and Testing antibacterial agents and mechanism of action: Drug resistance: Origin, Cell wall inhibitors Mechanisms and Inhibitors of Protein Transmission Synthesis Inhibitors of Nucleic Principles of Ayurveda Acid Synthesis Cell Membrane disruptors Antimetabolites  3 History of Chemotherapy and Antimicrobial Discovery Chemotherapy may involve drugs that target cancerous cells or tissues, or it may involve antimicrobial drugs that target infectious microorganisms. Broadly divided into two categories - cancer chemotherapy and antimicrobial chemotherapy  4 Use of Antimicrobials in Ancient Societies Chemical analyses of the skeletal remains of people from Nubia Showed tetracycline residue Indicates purposeful fermentation of tetracycline-producing Streptomyces during beer-making Traditional Medicine Indian and Chinese Herbalists: Long history of using plants with antimicrobial properties for medical purposes. Examples: Garlic, ginger, and other herbs used for their healing effects.  5 The First Antimicrobial drugs Paul Ehrlich & the "Magic Bullet" Concept Early 20th century: Envisioned a hypothetical drug that could target and destroy a microbe or a toxin without any collateral The principles of chemotherapy were founded damage by Paul Ehrlich in the early 1900s. The German Nobel Laureate was noted for his Collaboration with Sahachiro scientific concept of a “magic bullet” — that it Hata was possible to kill specific things in the body that cause disease without harming the body Developed itself a syphilis treatment by testing arsenic-  6 The First Antimicrobial drugs Discovery of Compound 606 Screened over 600 arsenic compounds; Compound 606 effectively targeted Treponema pallidum. Successfully cured syphilis in rabbits; marketed as Salvarsan (arsphenamine) for human treatment.  7 The First Antimicrobial drugs Domagk had been testing textile dyes, the firm’s main product. He found that one compound, a bright red dye called Prontosil, cured mice injected with lethal doses of haemolytic streptococci. Another early antibacterial was It proved successful in treating Prontosil. It was discovered in 1932 by streptococcal and staphylococcal Gerhard Domagk, the director of a German chemical company that infections in humans owing to its encompassed Bayer. bacteriostatic action  8 The First Antimicrobial drugs Jacques (1897–1977) and Therese (1892– 1978) Trefouel later showed that the body metabolized the dye to sulfanilamide Further research into Prontosil revealed that it is actually a prodrug – it is metabolised by the body into the active agent sulphanilamide.  9 The First Antimicrobial drugs On returning to the lab from summer holiday, Alexander Fleming noticed that one his bacteria cultures was contaminated with mould. The colonies of staphylococci immediately surrounding the mould had been destroyed, whereas other staphylococci colonies farther away were normal. Fleming observed it and famously remarked "That's 10 The long search for a treatment for tuberculosis began in 1882 when Robert Koch identified the cause of the disease: Mycobacterium tuberculosis. Antibiosis is an antagonistic association between two organisms (especially microorganisms), in which one is adversely affected. Laboratory observation of antibiosis While an antibiotic drug refers specifically to one intended to halt or eliminate a specific species of bacterium or range of bacteria, an antimicrobial drug may be intended to treat any of several classes of microorganism, including parasites, viruses, and fungi.  15 Terminology of chemotherapy Chemotherapeuti Any chemical used in treatment, relief, or c drug prophylaxis of disease Antimicrobial Use of chemotherapeutic drugs to control an chemotherapy infection Antimicrobials All-inclusive term for any antimicrobial drug, regardless of its origin  16 Terminology of chemotherapy Antibiotics Substances produced by the natural metabolic processes of some microorganisms that can inhibit or destroy other microorganisms Semisynthetic Drugs that are chemically modified in the drugs laboratory after being isolated from natural sources (ampicillin and amoxicillin) Synthetic drugs are derived in the laboratory from dyes or other organic compounds, through chemical reactions (ciprofloxacin, isoniazid) Several factors are important in choosing the most appropriate antimicrobial drug therapy, including bacteriostatic versus bactericidal mechanisms, spectrum of activity, dosage and route of administration, the potential for side effects, and the potential interactions between drugs. BACTERIOSTATIC VERSUS BACTERICIDAL Antimicrobial drugs are either bactericidal (they kill microbes directly) or bacteriostatic (they prevent microbes from growing). In bacteriostasis, the host’s own defenses, such as phagocytosis and antibody production, usually destroy the microorganisms. Spectrum of antimicrobial activity Antimicrobial agents are often classified as either narrow spectrum drugs—that is, they are effective only against a limited variety of pathogens—or broad-spectrum drugs that attack many different kinds of bacteria Spectrum of antimicrobial activity Antimicrobial agents are often classified as either narrow spectrum drugs—that is, they are effective only against a limited variety of pathogens—or broad-spectrum drugs that attack many different kinds of bacteria A narrow-spectrum antimicrobial targets only specific subsets of bacterial pathogens. For example, some narrow-spectrum drugs only target gram-positive bacteria, whereas others target only gram- negative bacteria. If the pathogen causing an infection has been identified, it is best to use a narrow-spectrum antimicrobial. Why? Spectrum of antimicrobial activity A broad-spectrum antimicrobial targets a wide variety of bacterial pathogens, including both gram-positive and gram-negative species. The risk associated with using broad-spectrum antimicrobials is that they will also target a broad spectrum of the normal microbiota, increasing the risk of a superinfection, a secondary infection in a patient having a preexisting infection. Spectrum of antimicrobial activity A superinfection develops when the antibacterial intended for the preexisting infection kills the protective microbiota, allowing another pathogen resistant to the antibacterial to proliferate and cause a secondary infection. Common examples of superinfections that develop as a result of antimicrobial usage include yeast infections (candidiasis) and pseudomembranous colitis caused by Clostridium difficile, which can be fatal. ANTIMICROBIA L CHEMOTHERAP SYBSc 2-3 pm 27th Nov, 2024 Y MECHANISMS OF DRUG ACTION Antimicrobial drugs function specifically in one of the following way: 1) Inhibition of cell wall synthesis 2) Breakdown of cell membrane structure or function 3) Inhibition of structures and functions of DNA and RNA 4) Inhibition of protein synthesis 5) Blocks on key metabolic pathways MECHANISMS OF DRUG ACTION ANTIMICROBIAL DRUGS THAT AFFECT CELL WALL SYNTHESIS ANTIMICROBIAL DRUGS THAT AFFECT CELL WALL SYNTHESIS The bacterial cell wall, also known as the peptidoglycan or murein layer Active cells must constantly synthesize new peptidoglycan and transport it to its proper place in the cell envelope. BETA LACTAMS Beta-lactam antibiotics have a four-member, nitrogen containing, beta- lactam ring at the core of their structure. BETA LACTAMS The beta-lactam ring is the key to the mode of action of these drugs. It is structurally similar to acyl-D-alanyl-D-alanine, the normal substrate required for synthesis of the linear glycopeptide in the bacterial cell wall. The enzymes essential for this function are anchored in the cell membrane and are referred to as penicillin-binding proteins (PBPs)  32 The two naturally occurring penicillins, penicillin G and penicillin V, are narrow- spectrum drugs.  33 Semisynthetic penicillins  34 Cephalosporins are a family of antibiotics originally isolated in 1948 from the fungus Cephalosporium. They contain a β-lactam structure very similar to that of the penicillins Carbapenems and Monobactam effective against a broad spectrum of bacteria  35 Vancomycin is a glycopeptide antibiotic produced by the bacterium Streptomyces orientalis. is bactericidal only for Gram- positive bacteria  36 Antimicrobial drugs that affect nucleic acid synthesis The most commonly used antibacterial drugs that inhibit nucleic acid synthesis function by inhibiting (1) DNA polymerase and topoisomerases (fluoroquinolones) or (2) RNA polymerase (rifamycins).  37 QUINOLONES a large family of bactericidal synthetic agents interfere with replication of the bacterial chromosome ability to inhibit the activity of bacterial DNA gyrase and topoisomerases FLUOROQUINOLONE They act by binding to the bacterial topoisomerases DNA gyrase and topoisomerase II  38 RIFAMYCIN The most commonly used member of the rifamycin class is the semisynthetic derivative rifampin. These agents block bacterial transcription by binding to the β- subunit of RNA polymerase. Rifampin is an important member of the multidrug regimen used to treat tuberculosis and other mycobacterial infections. ANTIMICROBIA L CHEMOTHERAP SYBSc 2-3 pm 4 Dec, 2024 th Y  40 Antimicrobial drugs that inhibit metabolic pathways Some drugs act by mimicking the normal substrate of an enzyme through a process called competitive inhibition. These drugs, termed metabolic analogs, are structurally similar to the natural substrate and compete with it for the active site on the enzyme. Antibacterial drugs called sulfonamides and trimethoprim provide an example of this mode of action  41 Sulfonamides and trimethoprim interfere with folate metabolism by blocking enzymes required for the synthesis of tetrahydrofolate, which is needed by bacterial cells for the synthesis of folic acid and the eventual production of DNA and RNA and amino acids  42 Sulfonamides’ competition with PABA (p-aminobenzoic acid) for the active site of the enzyme that synthesizes the folic acid precursor.  43 Sulfonamides’ competition with PABA (p-aminobenzoic acid) for the active site of the enzyme that synthesizes the folic acid precursor.  44 Antimicrobial drugs that affect protein synthesis Some antibiotics inhibit bacterial pathogens by disrupting protein synthesis (translation), often through interactions with the ribosome that may include binding to ribosomal RNA (rRNA). Most of these drugs target only bacterial ribosomes and, therefore, have no effect on the structurally distinct, cytoplasmic ribosomes of eukaryotic cells. However, because mitochondria and chloroplasts in Eukarya contain 70S ribosomes, many antibiotics that inhibit protein synthesis in Bacteria also inhibit protein synthesis in these organelles. Nevertheless, these drugs are still medically useful because the eukaryotic 70S ribosomes are affected only at higher concentrations than are used for antimicrobial therapy.  45 Aminoglycosides All contain a cyclohexane ring and amino sugars. Streptomycin, kanamycin, neomycin, and tobramycin are synthesized by different species of the bacterial genus Streptomyces, whereas gentamicin comes from a related bacterium, Micromonospora purpurea  46 Aminoglycosides - mode of action All aminoglycoside antibiotics disrupt peptide elongation during translation. This occurs as aminoglycosides bind to ribosomal RNA of the bacterial 30S ribosomal subunit, interfering with mRNA reading and/or causing early termination of peptide synthesis.  47 Aminoglycosides Aminoglycosides are bactericidal and are generally used to treat certain infections caused by Gram-negative bacteria. However, because they can be quite toxic, causing hearing and renal damage, loss of balance, nausea, and allergic reactions, they are used sparingly. Aminoglycoside side effects are thought to be due to their ability to bind to host mitochondrial ribosomes, which share the same binding site as their bacterial ancestors.  48 Tetracyclines The tetracyclines are a family of antibiotics with a common four- ring structure to which a variety of side chains are attached. Oxytetracycline and chlortetracycline are produced naturally by Streptomyces spp., whereas other tetracyclines are semisynthetic.  49 Tetracyclines - mode of action These antibiotics are similar to the aminoglycosides in that they target the 30S subunit of the ribosome, inhibiting protein synthesis. Bacteriostatic Broad-spectrum antibiotics that are active against most bacteria, including the intracellular pathogens rickettsias, chlamydiae, and mycoplasmas.  50 Macrolides The macrolide antibiotics contain a ring structure consisting of 12 to 22 carbons called a lactone ring. The lactone ring is linked to one or more sugars  51 mode of action inhibit translation by targeting the 50S (large) subunit of the bacterial ribosome Erythromycin is a relatively broad-spectrum antibiotic effective against Gram-positive bacteria, mycoplasmas, and some Gram negative bacteria, but it is usually only bacteriostatic. Newer semisynthetic macrolides include  52 Lincosamides Lincosamide antibiotics are produced by Streptomyces bacteria. They have a broad spectrum of activity against anaerobic bacteria and many Gram-positive cocci are inhibited. Chloramphenicol Chloramphenicol was first produced from cultures of Streptomyces venezuelae but is now synthesized chemically. Like erythromycin, this antibiotic binds the 50S ribosomal subunit to inhibit bacterial protein synthesis. It has a very broad spectrum of activity but, unfortunately, is quite toxic.  53 Antimicrobial drugs that inhibit cell membrane synthesis A cell with a damaged membrane invariably dies from disruption in metabolism or lysis and does not even have to be actively dividing to be destroyed Polymyxins interact with membrane phospholipids, distort the cell surface, and cause leakage of proteins and nitrogen bases, particularly in gram-negative bacteria The polyene antifungal antibiotics (amphotericin B and nystatin) form complexes with the sterols on fungal membranes; these complexes cause abnormal openings and seepage of small ions.  54 Antimicrobial drugs that inhibit cell membrane synthesis LIPOPEPTIDES Daptomycin exerts its antimicrobial effect by binding to and disrupting the cell membrane of gram positive bacteria. The drug binds to the cytoplasmic membrane and inserts its hydrophobic tail into the membrane, disrupting the cell membrane and increasing its permeability, which results in cell death Polymyxins (polymyxin B and colistin) are cyclic lipopeptide agents that disrupt bacterial cell membranes. The polymyxins act as detergents, interacting with phospholipids in the cell membranes to increase permeability LIPOPEPTIDES Daptomycin exerts its antimicrobial effect by binding to and disrupting the cell membrane of gram positive bacteria. The drug binds to the cytoplasmic membrane and inserts its hydrophobic tail into the membrane, disrupting the cell membrane and increasing its permeability, which results in cell death Polymyxins (polymyxin B and colistin) are cyclic lipopeptide agents that disrupt bacterial cell membranes. The polymyxins act as detergents, interacting with phospholipids in the cell membranes to increase permeability ANTIMICROBIA L CHEMOTHERAP SYBSc 1-2 pm 10 Dec, 2024 th Y  Drugs: Contents Anti-mycobacterial Antifungal Antiviral Antiprotozoal History Principal groups of Selection and Testing antibacterial agents and mechanism of action: Drug resistance: Origin, Cell wall inhibitors Mechanisms and Inhibitors of Protein Transmission Synthesis Inhibitors of Nucleic Principles of Ayurveda Acid Synthesis Cell Membrane disruptors Antimetabolites Antimicrobial drugs have selective toxicity An ideal antimicrobial agent exhibits selective toxicity, which means that the drug is harmful to a pathogen without being harmful to the host. The degree of selective toxicity may be expressed in terms of : THERAPEUTIC DOSE TOXIC DOSE The drug level required for The drug level at which treatment of a particular the agent becomes too infection toxic for the host  THERAPEUTIC DOSE TOXIC DOSE The drug level required for The drug level at which treatment of a particular the agent becomes too infection toxic for the host DRUG, MICROBE, HOST—SOME BASIC INTERACTIONS (1) The drug is administered to the host via a designated route. (2) The drug is dissolved in body fluids. (3) The drug is delivered to the infected area (extracellular or intracellular). (4) The drug destroys the infectious agent or inhibits its growth. (5) The drug is eventually excreted or broken down by the host’s organs, ideally without harming them. CHARACTERISTICS OF AN IDEAL ANTIMICROBIAL DRUG Considerations in Selecting an Antimicrobial Drug Before actual antimicrobic therapy can begin, it is important that at least three factors be known: (1) the nature of the microorganism causing the infection (2) the degree of the microorganism susceptibility (also called sensitivity) to various drugs; and (3) the overall medical condition of the patient. TESTING FOR THE DRUG SUSCEPTIBILITY OF MICROORGANISMS Testing is essential in those groups of bacteria commonly showing resistance, primarily Staphylococcus species, Neisseria gonorrhoeae, Streptococcus pneumoniae, and Enterococcus faecalis, and the aerobic gram-negative enteric bacilli. However, not all infectious agents require antimicrobial sensitivity testing. Dilution Susceptibility Tests: Minimum Inhibitory Concentration (MIC) The MIC is the lowest concentration of a drug that prevents growth of a particular pathogen. The minimal lethal concentration (MLC) is the lowest drug concentration that kills the pathogen. Dilution Susceptibility Tests: Minimum Inhibitory Concentration (MIC) Disk diffusion tests The Kirby-Bauer method is the disk diffusion test most often used. Disk diffusion tests - Etest Unlike the Kirby-Bauer test, the MIC is also determined by the Etest which uses a strip to produce the zone of inhibition. The advantage of the E-test is that the strip contains a gradient of drug calibrated in ug. This way, the MIC can be measured by observing the mark on the strip that corresponds to the edge of the zone of inhibition. ANTIMICROBIA L CHEMOTHERAP SYBSc 1-2 pm 17 Dec, 2024 th Y  70 Acquisition of drug resistance Microbes become newly resistant to a drug after one of the following events occurs: 1. Spontaneous mutations in critical chromosomal genes or 2. Acquisition of entire new genes or sets of genes via transfer from another species. Slight changes in microbial sensitivity, which can be overcome by larger doses of the drug, to complete loss of sensitivity The property of drug resistance can be intrinsic as well as acquired. Intrinsic drug resistance exists naturally and is not acquired through specific genetic changes. Resistance through intermicrobial transfer originates from chromosomal genes and plasmids called resistance (R) factors being transferred through the processes of conjugation, transformation, or transduction Plasmids encoded with drug resistance are naturally present in microorganisms before they have been exposed to the drug. Many bacteria also maintain transposable drug-resistance sequences (transposons) that are duplicated and inserted from one plasmid to another or from a plasmid to the chromosome. Faithfully replicated and inherited by all subsequent progeny  74 Specific mechanisms of drug resistance In general, a microorganism loses its sensitivity to a drug by expressing genes that stop the action of the drug. Gene expression takes the form of: (1) synthesis of enzymes that inactivate the drug, (2) decrease in cell permeability and uptake of the drug, (3) change in the number or affinity of the drug receptor sites, (4) modification of an essential metabolic pathway. Some bacteria can become resistant indirectly by lapsing into dormancy, or, in the case of penicillin, by converting to a cell-wall-deficient form (L form) that penicillin cannot affect. SPECIFIC MECHANISMS OF DRUG RESISTANCE The development of alternative enzymes that inactivate the drug (occurs only when new genes are acquired). Some strains of Neisseria gonorrhoeae, called PPNG (Penicillinase- producing Neisseria gonorrhoeae), have also acquired penicillinase genes, alternative drugs are required to treat gonorrhea Permeability or uptake of drug into bacterium is decreased or eliminated. The outer membrane of the cell wall of certain gram-negative bacteria is a natural blockade for some of the penicillin drugs. Aminoglycoside resistance is known to develop through changes in drug permeability caused by point mutations in proteins of the transport system or outer membrane The microbe engages special drug transport pumps that remove the drug Bacteria possess multidrug-resistant (MDR) pumps (proteins encoded by plasmids or chromosomes), MDRs lack selectivity Binding sites for drug are decreased in number or affinity (can occur via mutation or acquisition of new genes). Erythromycin and clindamycin resistance is associated with an alteration on the 50S ribosomal binding site. Penicillin resistance in Streptococcus pneumoniae and methicillin resistance in Staphylococcus aureus are related to an alteration in the binding proteins in the cell wall Some bacteria may become resistant by shedding their cell wall entirely, converting to a cell-wall-deficient form (L-form) that is unaffected by penicillin. An affected metabolic pathway is shut down or an alternate pathway is used (occurs due to mutation of original enzyme[s]). MULTIDRUG RESISTANCE Some bacteria are resistant to many different antibiotics - they are multidrug- resistant. Multidrug-resistant bacteria can be difficult to treat and facilitate spread of antibiotic resistance. When a bacterium is resistant to at least one antibiotic in three (or more) different antibiotic classes it is said to be multidrug- resistant Can carry one or more resistance mechanism(s), making them resistant to multiple antimicrobials CROSS RESISTANCE In cross-resistance, a single resistance mechanism confers resistance to multiple antimicrobial drugs. For example, having an efflux pump that can export multiple antimicrobial drugs is a common way for microbes to be resistant to multiple drugs by using a single resistance mechanism. Methicillin Resistant Staphylococcus aureus Methicillin, a semisynthetic penicillin, was designed to resist inactivation by β- lactamases MRSA acquires a new low- affinity penicillin-binding protein (PBP2a). This modified protein prevents all β- lactam antibiotics (including methicillin, penicillins, cephalosporins, and carbapenems) from effectively binding and disrupting the bacterial cell wall. Vancomycin Resistant Staphylococcus aureus Enterococcus faecalis and VRSA - a gene called vanA that wards off vancomycin Extended-Spectrum β-Lactamase–Producing Gram-Negative Pathogens Gram-negative pathogens that produce extended-spectrum β-lactamases (ESBLs) show resistance well beyond just penicillins. The spectrum of β- lactams inactivated by ESBLs genes encoding for ESBLs are usually provides for resistance to all found on mobile plasmids that also penicillins, cephalosporins, contain genes for resistance to other monobactams, and the β- drug classes (e.g., fluoroquinolones, aminoglycosides, tetracyclines), and lactamase-inhibitor may be readily spread to other bacteria combinations, but not the by horizontal gene transfer. carbapenems. This acronym refers to the names of the pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter spp.) but it is also fitting in that these pathogens are able to “escape” many conventional forms of antimicrobial therapy. As such, infections by ESKAPE pathogens can be difficult to treat and they cause a large number of nosocomial infections. BIOFILMS AND DRUG RESISTANCE Biofilms can form on natural tissues in the body, like heart valves and teeth, but they also develop on medical devices such as catheters and artificial valves. These devices provide surfaces where bacteria can easily attach and grow. Microbes in infectious biofilms may be hundreds of times more drug resistant than the same free, unattached microbes: microbes are protected by the impenetrable nature of the extracellular matrix microbes within the biofilm communicate with one another if one microbe in a biofilm can inactivate a particular drug, nearby microbes may benefit from the drug-free environment NON-GENETIC BASIS OF RESISTANCE There are several non genetic reasons for the failure of drugs to inhibit the growth of bacteria: Bacteria can be walled off within an abscess cavity that the drug cannot penetrate effectively Bacteria can be in a resting state (i.e., not growing); they are therefore insensitive to cell wall inhibitors such as penicillins and cephalosporins Under certain circumstances, organisms that would ordinarily be killed by penicillin can lose their cell walls, survive as protoplasts, and be insensitive to cell wall–active drugs. The presence of foreign bodies makes successful antibiotic treatment more difficult. NATURAL SELECTION AND DRUG RESISTANCE ANTIMYCOBACTERIAL DRUGS M. tuberculosis and other mycobacterial infections need prolonged treatment have a waxy outer layer have an intracellular location grow and multiply extremely slowly are common and increasing in the wake of the AIDS epidemic in resource-poor countries ANTIMYCOBACTERIAL DRUGS DRUG MECHANISM OF ACTION Isoniazid Inhibits mycolic acid synthesis in mycobacteria Ethambutol Inhibits polymerization of arabinogalactan in the mycobacterial cell wall Pyrazinamide Targets mycolic acid synthesis  94 Antiparasitic chemotherapy Antimalarial Drugs: Quinine and Its Relatives Quinine, extracted from the bark of the cinchona tree, was the principal treatment for malaria for hundreds of years, but it has been replaced by the synthesized quinolines, mainly chloroquine and primaquine, which have less toxicity to humans.  95 Antiparasitic chemotherapy Because there are several species of Plasmodium (the malaria parasite) and many stages in its life cycle, no single drug is universally effective for every species and stage, and each drug is restricted in application. Eg: Primaquine eliminates the liver phase of infection Chloroquine suppresses acute attacks associated with infection of red blood cells.  96 Chemotherapy for Other Protozoan Infections Because there are several species of Plasmodium (the malaria parasite) and many stages in its life cycle, no single drug is universally effective for every species and stage, and each drug is restricted in application. Eg: Primaquine eliminates the liver phase of infection Chloroquine suppresses acute attacks associated with infection of red blood cells.