MIC1204: Antibiotics, Resistance and Superbugs PDF

MIC1204: Introduction to Microbiology and Immunology Antibiotics, resistance, and superbugs JS Cummings Antimicrobial Terms  Chemical Disinfectants, Antiseptics, and Topical Antimicrobial Agents Modes of Antimicrobial Action Understanding that microbes have adapted to living in...

MIC1204: Introduction to Microbiology and Immunology Antibiotics, resistance, and superbugs JS Cummings Antimicrobial Terms  Chemical Disinfectants, Antiseptics, and Topical Antimicrobial Agents Modes of Antimicrobial Action Understanding that microbes have adapted to living in particular environments, how can they be targeted in situ or in vivo? The following are actions used to target microbes: DNA Damage Denaturation of Proteins Disruption of Cell membrane/wall Disruption of free sulfhydryl groups Chemical antagonism DNA Damage A number of physical and chemical agents act by damaging DNA; these include ionizing radiations, ultraviolet light, and DNA- reactive chemicals. Among the last category are alkylating agents and other compounds that react covalently with purine and pyrimidine bases to form DNA adducts or interstrand cross-links. Radiation damage DNA in several ways: Ultraviolet light, for example, induces cross-linking between adjacent pyrimidines on one or the other of the two polynucleotide strands, forming pyrimidine dimers; ionizing radiations produce breaks in single and double strands. Radiation-induced and chemically-induced DNA lesions kill the cell mainly by interfering with DNA replication.  Protein Structure Levels Denaturation of Proteins Proteins exist in a folded, three-dimensional state determined primarily by intramolecular noncovalent interactions such as ionic, hydrophobic, and hydrogen bonds or covalent disulfide linkages. This state is called the tertiary structure of the protein; it is readily disrupted by a number of physical (eg, heat) or chemical (eg, alcohol) agents, causing the protein to become nonfunctional. The disruption of the tertiary structure of a protein is called protein denaturation. Disruption of the Cell Membrane or Wall The cell membrane acts as a selective barrier, allowing some solutes to pass through and excluding others. Many compounds are actively transported through the membrane, becoming concentrated within the cell. The membrane is also the site of enzymes involved in the biosynthesis of components of the cell envelope. Substances that concentrate at the cell surface may alter the physical and chemical properties of the membrane, preventing its normal functions and therefore killing or inhibiting the cell. The cell wall acts as a corseting structure (best characterized as a fishing net), protecting the cell against osmotic lysis. Thus, agents that destroy the wall (eg, lysozyme, which cleaves the sugar linkages) or prevent its normal synthesis (eg, penicillin, which interrupts peptidyl crosslinkages) may bring about lysis of the cell. -lactam known as Carbapenem as a classic example, blocking enzymes that construct the cell wall. Disruption of Free Sulfhydryl Groups Enzymes containing cysteine have side chains terminating in sulfhydryl groups. In addition to these, coenzymes such as coenzyme A and dihydrolipoate contain free sulfhydryl groups. Such enzymes and coenzymes cannot function unless the sulfhydryl groups remain free and reduced. Oxidizing agents thus interfere with metabolism by forming disulfide linkages between neighboring sulfhydryl groups: R—SH + HS—R –––> R—S—S—R (+2H) Many metals such as mercuric ion likewise interfere by combining with sulfhydryls. There are sulfhydryl- containing enzymes in the cell, so oxidizing agents and heavy metals do widespread damage. Chemical Antagonism Enzymes are known to perform critical functions enabling cellular activities that contribute to an organism’s ability to survive in its environment. The interference by a chemical agent with the normal reaction between a specific enzyme and its substrate is known as chemical antagonism. The antagonist acts by combining with some part of the enzyme thereby preventing attachment of the normal substrate. An antagonist combines with an enzyme because of its chemical affinity for an essential site on that enzyme. Enzymes perform their catalytic function by virtue of their affinity for their natural substrates; hence any compound structurally resembling a substrate in essential aspects may also have an affinity for the enzyme. If this affinity is great enough, the “analog” will displace the normal substrate and prevent the proper reaction from taking place. Antibiotics Definition - Naturally occurring and synthetically derived organic compounds that inhibit or destroy selective bacteria, generally at low concentrations. The rational selection of antimicrobial drugs depends on two specific considerations: diagnosis susceptibility Diagnosis Specific etiologic diagnosis must be formulated. This can often be done on the basis of a clinical impression. Thus, in typical lobar pneumonia or acute urinary tract infection, the relationship between the clinical picture and causative agent is sufficiently constant to permit selection of the antibiotic of choice on the basis of clinical impression alone. Even in these cases, however, as a safeguard against diagnostic error, it is preferable to obtain a representative specimen for bacteriologic study before giving antimicrobial drugs. In most infections, the relationship between causative agent and clinical picture is not constant. It is therefore important to obtain proper specimens for bacteriologic identification of the causative agent. As soon as such specimens have been secured, chemotherapy can be started on the basis of the “best guess.” After the causative agent has been identified by laboratory procedures, the initial regimen can be modified as necessary.  The “best guess” of a causative organism is based on the Diagnosis following considerations, among others: the site of infection (eg, pneumonia, urinary tract Continued infection) the age of the patient (eg, meningitis: neonatal, young child, adult), the place where the infection was acquired (hospital versus community), mechanical predisposing factors (indwelling vascular catheter, urinary catheter, ventilator, exposure to vector), and predisposing host factors (immunodeficiency, corticosteroids, transplant, cancer chemotherapy). When the causative agent of a clinical infection is known, the drug of choice can often be selected on the basis of current clinical experience. At other times, laboratory tests for antibiotic susceptibility are necessary to determine the drug of choice. Susceptibility Laboratory tests for antibiotic susceptibility are indicated in the following circumstances: 1. when the microorganism recovered is of a type that is often resistant to antimicrobial drugs (eg, gram-negative enteric bacteria); 2. when an infectious process is likely to be fatal unless treated specifically (eg, meningitis, septicemia); and 3. in certain infections where eradication of the infectious organisms requires the use of drugs that are rapidly bactericidal, not merely bacteriostatic (eg, infective endocarditis).  The indications for administration of antibiotics must sometimes be Dangers Of qualified by the following concerns: 1. Widespread sensitization of the population, with resulting Indiscriminate Use hypersensitivity, anaphylaxis, rashes, fever, blood disorders, cholestatic hepatitis, and perhaps collagen-vascular diseases 2. Changes in the normal microbiota of the body, with disease resulting from “superinfection” caused by overgrowth of drug- resistant organisms 3. Masking serious infection without eradicating it (eg, the clinical manifestations of an abscess may be suppressed while the infectious process continues) 4. Direct drug toxicity (eg, granulocytopenia or thrombocytopenia with cephalosporins and penicillins and renal damage or auditory nerve damage from aminoglycosides). 5. Development of drug resistance in microbial populations, chiefly through the elimination of drug-sensitive microorganisms from antibiotic-saturated environments (eg, hospitals) and their eplacement by drug-resistant microorganisms. Major Antibiotic Classes PENICILLINS CEPHALOSPORINS OTHER -LACTAM DRUGS TETRACYCLINES GLYCYLCYCLINES CHLORAMPHENICOL ERYTHROMYCINS CLINDAMYCIN AND LINCOMYCIN GLYCOPEPTIDES AND LIPOPEPTIDES Penicillin The story of penicillin's discovery is often cited as a prime example of scientific serendipity. In 1928, Scottish bacteriologist Alexander Fleming noticed mold growing on a petri dish containing Staphylococcus aureus bacteria. The mold, later identified as Penicillium notatum, had inhibited the growth of the bacteria. This sparked his curiosity, leading to the isolation and characterization of the world's first antibiotic – penicillin. Prokaryotic Genetic Structure Central Dogma of Molecular Biology Nucleus Cytoplasm Transcription Translation DNA RNA Protein  DNA Structure Antibiotic Resistance Review of common antimicrobial actions of antibiotics DNA Damage Denaturation of Proteins Disruption of cell membrane or cell wall Disruption of free sulfhydryl groups Chemical antagonism These methods of elimination are all designed to target a physical characteristic of the microbe The development of resistance to antibiotic compounds can occur in several ways including: Non-genetic origin of resistance Chromosomal resistance Extrachromosomal resistance Most drug-resistant microbes emerge as a result of genetic change and subsequent selection processes by antimicrobial drugs. Nongenetic Origin of Drug Resistance Active replication of bacteria is required for most antibacterial drug actions. Consequently, microorganisms that are metabolically inactive (nonmultiplying) may be phenotypically resistant to drugs. However, their offspring are fully susceptible. Example: Mycobacteria often survive in tissues for many years after infection yet are restrained by the host’s defenses and do not multiply. Such “persisting” organisms are resistant to treatment and cannot be eradicated by drugs. Yet if they start to multiply (eg, after suppression of cellular immunity in the patient), they are fully susceptible to the same drugs. Microorganisms may lose the specific target structure for a drug for several generations and thus be resistant. Example: Penicillin-susceptible organisms may change to cell wall–deficient L forms during penicillin administration. Lacking cell walls, they are resistant to cell wall–inhibitor drugs (penicillins, cephalosporins) and may remain so for several generations. Microorganisms may infect the host at sites where antimicrobials are excluded or are not active. Example: Aminoglycosides such as gentamicin are not effective in treating Salmonella enteric fevers because the salmonellae are intracellular and the aminoglycosides do not enter the cells  This develops as a result of spontaneous mutation in a locus that controls susceptibility to a given antimicrobial drug. The presence of the antimicrobial drug serves as a selecting mechanism to suppress susceptible organisms and favor the growth of drug-resistant mutants. Spontaneous mutation occurs with a frequency of ~10–12–10–7 and thus is an infrequent cause of the emergence of clinical drug resistance in a given patient. Chromosomal However, chromosomal mutants resistant to rifampin occur with high frequency (~10–7–105). Consequently, treatment of bacterial Resistance infections with rifampin as the sole drug often fails. Chromosomal mutants are most commonly resistant by virtue of a change in a structural receptor for a drug. Example, the P 12 protein on the 30S subunit of the bacterial ribosome serves as a receptor for streptomycin attachment. Mutation in the gene controlling that structural protein results in streptomycin resistance. Mutation can also result in the loss of Penicillin Binding Proteins (PBP), making such mutants resistant to β-lactam drugs.  Bacteria often contain extrachromosomal genetic elements called plasmids. Some plasmids carry genes for resistance to one—and often several— antimicrobial drugs. Plasmid genes for antimicrobial resistance often control the formation of enzymes capable of Extrachromosomal destroying the antimicrobial drugs. Thus, plasmids Resistance determine resistance to penicillins and cephalosporins by carrying genes for the formation of β-lactamases. Plasmids code for enzymes that acetylate, adenylate, or phosphorylate various aminoglycosides; for enzymes that determine the active transport of tetracyclines across the cell membrane; and for others. Genetic material and plasmids can be transferred by transduction, transformation, and conjugation.  Antimicrobial Resistance that arises from any of the above-mentioned origins can be effected through different mechanisms including each of the following: 1. Inactivation of -lactam molecules by the microbial production of -lactamases 2. Decreased penetration to the target site Resistance (e.g., the resistance of Pseudomonas Mechanisms aeruginosa) 3. Alteration of target site PBPs (e.g., penicillin resistance in pneumococci) 4. Efflux from the periplasmic space through specific pumping mechanisms Notable examples of Superbugs Methicillin-resistant Staphylococcus aureus (MRSA) Carbapenem-resistant Enterobacteriaceae (CRE) Extensively drug-resistant tuberculosis (XDR- TB) Antibiotic Resistant Klebsiella (Nakry) Methicillin-resistant Staphylococcus aureus (MRSA) Most specimens of SA circulating in 1944 were susceptible to penicillin G Widespread use of penicillin lead to population level resistance that accounted for 65-85% of hospital samples collected -lactamase chief method of resistance Advent of lactamase resistant penicillins provided respite (nafcillin, methicillin, oxacillin) Strains eventually mutated into resistance Vancomycin remains chief option in treating MRSA though evidence of genetic mutation conferring resistance has been observed (VRSA)  Given the frequency of the emergence of drug Limitation Of Drug resistance among microbes, the following Resistance considerations should be applied in the healthcare setting: 1. maintaining sufficiently high levels of the drug in the tissues to inhibit both the original population and first-step mutants; 2. simultaneously administering two drugs that do not give cross-resistance, each of which delays the emergence of mutants resistant to the other drug (eg, rifampin and isoniazid [INH] in the treatment of tuberculosis); and 3. avoiding exposure of microorganisms to a particularly valuable drug by limiting its use, especially in hospitals.