Antimicrobial Chemotherapy Lecture Notes PDF

ANTIMICROBIAL CHEMOTHERAPY By: Dr. Haidy Samir Mohamed khalil Professor of Medical Microbiology & Immunology Faculty of Medicine Helwan University Let us start by …..  Objectives Explain important definitions Identify Mechanisms of Action of Chemoth...

ANTIMICROBIAL CHEMOTHERAPY By: Dr. Haidy Samir Mohamed khalil Professor of Medical Microbiology & Immunology Faculty of Medicine Helwan University Let us start by …..  Objectives Explain important definitions Identify Mechanisms of Action of Chemotherapeutics Identify Mechanisms of Resistance to Antimicrobial Agents Compare between non-genetic and genetic Origin of Resistance to Antimicrobial Agents Explain Complications of Antibacterial Chemotherapy Define Chemoprophylaxis and its uses Enumerate the clinical uses of antibiotics Enumerate the effect of combination of 2 antibiotics  Chemotherapy It is the treatment of infectious diseases by administration of drugs which are lethal or inhibitory to the causative organisms.  An antibiotic It is an antimicrobial substance produced by a living microorganism and is active in high dilutions. Many of these antibiotics have been chemically synthesized. The term is used for any antimicrobial chemotherapeutic agent whether naturally produced or chemically synthesized. Synthetic modifications of previously discovered drugs allowed the development of several new antimicrobial agents.  Bactericidal drugs They have a rapid killing action of bacteria, which is irreversible. Those are particularly useful in certain infections, e.g. those that are immediately life-threatening as in severe leucopenic patients and endocarditis. Examples include penicillins, cephalosporins and aminoglycosides.  Bacteriostatic drugs They inhibit bacterial multiplication, but do not kill them. The bacteria can grow again when the drug is withdrawn. In this case, host defence mechanisms, such as phagocytosis, are required to kill bacteria. Examples include sulphonamides, tetracyclines and chloramphenicol.  Spectrum of Action of Chemotherapeutics: Broad-spectrum antibiotics Narrow-spectrum antibiotics  Cont. Broad-spectrum antibiotics: those are active against several types of microorganisms, both gram positive and gram negative e.g. tetracyclines, chloramphenicol and ampicillin. Narrow-spectrum antibiotics are active against one or very few types, e.g. vancomycin is primarily used against certain gram positive cocci i.e. Staphylococci and Enterococci.  Mechanisms of Action of Chemotherapeutics An ideal antimicrobial agent should have selective toxicity, i.e. it can kill or inhibit the growth of a microorganism in concentrations that are not harmful to the cells of the host. Disinfectants: e.g. phenol and antiseptics, e.g. alcohol and iodine, destroy bacteria but they are highly toxic to tissue cells and are unsuitable for use as chemotherapeutic agents.  Thus, the mechanism of action of a chemotherapeutic must depend on the inhibition of a metabolic channel or a structure that is present in the microbe (e.g. peptidoglycan) but not present in the host cell. Several mechanisms are known: 1- Inhibition of Bacterial Cell Wall Synthesis 2- Inhibition of Bacterial Cytoplasmic Membrane Functions 3- Inhibition of Bacterial Protein Synthesis 4- lnhibition of Bacterial Nucleic Acid Synthesis 5- Competitive Inhibition  1- Inhibition of Bacterial Cell Wall Synthesis: Due to its unique structure and function, the bacterial cell wall is an ideal point of attack by selective toxic agents. Penicillin, cephalosporins and vancomycin, interfere with cell wall synthesis and cause bacteriolysis. Beta-lactams e.g. penicillin and cephalosporins and the glycopeptides e.g. vancomycin inhibit peptidoglycan synthesis.  However, beta-lactams inhibit the final steps in synthesis of peptidoglycan by binding to receptors called penicillin- binding proteins (PBPs) in the cell wall. Vancomycin on the other hand, inhibits the early steps in peptidoglycan synthesis by a different mechanism. That is why vancomycin is effective in treatment of beta- lactam resistant staphylococcal infections (MRSA).  2- Inhibition of Bacterial Cytoplasmic Membrane Functions: Some antibiotics cause disruption of the cytoplasmic membrane and leakage of cellular proteins and nucleotides leading to cell death. Polymyxins, amphotericin B, and nystatin are examples. These drugs are highly toxic as they have a narrow margin of selective toxicity.  3- Inhibition of Bacterial Protein Synthesis: Several drugs inhibit protein synthesis in bacteria without significantly interfering with protein synthesis in human cells. This selectivity is due to the differences between bacterial and human ribosomal proteins, RNA, and associated enzymes.  Bacteria have 70S ribosomes (with 50S and 30S subunits), whereas human cells have 80S ribosomes (with 60S and 40S subunits). Chloramphenicol, erythromycin, linezolid and streptogramins (qumupristin/dalfopristin) act on 50S subunits, while tetracycline and aminoglycosides (gentamicin and arnikin) act on 3 OS subunits.  4- lnhibition of Bacterial Nucleic Acid Synthesis: These can act on any of the steps of DNA or RNA replication. Quinolones and novobiocin inhibit DNA synthesis by blocking DNA gyrase. Rifampicin inhibits RNA synthesis by binding to RNA polymerase. Trimethoprim and sulfonamides inhibit nucleotide synthesis.  5- Competitive Inhibition: In which the chemotherapeutic agent competes with an essential metabolite for the same enzyme. Para-aminobenzoic acid (PABA) is an essential metabolite for many organisms. They use it as a precursor in folic acid synthesis which is essential for nucleic acid synthesis. Sulphonamides are structural analogues to PABA so they enter into the reaction in place of PABA and compete for the active center of the enzyme thus inhibiting folic acid synthesis.  Mechanisms of Resistance to Antimicrobial Agents: The mechanisms by which the organism develops resistance may be one of the following: 1. Bacteria produce enzymes that inactivate the drug 2. Synthesize modified targets 3. New metabolic pathways 4. Decrease their permeability 5. Bacteria actively pump the drug out  1- Bacteria produce enzymes that inactivate the drug Production of beta-lactamases that cleave beta- lactam ring in penicillins and cephalosporins. Esterases hydrolyze the lactone ring of macrolides. Acetyl-transferase produced by gram negative bacilli inactivates chloramphenicol.  2- Bacteria synthesize modified targets against which the drug has no effect Resistance to aminoglycosides is associated with alteration of a specific protein in the 30S subunit of the bacterial ribosome. Alteration of the penicillin-binding proteins is a mode of resistance to penicillin and methicillin by MRSA.  3- Bacteria develop new metabolic pathways that bypass the reaction inhibited by the drug Sulphonamide resistant bacteria acquire the ability to use preformed folic acid with no need for extracellular PABA.  4- Bacteria decrease their permeability to the drug Such that an effective intracellular concentration is not achieved. Changes in porins (hollow membrane proteins) can reduce the amount of penicillin entering bacteria.  5- Bacteria actively pump the drug out Bacteria actively pump the drug out across the cytoplasmic membrane using efflux pump or "multidrug resistance pump" (MDR). The MDR pump imports protons and, in an exchange-type reaction, exports a variety of foreign molecules including certain antibiotics, such that an effective intracellular concentration of the drug is not achieved, as in resistance to quinolones and macrolides.  Origin of Resistance to Antimicrobial Agents A- Non-genetic Drug Resistance: B- Genetic Acquired Drug Resistance: 1- Metabolic inactivity 1- Plasmid mediated resistance 2- Loss of target structure: 2- Transposon and integron 3- Bacteria may be walled off mediated resistance within an abscess 3- Chromosomal drug 4- Intrinsic natural resistance resistance  CLSI … Do you know it?  Origin of Resistance to Antimicrobial Agents A- Non-genetic Drug Resistance: 1- Metabolic inactivity: Most antimicrobial agents act effectively only on replicating cells. Non-multiplying organisms are phenotypically resistant to drugs. Tubercle bacilli survive for years in tissues and then- resistance to anti-tuberculous drugs is due in part to their metabolic inactivity (dormancy). 2- Loss of target structure: Protoplasts or L-forms of bacteria are penicillin resistant, having lost their cell wall which is the structural target site of the drug. 3- Bacteria may be walled off within an abscess cavity that the drug can not penetrate effectively. 4- Intrinsic natural resistance, e.g. mycoplasma are naturally resistant to penicillin because they lack a cell wall enterococci are naturally resistant to cephalosporins as they lack the receptor for the drug.  B- Genetic Acquired Drug Resistance: 1- Plasmid mediated resistance Resistance (R) factors are a class of plasmids that mediate resistance to one or more antimicrobial agent. Plasmids frequently carry genes that code for the production of enzymes that inactivate antimicrobial agents, e.g. beta-lactamase which destroys the beta lactam ring in penicillin and cephalosporins. Plasmids may result in epidemic resistance among bacteria by moving from one to the other by conjugation, transduction or transformation. 2- Transposon and integron mediated resistance Many transposons carry genes that code for drug resistance. As they move between plasmids and chromosomes they can transfer this property to bacteria. The process is called transposition. Five or more resistance genes may be contained in a single integron causing dissemination of drug resistance genes among gram negative bacteria. 3- Chromosomal drug resistance This develops as a result of spontaneous mutation in a gene that controls susceptibility to an antimicrobial agent. The most common result of chromosomal mutation is alteration of the receptors for a drug. For example, streptomycin resistance can result from a mutation in the chromosomal gene that controls the receptor for streptomycin, located in the 30s bacterial ribosome.  Complications of Antibacterial Chemotherapy 1- Development of drug resistance 2- Drug toxicity 3- Superinfection 4- Hypersensitivity  Complications of Antibacterial Chemotherapy 1- Development of drug resistance: This is one of the most serious complications of chemotherapy. The emergence of resistant mutants is encouraged by inadequate dosage, prolonged treatment, the presence of a closed focus of infection and the abuse of antibiotics without in vitro susceptibility testing. The problem is more serious when resistant strains develop in hospitals. About 90% of hospital strains of Staph, aureus are resistant to penicillin. 2- Drug toxicity: Many of the antibacterial drugs have toxic side effects. This can be due to overdosage, prolonged use or narrow margin of selective toxicity, Streptomycin affects the 8th cranial nerve leading to deafness. Chloramphenicol may cause bone marrow depression. Aminoglycosides, (e.g. gentamicin, tobramycin) are nephrotoxic. Tetracyclines inhibit growth and development of bones and teeth in the developing foetus and infants. 3- Superinfection: a- Superinfection may occur by pre-existing resistant strain present in the environment e.g. penicillin resistant Staph, aureus in hospital infections. b- Another type of superinfection is due to suppression of normal flora by the antibiotic used and their replacement with drug resistant organisms which cause disease, e.g.: i- Overgrowth of Candida in the vagina causing vaginitis or in the mouth causing oral thrush. ii- Prolonged oral chemotherapy leading to suppression of intestinal flora and overgrowth of staphylococci causing staphylococcal enterocolitis orCV. difficile which causes pseudomembranous colitis iii- Overgrowth of naturally drug resistant gram negative organisms, e.g. pseudomonas, proteus or enterobacter, may account for respiratory tract superinfection. 4- Hypersensitivity: The drug may act as a hapten, binds to tissue proteins, and stimulates an exaggerated immune response leading to tissue damage, i.e. hypersensitivity. Any type of hypersensitivity reaction can occur with several antibiotics. The most serious is anaphylactic shock, this may occur with penicillin or cephalosporins. Milder manifestations may be urticaria, purpural eruptions, skin rash, diarrhoea, vomiting and jaundice.  Chemoprophylaxis: Chemoprophylaxis is the use of antimicrobial agents to prevent rather than to treat infectious diseases. The following are principal conditions for which prophylactic antibiotics are positively indicated: 1- The use of benzathine penicillin G injections every 3- 4 weeks to prevent reinfection with Strept. pyogenes in rheumatic patients. 2- A single large dose of amoxicillin given immediately prior to dental procedures is recommended for patients with congenital or rheumatic heart disease to prevent endocarditis. 3- The oral administration of rifampicin 600 mg twice a day for 2 days to exposed persons during epidemics of meningococcal meningitis. 4- Oral administration of tetracycline to prevent cholera.  Cont. 5- Women identified as vaginal carriers of Str. agalactiae should receive ampicillin intravenously at least 4 hours before delivery to prevent occurrence of neonatal sepsis and meningitis. 6- Chemoprophylaxis in surgery is indicated in the following conditions: a- Large bowel surgery b- Major orthopedic and cardiac surgery. c- Amputation of an ischaemic limb.  Clinical Use of Antibiotics: The objective of antibiotic therapy is to cure the patient with minimal complications. At the same time, it is important to discourage the emergence of drug-resistant organisms. The following principles should be observed: 1- Antibiotics should not be given for trivial infections. 2- They should be used for prophylaxis only in special circumstances. 3- Treatment should be based on a clear clinical and bacteriological diagnosis. Suitable specimens should be sent to the laboratory before treatment is begun. However, "empirical treatment" can be started after taking the sample; but should be modified later according to results of antibiotic sensitivity testing in vitro. 4- Antibiotics for systemic treatment should be given in full therapeutic doses, by the proper route of administration and for adequate periods. 5- Combined therapy with two or more antibiotics is required in some conditions, e.g.: a- Serious infections e.g. infective endocarditis or meningitis. b-In the treatment of T.B. to delay emergence of drug resistant mutants c- Sepsis by resistant organisms in immunocompromised patients d- Febrile neutropaenic patients. e-Severe mixed infections e.g. peritonitis following perforation of the colon or compound fractures.  Combination of two drugs may result in one of several interactions:- 1- Indifference (1+1=1) 2- Addition (1+1=2) 3- Synergism (1+1= >2) 4- Antagonism (1+1 = < 1) 1- Indifference, i.e., the combined action is no greater than that of the most effective agent when used alone. (1+1=1) 2- Addition, i.e., the combined action is equivalent to the sum of the actions of each drug when used alone. (1+1=2) 3- Synergism, i.e., the combined action is significantly greater than the sum of the two drugs acting separately, e.g. combination of penicillin and an aminoglycoside against enterococci, because penicillin damages the cell wall sufficiently to enhance the entry of the aminoglycoside. (1+1= >2) 4- One drug may antagonize the action of the other e.g. the use of penicillin combined with the bacteriostatic drug tetracycline in the treatment of meningitis caused by pneumococci. Tetracycline inhibits the growth of the organism, thereby preventing the bactericidal effect of penicillin, which kills multiplying organisms only. (1+1 = < 1) Development of antimicrobial stewardship …. Is it a must ? The more you use it , the faster you loose it Q&A  References Manual of medical microbiology and immunology- Abla M. El- Mishad, ninth edition, 2015. Jawetz, Melnick & Adelberg's Medical Microbiology, 27th Edition, 2017. Review of Medical Microbiology and Immunology (LANGE Basic Science) - Warren Levinson, 14th Edition, 2016.  Personal contact data [email protected] [email protected] 01091584654 Haidy Samir Mohamed Khalil 4/1/24