Lec09 Ch12 Antibiotics and Resistance PDF

Antibiotics and resistance Controlling microbes in the host Principles of Antimicrobial Therapy Goal of antimicrobial chemotherapy: – Administer a drug to an infected person that destroys the infective agent without harming the host’s cells or normal flora The perfect drug does not...

Antibiotics and resistance Controlling microbes in the host Principles of Antimicrobial Therapy Goal of antimicrobial chemotherapy: – Administer a drug to an infected person that destroys the infective agent without harming the host’s cells or normal flora The perfect drug does not exist! Principles of Antimicrobial Therapy Origins of Antimicrobial Drugs Antibiotics are common metabolic products of bacteria and fungi. – Inhibiting the growth of other microorganisms in the same habitat reduces competition for nutrients – Selective advantage has allowed the genes for antibiotics production to be preserved in evolution  Salvarsan – the “Magic Bullet” Arsphenamine Introduced in 1910 First modern chemotherapeutic agent Used to treat syphilis and trypanosomiasis Demonstrated need for selective toxicity Antibacterials target a diverse set of cellular processes Testing for drug susceptibility Kirby Bauer disc diffusion MIC – Minimum Inhibitory Concentration Metabolic activity (Resazurin) Choosing an antimicrobial for treatment Three factors must be known before starting antimicrobial therapy: – The identity of the microorganism causing the infection – The degree of the microorganism’s susceptibility (sensitivity) to various drugs – The overall medical condition of the patient Why is a diagnosis desired before starting treatment? When would treatment be started before a diagnosis, and what characteristics in a drug would you want? Choosing an antimicrobial for treatment Patient History – Preexisting conditions that will influence drug activity – Kidney or Liver disease – Why? – Old, young, pregnant, etc. Type and location of infection Therapeutic Index (TI) of available drugs – Ratio of dose toxic to humans to MIC Why might the treatment fail? Survey of Major Antimicrobial Drug Groups Targeting the Cell Wall: Beta-Lactams Penicillins, Cephalosporins, Carbapenems Penicillin discovered by Alexander Fleming in the 1930s Brought to US for commercialization by Florey and Chain at start of WWII Targeting the Cell Wall: Beta-Lactams Penicillins, Cephalosporins, Carbapenems Beta lactams bind the PG transpeptidase, preventing cell wall synthesis Cell wall is a dynamic structure – new PG is constantly being made Causes cell wall failure and cell rupture Targeting Protein Synthesis: Aminoglycosides, Macrolides, etc. Macrolides – Erythromycin Aminoglycosides – Streptomycin, kanamycin Tetracyclines – Tetracycline, doxycycline – Glycylcyclines Tigecycline Targeting Protein Synthesis: Aminoglycosides, Macrolides, etc. Macrolides (Erythromycin) – bind 50s ribosomal subunit, preventing ribosome translocation Aminoglycosides (Streptomycin, kanamycin) – bind 30s ribosomal subunit, causing misreading of mRNA Tetracyclines – Block tRNA entry to ribosome Targeting Nucleic acid synthesis: Fluoroquinolones, Rifampicin Fluoroquinolones block Ciprofloxacin activity of DNA gyrase and topoisomerase – Prevents DNA synthesis – DNA can’t be unwound – polymerases can’t transit down DNA strand Rifampicin binds RNA polymerase – Prevents mRNA synthesis – Sits in channel and physically blocks growing Rifampicin RNA chain Targeting Cell Membranes Polycationic Anti-Microbial Peptides (AMPs) Post-translationally modified peptide chains Bind to membranes and LPS Polymyxin B Disrupt membrane stability – Holes are formed – Cellular contents leak out – Cell lyses Colistin  Targets of anti-fungals Many target ergosterol and cell wall synthesis Few if any target ribosomes or other core metabolic processes – Why? Targeting Fungal infections Azoles and Allylamines Allylamines Azoles and Allylamines block ergosterol formation – Work at different steps in pathway – Ultimately destabilizes Azoles membrane fluidity Many available OTC Azoles – Terbinafine (allylamine) = Lamisil – Clotrimazole (azole) = Lotrimin AF Using phage to target bacteria Phage can specifically target specific species There is a phage for every bacteria Leaves microbiome intact Phage being engineered with CRISPR to improve efficacy Targeting viral infections Limited targets – viruses use host machinery viral polymerase inhibitors – Reverse transcriptase – RNA-Dependent RNA Pol Nucleoside analogs – Error prone viral polymerases will incorporate in viral genomes – Leads to “error catastrophe” Integrase inhibitors Protease inhibitors Antiprotozoal and Antihelminthic Chemotherapy Antimalarial drugs: – Quinine: principal treatment for hundreds of years Anti-malarial treatment – now consumed in tonic water Replaced by chloroquine and primaquine: less toxicity Chemotherapy for other protozoan infections: – Metronidazole: amoebicide Antihelminthic drug therapy: – Mebendazole and albendazole block microtubule assembly and stability – Pyrantel and Praziquantel Alter calcium influx Cause contraction and detachment from intestinal wall Side effects of antibiotic use Allergy – Immune system recognizes drug as foreign Organ toxicity – Kidney, liver, bones, others Disrupt normal flora – Antibiotics don’t distinguish between pathogens and microbiota – Results in superinfection Clostridium difficile (C. diff) in GI tract – has implications for immune tolerance of microbiota Development of antibiotic resistance Destroy the drug Block drug entry Efflux drug Alter target to prevent binding Bypass targeted pathway Antibiotic resistance rises through Natural Selection Random mutations in a population give rise to resistant individuals Antibiotic selection kills susceptible cells Resistant cells replicate to fill niche  Resistance genes found in Klebsiella pneumoniae BAA-2146 – Aminoglycoside acetyltransferase – Macrolide exporter MacAB – Fosfomycin resistance FosA – DNA gyrase, DNA topoisomerase (Quinolones) – Strept/Spectinomycin O-adenylyltransferases – Efflux systems CmeABC (RND) Multidrug and Toxin Extrusion (MATE) AcrAB/TolC MdtABCD Others… – Colistin V, E2, and other Bacteriocin resistance (multiple systems) – Metal resistance – Chromium, Copper, Mercury, Arsenic, Zinc, Cobalt… – “Classic” b-lactamases – NDM-1 New Delhi Metallo-b-lactamase Antibiotic resistance timeline New antibiotics are not in the pipeline for development  Spontaneous evolution of antibiotic resistance https://www.youtube.com/watch?v=plVk4NVIUh8 Kishony Lab, Harvard Medical Center