chapter 12 drugs, microbes, host – the elements of chemotherapy
TRANSCRIPT
Chapter 12Chapter 12
Drugs, Microbes, Host – The Drugs, Microbes, Host – The Elements of ChemotherapyElements of Chemotherapy
IntroductionIntroduction
TerminologyTerminologyBacteriostatic (inhibits growth) vs. Bactericidal (kills Bacteriostatic (inhibits growth) vs. Bactericidal (kills organisms)organisms)Broad spectrum vs. Narrow spectrumBroad spectrum vs. Narrow spectrumGeneric (chemical composition of drug) vs. Trade name Generic (chemical composition of drug) vs. Trade name (name given to a drug by a manufacture(name given to a drug by a manufactureSources of antimicrobial agentsSources of antimicrobial agentsAntibiotics - growth products of organismsAntibiotics - growth products of organismsChemosynthetic agents - synthesized in a laboratoryChemosynthetic agents - synthesized in a laboratorySemi-synthetic agents - growth products of organisms Semi-synthetic agents - growth products of organisms that have been chemically altered in laboratorythat have been chemically altered in laboratory
Colony of Colony of StreptomycesStreptomyces, one of nature’s , one of nature’s most prolific antibiotic producersmost prolific antibiotic producers
Major targets of drugs acting on Major targets of drugs acting on bacterial cellsbacterial cells
Mode of action of antimicrobial agents: Mode of action of antimicrobial agents: competitive inhibition competitive inhibition
Antibacterial drugsAntibacterial drugsInvolves inhibiting Involves inhibiting synthesis of a critical synthesis of a critical metabolitemetaboliteBacteriostatic in activityBacteriostatic in activityExamplesExamples
Sulfa - competition based Sulfa - competition based on chemical similarity on chemical similarity between sulfas and PABA between sulfas and PABA which is needed for folic which is needed for folic acid synthesis; folic acid acid synthesis; folic acid critical in protein and NA critical in protein and NA synthesis; side effect = synthesis; side effect = possible kidney damage possible kidney damage from crystalsfrom crystals
Mode of action of antimicrobial agents: Mode of action of antimicrobial agents: inhibition of cell wall synthesisinhibition of cell wall synthesis
Inhibition of cell wall Inhibition of cell wall synthesissynthesisInterfere with one of Interfere with one of multiple steps in CW multiple steps in CW synthesissynthesisGreater effect Greater effect demonstrated against demonstrated against G(+) than G(-) – most G(+) than G(-) – most act against synthesis of act against synthesis of peptidoglycan; many peptidoglycan; many drugs cannot penetrate drugs cannot penetrate LPSLPSGenerally considered to Generally considered to be bactericidal - cause be bactericidal - cause lysislysis
Mode of action of antimicrobial agents: Mode of action of antimicrobial agents: inhibition of cell wall synthesisinhibition of cell wall synthesis
Drugs with beta lactam ring - prevent Drugs with beta lactam ring - prevent cross-linking in last step of synthesiscross-linking in last step of synthesis
PenicillinsPenicillins Original penicillins effective only Original penicillins effective only
against G(+) - esp. staph & strepagainst G(+) - esp. staph & strep Semisynthetic penicillins Semisynthetic penicillins
developed to overcome developed to overcome penicillinase (beta lactamase) penicillinase (beta lactamase) producing strains (i.e.methicillin, producing strains (i.e.methicillin, oxacillin & nafcillin) and broaden oxacillin & nafcillin) and broaden spectrum to G(-) (i.e. ampicillin & spectrum to G(-) (i.e. ampicillin & carbenicillin)carbenicillin)
Toxicity involves hypersensitivityToxicity involves hypersensitivity
Mode of action of antimicrobial agents: Mode of action of antimicrobial agents: inhibition of cell wall synthesisinhibition of cell wall synthesis
CephalosporinsCephalosporins Broad spectrum antibioticsBroad spectrum antibiotics Used when individuals are Used when individuals are
allergic to penicillinallergic to penicillin Modifications to original drug Modifications to original drug
increased spectrum increased spectrum (generations 1, 2, and 3)(generations 1, 2, and 3)
MoxalactamsMoxalactamsOther drugs - interfere at other Other drugs - interfere at other stepssteps
CycloserineCycloserine Bacitracin - used primarily as Bacitracin - used primarily as
topical (on the skin - not taken topical (on the skin - not taken internally because of toxicity)internally because of toxicity)
VancomycinVancomycin
Mode of action of antimicrobial agents: Mode of action of antimicrobial agents: Inhibition of protein synthesis Inhibition of protein synthesis
Inhibit one of many steps in protein synthesisInhibit one of many steps in protein synthesisExamplesExamples
Aminoglycosides - includes streptomycin, Aminoglycosides - includes streptomycin, kanamycin, tobramycin, gentamicin, kanamycin, tobramycin, gentamicin, neomycin, and amikacin; causes neomycin, and amikacin; causes misreading of mRNA; bactericidal; misreading of mRNA; bactericidal; synergistic with penicillinssynergistic with penicillins
Tetracyclines - block binding to tRNA; Tetracyclines - block binding to tRNA; bacteriostatic; broad spectrumbacteriostatic; broad spectrum
Chloramphenicol - prevent peptide bond Chloramphenicol - prevent peptide bond formation; bacteriostatic; excellent formation; bacteriostatic; excellent penetration of CNS (useful for treating penetration of CNS (useful for treating meningitis); side effect = aplastic anemiameningitis); side effect = aplastic anemia
Erythromycin - antimicrobial spectrum Erythromycin - antimicrobial spectrum similar to penicillin; used especially in similar to penicillin; used especially in penicillin-allergicpenicillin-allergic
Lincomycin and clindamycin - Lincomycin and clindamycin - bacteriostatic; clindamycin useful for bacteriostatic; clindamycin useful for anaerobes; may cause anaerobes; may cause pseudomembranous colitispseudomembranous colitis
Structure of aminoglycoside, tetracycline, Structure of aminoglycoside, tetracycline, chloramphenicol, and erythromycinchloramphenicol, and erythromycin
Effects of drugs on bacterial cell Effects of drugs on bacterial cell membranemembrane
Act directly on cell Act directly on cell membranes (do not need membranes (do not need to enter cell to cause to enter cell to cause damage)damage)
Bactericidal - produce Bactericidal - produce irreversible damage to irreversible damage to membrane permeabilitymembrane permeability
Example = polymyxins Example = polymyxins and colistinand colistin
Highly toxic to kidneys Highly toxic to kidneys and nervesand nerves
Antifungal drugs Antifungal drugs Damage cell membranesDamage cell membranes
Bind or interfere with ergosterol Bind or interfere with ergosterol (unique fungal sterol in (unique fungal sterol in membrane)membrane)
ExamplesExamplesPolyenes - nystatin and Polyenes - nystatin and amphotericin Bamphotericin BImidazoles - clotrimazole, Imidazoles - clotrimazole, miconazole (topical) and miconazole (topical) and ketoconazole (systemic)ketoconazole (systemic)
Inhibition of nucleic acid Inhibition of nucleic acid synthesissynthesis
Griseofulvin - interferes with Griseofulvin - interferes with mitosis; selectively binds to mitosis; selectively binds to keratin in skin, hair & nails; keratin in skin, hair & nails; used primarily with fungi used primarily with fungi classified as dermatophyticclassified as dermatophytic
5-fluorocytosine5-fluorocytosine
Antiviral drugs Antiviral drugs
Interference with uptake or uncoating of virus - Interference with uptake or uncoating of virus - amantadine (used to prevent influenza A)amantadine (used to prevent influenza A)
Inhibition of nucleic acid synthesisInhibition of nucleic acid synthesis
Ribavirin - effective in vitro against a wide range Ribavirin - effective in vitro against a wide range of viruses; highly toxicof viruses; highly toxic
Acyclovir - Herpes-specific (genital herpes, cold Acyclovir - Herpes-specific (genital herpes, cold sores, chickenpox)sores, chickenpox)
Azidothymidine (AZT) - treatment of HIV Azidothymidine (AZT) - treatment of HIV infectionsinfections
Antibiotic resistance in bacteria Antibiotic resistance in bacteria
Sites of resistanceSites of resistance
Membrane transport - Membrane transport - LPS layer of G(-) LPS layer of G(-) prevent entry of many prevent entry of many drugsdrugs
Targets of Targets of antimicrobial agents antimicrobial agents (e.g. ribosomes)(e.g. ribosomes)
Presence of antibiotic-Presence of antibiotic-destroying enzymes destroying enzymes (e.g. beta lactamase)(e.g. beta lactamase)
Antibiotic resistance in bacteriaAntibiotic resistance in bacteriaMechanisms of changing Mechanisms of changing resistanceresistanceMutations - permanent Mutations - permanent changes in chromosomes; changes in chromosomes; not caused by antibioticsnot caused by antibioticsAcquisition of new genetic Acquisition of new genetic information - methodsinformation - methodsTransformation = naked Transformation = naked DNADNATransduction = via virusesTransduction = via virusesConjugation = via sex pili Conjugation = via sex pili (sexual recombination) – (sexual recombination) – most rapid methodmost rapid methodSelective pressures of Selective pressures of antimicrobial therapy - use antimicrobial therapy - use of antibiotics select for of antibiotics select for bacteria that are resistant - bacteria that are resistant - sensitive bacteria are sensitive bacteria are destroyeddestroyed
Antibiotic susceptibility testing Antibiotic susceptibility testing
Susceptibility no longer predictableSusceptibility no longer predictableVariables affecting outcome of therapyVariables affecting outcome of therapyCondition of host (immune status - Condition of host (immune status - underlying diseases)underlying diseases)Site of infection (can drugs get to site?)Site of infection (can drugs get to site?)Properties of antimicrobial agentProperties of antimicrobial agentOther drugs taken concurrentlyOther drugs taken concurrentlySusceptibility of organism to drugSusceptibility of organism to drug
The role of antimicrobials in disrupting The role of antimicrobials in disrupting microbial flora and causing superinfectionsmicrobial flora and causing superinfections
Broth dilution methods (Minimal Inhibitory Broth dilution methods (Minimal Inhibitory Concentration) – Quantitative MethodConcentration) – Quantitative Method
ProcedureProcedure Uses decreasing concentrations of antimicrobial agents prepared Uses decreasing concentrations of antimicrobial agents prepared
in 2-fold dilutions of broth that will support growth of test organismin 2-fold dilutions of broth that will support growth of test organism Standard inoculum is added to broth containing dilutions of Standard inoculum is added to broth containing dilutions of
antimicrobial agent, incubated overnight and then examined for antimicrobial agent, incubated overnight and then examined for growth (turbidity)growth (turbidity)
Lowest concentration of agent that inhibits growth as detected by Lowest concentration of agent that inhibits growth as detected by lack of visible turbidity = Minimal Inhibitory Concentration (MIC)lack of visible turbidity = Minimal Inhibitory Concentration (MIC)
Susceptibility and resistance determined by break point of Susceptibility and resistance determined by break point of drug (highest conc. of drug in the blood that can be drug (highest conc. of drug in the blood that can be achieved with maximal therapy); if MIC is lower than achieved with maximal therapy); if MIC is lower than breakpoint, organism = susceptible; if MIC is higher than breakpoint, organism = susceptible; if MIC is higher than breakpoint, organism = resistant breakpoint, organism = resistant
Minimum Inhibitory ConcentrationMinimum Inhibitory Concentration
Disk diffusion (Kirby/Bauer) - QualitativeDisk diffusion (Kirby/Bauer) - Qualitative
Advantage - rapid testing of several antibiotics Advantage - rapid testing of several antibiotics simultaneouslysimultaneouslyUses antimicrobial agents incorporated into filter paper Uses antimicrobial agents incorporated into filter paper disks placed on agar media causing drug to diffuse disks placed on agar media causing drug to diffuse creating a concentration gradient (conc. highest closest creating a concentration gradient (conc. highest closest to disk)to disk)Susceptibility/resistance determined by measuring Susceptibility/resistance determined by measuring diameter of zone of inhibition around disk and comparing diameter of zone of inhibition around disk and comparing to established zones for each antibioticto established zones for each antibioticStandardization - Bauer, Kirby, Sherris & Turck Standardization - Bauer, Kirby, Sherris & Turck (correlated with MIC’s using large numbers strains & (correlated with MIC’s using large numbers strains & regression analysis)regression analysis)
Techniques for preparation and Techniques for preparation and interpretation of disc diffusion testsinterpretation of disc diffusion tests
Antimicrobial gradient strip method (E Antimicrobial gradient strip method (E test)test)
Disk diffusion method that Disk diffusion method that allows determination of MIC in allows determination of MIC in agaragarConsists of plastic strip Consists of plastic strip containing gradient of containing gradient of antimicrobial agent along with antimicrobial agent along with an interpretive scalean interpretive scalePerformed similar to disk Performed similar to disk diffusiondiffusionOrganisms grow in elliptical Organisms grow in elliptical zone of inhibition around strip zone of inhibition around strip relative to concentration of relative to concentration of antibiotic along its lengthantibiotic along its lengthMIC determined by reading MIC determined by reading scale at point where zone of scale at point where zone of inhibition intersects stripinhibition intersects strip