1、Antimicrobial AgentsMartin VotavaOlga KroftovOverview If bacteria make it past our immune system and start reproducing inside our bodies,they cause disease.Certain bacteria produce chemicals that damage or disable parts of our bodies.Antibiotics work to kill bacteria.Antibiotics are specific to cert
2、ain bacteria and disrupt their function.What is an Antibiotic?An antibiotic is a selective poison.It has been chosen so that it will kill the desired bacteria,but not the cells in your body.Each different type of antibiotic affects different bacteria in different ways.For example,an antibiotic might
3、 inhibit a bacterias ability to turn glucose into energy,or the bacterias ability to construct its cell wall.Therefore the bacteria dies instead of reproducing.Antibiotics Substances produced by various speciesof microorganisms:bacteria,fungi,actinomycetes-to suppress the growth of other microorgani
4、sms and to destroy them.Today the term ATB extends to include synthetic antibacterial agents:sulfonamides and quinolones.History The German chemist Paul Ehrlich developed the idea of selective toxicity:that certain chemicals that would be toxic to some organisms,e.g.,infectious bacteria,would be har
5、mless to other organisms,e.g.,humans.In 1928,Sir Alexander Fleming,a Scottish biologist,observed that Penicillium notatum,a common mold,had destroyed staphylococcus bacteria in culture.Sir Alexander FlemingFlemings Petri DishZone of Inhibition Around the fungal colony is a clear zone where no bacter
6、ia are growing Zone of inhibition due to the diffusion of a substance with antibiotic properties from the fungusHistory Penicillin was isolated in 1939,and in 1944 Selman Waksman and Albert Schatz,American microbiologists,isolated streptomycin and a number of other antibiotics from Streptomyces gris
7、eus.Susceptibility vs.Resistanceof microorganisms to Antimicrobial Agents Success of therapeutic outcome depends on:Achieving concentration of ATB at the site of infection that is sufficient to inhibit bacterial growth.Host defenses maximally effective MI effect is sufficient bacteriostatic agents(s
8、low protein synthesis,prevent bacterial division)Host defenses impaired-bactericidal agents Complete ATB-mediated killing is necessarySusceptibility vs.Resistance(cont.)Dose of drug has to be sufficient to produce effect inhibit or kill the microorganism:However concentration of the drug must remain
9、 below those that are toxic to human cells If can be achieved microorganism susceptible to the ATB If effective concentration is higher than toxic-microorganism is resistantSusceptibility vs.Resistance(cont.)Limitation of in vitro tests In vitro sensitivity tests are based on non-toxic plasma concen
10、trations cut off Do not reflect concentration at the site of infection E.g.:G-aer.bacilli like Ps.aeruginosa inhibited by 2 4 ug/ml of gentamycin or tobramycin.Susceptible!?Antibiotic Susceptibility Testing84021 Tetracycline(g/ml)MIC=2 g/mlDetermination of MICChlAmpEryStrTetDisk Diffusion TestSuscep
11、tibility vs.Resistance(cont.)Plasma concentration above 6-10 ug/ml may result in ototoxicity or nephrotoxicity Ration of toxic to therapeutic concentration is very low agents difficult to use.Concentration in certain compartments vitreous fluid or cerebrospinal fluid much lower than those in plasma.
12、Therefore can be only marginally effective or ineffective even those in vitro test states sensitive.Susceptibility vs.Resistance(cont.)Therefore can be only marginally effective or ineffective even those in vitro test states sensitive“.Conversely concentration of drug in urine may be much higher tha
13、n in plasma,so resistant“agents can be effective in infection limited to urine tractResistance To be effective ATB must reach the target and bind to it.Resistance:Failure to reach the target The drug is inactivated The target is alteredResistance(cont.)Bacteria produce enzymes at or within the cell
14、surface inactivate drug Bacteria possess impermeable cell membrane prevent influx of drug.Transport mechanism for certain drug is energy dependent-not effective in anaerobic environment.ATB as organic acids penetration is pH dependent.Resistance(cont.)Acquired by mutation and passed vertically by se
15、lection to daughter cells.More commonly horizontal transfer of resistance determinant from donor cell,often another bacterial species,by transformation,transduction,or conjugation.Horizontal transfer can be rapidly disseminated By clonal spread or resistant strain itself Or genetic exchange between
16、resistant and further susceptible strains.Resistance(cont.)Methicilin resistant strains of Staphylococcus aureus clonally derived from few ancestral strains with mecA gene Encodes low-affinity penicillin-binding protein that confers methicillin resistance.Staphylococcal beta-lactamase gene,which is
17、plasmid encoded,presumambly transferred on numerous occasions.Because is widely distributed among unrelated strains,identified also in enterococciSelection of the ATB Requires clinical judgment,detailed knowledge of pharmacological and microbiological factors.Empirical therapy initial infecting orga
18、nism not identified single broad spectrum agent Definitive therapy-microorganism identified a narrow spectrum low toxicity regiment to complete the course of treatment Empirical and Definite Therapy Knowledge of the most likely infecting microorganism and its susceptibility Gram stain Pending isolat
19、ion and identification of the pathogen Specimen for culture from site of infection should be obtain before initiation of therapy Definite therapyPenicillins Penicillins contain a b b-lactam ring which inhibits the formation of peptidoglycan crosslinks in bacterial cell walls(especially in Gram-possi
20、tive organisms)Penicillins are bactericidal but can act only on dividing cells They are not toxic to animal cells which have no cell wallSynthesis of Penicillinb-Lactams produced by fungi,some ascomycetes,and several actinomycete bacteriab-Lactams are synthesized from amino acids valine and cysteine
21、b b Lactam Basic StructurePenicillins(cont.)Clinical Pharmacokinetics Penicillins are poorly lipid soluble and do not cross the blood-brain barrier in appreciable concentrations unless it is inflamed(so they are effective in meningitis)They are actively excreted unchanged by the kidney,but the dose
22、should be reduced in severe renal failurePenicillins(cont.)Resistance This is the result of production of b b-lactamase in the bacteria which destroys the b b-lactam ring It occurs in e.g.Staphylococcus aureus,Haemophilus influenzae and Neisseria gonorrhoeaPenicillins(cont.)Examples There are now a
23、wide variety of penicillins,which may be acid labile(i.e.broken down by the stomach acid and so inactive when given orally)or acid stable,or may be narrow or broad spectrum in actionPenicillins(cont.)Examples Benzylpenicillin(Penicillin G)is acid labile and b-lactamase sensitive and is given only pa
24、renterally It is the most potent penicillin but has a relatively narrow spectrum covering Strepptococcus pyogenes,S.pneumoniae,Neisseria meningitis or N.gonorrhoeae,treponemes,Listeria,Actinomycetes,ClostridiaPenicillins(cont.)Examples Phenoxymethylpenicillin(Penicillin V)is acid stable and is given
25、 orally for minor infections it is otherwise similar to benzylpenicillinPenicillins(cont.)Examples Ampicillin is less active than benzylpenicillin against Gram-possitive bacteria but has a wider spectrum including(in addition in those above)Strept.faecalis,Haemophilus influenza,and some E.coli,Klebs
26、iella and Proteus strains It is acid stable,is given orally or parenterally,but is b-laclamase sensitivePenicillins(cont.)Examples Amoxycillin is similar but better absorbed orally It is sometimes combined with clavulanic acid,which is a b-lactam with little antibacterial effect but which binds stro
27、ngly to b-lactamase and blocks the action of b-lactamase in this way It extends the spectrum of amoxycillinPenicillins(cont.)Examples Flucloxacillin is acid stable and is given orally or parenterally It is b-lactamase resistant It is used as a narrow spectrum drug for Staphylococcus aureus infection
28、sPenicillins(cont.)Examples Azlocillin is acid labile and is only used parenterally It is b-lactamase sensitive and has a broad spectrum,which includes Pseudomonas aeruginosa and Proteus species It is used intravenously for life-threatening infections,i.e.in immunocompromised patients together with
29、an aminoglycoside Penicillins(cont.)Adverse effects Allergy(in 0.7%to 1.0%patients).Patient should be always asked about a history of previous exposure and adverse effects Superinfections(e.g.caused by Candida)Diarrhoea:especially with ampicillin,less common with amoxycillin Rare:haemolysis,nephriti
30、sPenicillins(cont.)Drug interactions The use of ampicillin(or other broad-spectrum antibiotics)may decrease the effectiveness of oral conraceptives by diminishing enterohepatic circulationAntistaphylococcus penicillins Oxacillin,cloxacillin Resistant against staphylococcus penicillinasisCephalospori
31、ns They also owe their activity to b-lactam ring and are bactericidal.Good alternatives to penicillins when a broad-spectrum drug is required should not be used as first choice unless the organism is known to be sensitiveCephalosporins BACTERICIDAL-modify cell wall synthesis CLASSIFICATION-first gen
32、eration are early compounds Second generation-resistant to-lactamases Third generation-resistant to-lactamases&increased spectrum of activity Fourth generation-increased spectrum of activity Cephalosporins FIRST GENERATION-eg cefadroxil,cefalexin,Cefadrine-most active vs gram+ve cocci.An alternative
33、 to penicillins for staph and strep infections;useful in UTIs SECOND GENERATION-eg cefaclor and cefuroxime.Active vs enerobacteriaceae eg E.coli,Klebsiella spp,proteus spp.May be active vs H influenzae and N meningtidisCephalosporins THIRD GENERATION-eg cefixime and other I.V.s cefotaxime,ceftriaxon
34、e,ceftazidine.Very broad spectrum of activity inc gram-ve rods,less activity vs gram+ve organisms.FOURTH GENERATION-cefpirome better vs gram+ve than 3rd generation.Also better vs gram-ve esp enterobacteriaceae&pseudomonas aerugenosa.I.V.route onlyCephalosporins(cont.)Adverse effects Allergy(10-20%of
35、 patients wit penicillin allergy are also allergic to cephalosporins)Nephritis and acute renal failure Superinfections Gastrointestinal upsets when given orallyAminoglycosides(bactericidal)streptomycin,kanamycin,gentamicin,tobramycin,amikacin,netilmicin,neomycin(topical)Mode of action-The aminoglyco
36、sides irreversibly bind to the 16S ribosomal RNA and freeze the 30S initiation complex(30S-mRNA-tRNA)so that no further initiation can occur.They also slow down protein synthesis that has already initiated and induce misreading of the mRNA.By binding to the 16 S r-RNA the aminoglycosides increase th
37、e affinity of the A site for t-RNA regardless of the anticodon specificity.May also destabilize bacterial membranes.Spectrum of Activity-Many gram-negative and some gram-positive bacteriaResistance-CommonSynergy-The aminoglycosides synergize with-lactam antibiotics.The-lactams inhibit cell wall synt
38、hesis and thereby increase the permeability of the aminoglycosides.AminoglycosidesClinical pharmacokinetics These are poorly lipid soluble and,therefore,not absorbed orally Parenteral administration is required for systemic effect.They do not enter the CNS even when the meninges are inflamed.They ar
39、e not metabolized.Aminoglycosides(cont.)Clinical pharmacokinetics They are excreted unchanged by the kidney(where high concentration may occur,perhaps causing toxic tubular demage)by glomerular filtration(no active secretion).Their clearance is markedly reduced in renal impairment and toxic concentr
40、ations are more likely.Aminoglycosides(cont.)Resistance Resistance results from bacterial enzymes which break down aminoglycosides or to their decreased transport into the cells.Aminoglycosides(cont.)Examples Gentamicin is the most commonly used,covering Gram-negative aerobes,e.g.Enteric organisms(E
41、.coli,Klebsiella,S.faecalis,Pseudomonas and Proteus spp.)It is also used in antibiotic combination against Staphylococcus aureus.It is not active against aerobic Streptococci.Aminoglycosides(cont.)Examples In addition to treating known sensitive organisms,it is used often blindly with other antibiot
42、ics in severe infections of unknown cause.Streptomycin was formerly the mainstay of antituberculous therapy but is now rarely used in the developed world.Aminoglycosides(cont.)Examples Tobramycin:used for pseudomonas and for some gentamicin-resistant organisms.Some aminoglycosides,e.g.Gentamicin,may
43、 also be applied topically for local effect,e.g.In ear and eye ointments.Neomycin is used orally for decontamination of GI tract.Aminoglycosides(cont.)Adverse effects Although effective,aminoglycosides are toxic,and this is plasma concentration related.It is essential to monitor plasma concentration
44、s(shortly before and after administration of a dose)to ensure adequate concentrations for bactericidal effects,while minimising adverse effects,every 2-3 days.Aminoglycosides(cont.)Adverse effects The main adverse effects are:NephrotoxicityToxic to the 8th cranial nerve(ototoxic),especially the vest
45、ibular division.Other adverse effects are not dose related,and are relatively rare,e.g.Allergies,eosinophilia.Macrolides(bacteriostatic)erythromycin,clarithromycin,azithromycin,spiramycin Mode of action-The macrolides inhibit translocation by binding to 50 S ribosomal subunit Spectrum of activity-Gr
46、am-positive bacteria,Mycoplasma,Legionella(intracellular bacterias)Resistance-CommonMacrolides(cont.)Examples and clinical pharmacokinetics Erythromycin is acid labile but is given as an enterically coated tablet Absorption is erratic and poor.It is excreted unchanged in bile and is reabsorbed lower
47、 down the gastrointestinal tract(enterohepatic circulation).It may be given orally or parenterallyMacrolides(cont.)Examples and clinical pharmacokinetics Macrolides are widely distributed in the body except to the brain and cerebrospinal fluid The spectrum includes Staphylococcus aureus,Streptococcu
48、ss pyogenes,S.pneumoniae,Mycoplasma pneumoniae and Chlamydia infections.Macrolides(cont.)Examples and clinical pharmacokinetics Newer macrolides such as clarithromycin and azithromycin may have fewer adverse effects.Macrolides side effects Nauzea,vomitus Allergy Hepatitis,ototoxicity Interaction wit
49、h cytochrome P450 3A4(inhibition)Chloramphenicol,Lincomycin,Clindamycin(bacteriostatic)Mode of action-These antimicrobials bind to the 50S ribosome and inhibit peptidyl transferase activity.Spectrum of activity-Chloramphenicol-Broad range;Lincomycin and clindamycin-Restricted range Resistance-Common
50、 Adverse effects-Chloramphenicol is toxic(bone marrow suppression)but is used in the treatment of bacterial meningitis.Clindamycin Clindamycin,although chemically distinct,is similar to erythromycin in mode of action and spectrum.It is rapidly absorbed and penetrates most tissues well,except CNS.It
