Antimicrobial Agents. Prof. Khaled H. Abu-Elteen. 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.
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Antimicrobial Agents Prof. Khaled H. Abu-Elteen
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 inhibit a bacteria's ability to turn glucose into energy, or the bacteria's ability to construct its cell wall. Therefore the bacteria dies instead of reproducing.
Antibiotics • Substances produced by various species of microorganisms: bacteria, fungi, actinomycetes- to suppress the growth of other microorganisms and to destroy them. Today the term antibiotics extends to include synthetic antibacterial agents: sulfonamides and quinolones.
Where do antibiotics come from? • Several species of fungi including Penicillium and Cephalosporium • E.g. penicillin, cephalosporin • Species of actinomycetes, Gram positive filamentous bacteria • Many from species of Streptomyces • Also from Bacillus, Gram positive spore formers • A few from myxobacteria, Gram negative bacteria • New sources explored: plants, herps, fish
Selective toxicity • The more distantly related the invader, the more targets available for the drug to hit • The less likelihood of direct toxic effects. • Prokaryotes biochemically least similar • Fungi and Protozoa are eukaryotes, so more closely related to humans. • Helminths (worms) also animals • Viruses use our own cell machinery • Cancer cells ARE our cells.
History of Antimicrobial Therapy • 1909 Paul Ehrlich • Differential staining of tissue, bacteria • Search for magic bullet that would attack bacterial structures, not ours. • Developed salvarsan, used against syphilis.
1929 Penicillin discovered by Alexander Fleming • 1940 Florey and Chain mass produce penicillin for war time use, becomes available to the public. • 1935 Sulfa drugs discovered • 1944 Streptomycin discovered by Waksman from Streptomyces griseus
Historical distinctions • Antibiotics: substances produced by organisms that have inhibitory effects on other organisms. • Penicillin, streptomycin • Synthetic drugs: produced in a lab. • Salvarsan, sulfa drugs • Nowadays, most antimicrobials are semi-synthetic • Distinction between “antibiotics” and “synthetic drugs” slowly being abandoned.
Selective toxicity means safer for host • Antibiotics generally have a low MIC • Minimum inhibitory concentration • Effective at lower doses • Good therapeutic index ( Ti) • Safer; larger quantity must be administered before harmful side effects occur. e.g. Ti = LD50 / ED50 Where LD = lethal dose ED = effective dose
Bacteriostatic vs. Bactericidal • Antibiotics differ by mode of action • Bacteriostatic compounds inhibit the growth of bacteria • Holds invaders in check; host immune system does the killing • Bactericidal compounds directly kill the bacteria • Location and severity of infection affect choice of antibiotic • E.g. CNS infection calls for bactericidal treatment.
VI. Antibacterial Agents • A. Inhibitors of cell wall synthesis • 1. Penicillins • 2. Cephalosporins • 3. Other antibacterial agents that act on cell walls • B. Disrupters of cell membranes • 1. Polymyxins • 2. Tyrocidins • C. Inhibitors of protein synthesis • 1. Aminoglycosides • 2. Tetracyclines • 3. Chloramphenicol • 4. Other antibacterial agents that affect protein synthesis • a. Macrolides • b. Lincosamides • D. Inhibitors of nucleic acid synthesis • 1. Rifampin • 2. Quinolones • E. Antimetabolites and other antibacterial agents • 1. Sulfonamides • 2. Isoniazid • 3. Ethambutol • 4. Nitrofurans
Antibiotic Mechanisms of Action Alteration of Cell Membrane Polymyxins Bacitracin Neomycin Transcription Translation Translation
Review and Overview of Bacterial Targets • Bacterial cell walls • Except for Mycoplasma and relatives, all bacteria of the Domain Eubacteria possess peptidoglycan • Peptidoglycan provides shape and structural support to bacterial cells • Bacterial cytoplasm is generally hypertonic compared to their environment • Net flow of water: into cell • Wall under high osmotic pressure
Cell walls continued • Chemical structure of peptidoglycan contributes to its function • Polysaccharide chains composed of 2 alternating sugars, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) • Cross-linked in 3 dimensions with amino acid chains • A breach in peptidoglycan endangers the bacterium
Peptidoglycan Molecule Cross links are both horizontal and vertical between glycan chains stacked atop one another.
There is no molecule similar to peptidoglycan in humans, making drugs that target cell wall synthesis very selective in their toxicity against bacteria.
Gram positive & Gram Negative • Gram positive bacteria have a thick cell wall • Peptidoglycan directly accessible from environment • Gram negative bacteria have a different wall • Thin layer of peptidoglycan • Surrounded by an outer membrane composed of lipopolysaccharide, phospholipids, and proteins • Outer membrane is a barrier to diffusion of molecules including many antibiotics • Some hydrophobic antibiotics may diffuse in. • Porins allow passage of only some antibiotics
1-Inhibition of cell wall synthesis • beta-lactam containing antibiotics inhibit transpeptidase; bacteria cannot synthesize reinforced cell wall and they lyse when they try to grow • Vancomycin and cyclo-Ser inhibit specific binding of Ala’s in crossbridges to transpeptidase in many gram+ bacteria • Bacitracin inhibits secretion of NAG and NAM subunits • All of these only kill growing bacteria
Penicillins • Penicillins contain a b-lactam ring which inhibits the formation of peptidoglycan crosslinks in bacterial cell walls (especially in Gram-possitive organisms) • Penicillins are bactericidal but can act only on dividing cells • They are not toxic to animal cells which have no cell wall
Synthesis of Penicillin • b-Lactams produced by fungi, some ascomycetes, and several actinomycete bacteria • b-Lactams are synthesized from amino acids valine and cysteine
Penicillins 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 should be reduced in severe renal failure
Penicillins Resistance • This is the result of production of b-lactamase enzyme in the bacteria which destroys the b-lactam ring • It occurs in e.g. Staphylococcus aureus, Haemophilus influenzae and Neisseria gonorrhoea
Penicillins Examples • There are now a 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 action
Penicillins Examples • Benzylpenicillin (Penicillin G) is acid labile and b-lactamase sensitive and is given only parenterally • 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, Clostridia
Penicillins Examples • Phenoxymethylpenicillin (Penicillin V) is acid stable and is given orally for minor infections • it is otherwise similar to benzylpenicillin
Penicillins 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, Klebsiella and Proteus strains • It is acid stable, is given orally or parenterally, but is b-laclamase sensitive
Penicillins 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 strongly to b-lactamase and blocks the action of b-lactamase in this way • It extends the spectrum of amoxycillin
Penicillins Adverse effects • Allergy : 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, nephritis
Penicillins Drug interactions • The use of ampicillin (or other broad-spectrum antibiotics) may decrease the effectiveness of oral conraceptives by diminishing enterohepatic circulation
Cephalosporins • They also owe their activity to b-lactam ring and are bactericidal. • Produced from a fungus Cephalosporiumacremonium. • 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 sensitive
Cephalosporins • BACTERICIDAL- modify cell wall synthesis • Interfere at the final step of peptidoglycan synthesis ( Transpeptidation) • CLASSIFICATION- first generation are early compounds • Second generation- resistant to β-lactamases • Third generation- resistant to β-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 to penicillins for staph and strep infections; useful in UTIs • SECOND GENERATION- eg cefaclor and cefuroxime. Active vs enerobacteriaceae eg E. coli, Klebsiellaspp,proteus spp. May be active vs H influenzae and N meningtidis
Cephalosporins • THIRD GENERATION- eg cefixime and other I.V.scefotaxime,ceftriaxone,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 only
Cephalosporins Adverse effects • Allergy (10-20% of patients with penicillin allergy are also allergic to cephalosporins) • Nephritis and acute renal failure • Thrombophlebitis • Superinfections • Gastrointestinal upsets when given orally
Vancomycin • This interferes with bacterial cell wall formation and is not absorbed after oral administration and must be given parenterally. • It is excreted by the kidney. • It is used i.v. to treat serious or resistant Staph. aureus infections and for prophylaxis of endocarditis in penicillin-allergic people.
Vancomycin Adverse effects • Its toxicity is similar to aminoglycoside and likewise monitoring of plasma concentrations is essential. • Nephrotoxicity • Allergy
2- Inhibition of protein synthesis • Aminoglycosides (bactericidal): streptomycin, kanamycin, gentamicin, tobramycin, amikacin, netilmicin, neomycin • Macrolides • Chloramphenicol, Lincomycin, Clindamycin (bacteriostatic)
Protein synthesis inhibitors • Need to affect bacteria, not mitochondria • Aminoglycosides (streptomycin, gentamicin) change shape of 30S ribosome subunit • Tetracycline blocks access to A site of 30S subunit • Chloramphenicol block peptide bond formation from 50S subunit • Macrolides (erythromycin) block 50S subunit action • Antisense NAs bind to beginning of mRNA and block translation
Ribosomes: site of protein synthesis • Prokaryotic ribosomes are 70S; • Large subunit: 50 S • 33 polypeptides, 5S RNA, 23 S RNA • Small subunit: 30 S • 21 polypeptides, 16S RNA • Eukaryotic are 80SLarge subunit: 60 S • 50 polypeptides, 5S, 5.8S, and 28S RNA • Small subunit: 40S • 33 polypeptides, 18S RNA