lecture 02

Bio 319: Antibiotics

Lecture TwoTopic: Characteristics of antibiotics and their classification

Lecturer: Dr. G. Kattam Maiyoh

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GKM/BIO319:Antibiotics/Sem02/2013 2

Brief overview• General aspects• Classification

– Cidal vs. Static– Target organism– Spectrum of activity– Mechanism of action

• Cell synthesis inhibitors– Beta lactams

» Penicillins » cephalosporins

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• Principle:Inhibit growth of bacteria without harming the host– Drug must penetrate body tissue to reach bacteria (exception: GI infection)

(unique targets: cell wall, protein synthesis, metabolic pathways…)– Bacteria targeted must be within the spectrum of the Antibiotic– Drug can be bactericidal or bacteriostatic– Different agents can be combined for synergistic effect

(Note: not all combinations are useful, e.g. cell wall synthesis inhibitors loose effectiveness when combined with bacteriostatic drugs)– Identification of the invasive microorganism necessary for optimal treatment

• General side effect:Alteration in normal body flora

– GI tract harbors symbiotic bacteria which are killed by antibiotic =>resistant bacteria repopulate the niche = secondary or superinfection result(most common: overgrowth of Clostridium difficile)

General Aspects of antibiotics

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Classification of antibiotics• At the highest level, antibiotics can be classified as either

bactericidal or bacteriostatic. • Bactericidals kill bacteria directly while bacteriostatics

prevent cell division. • However, these classifications are based on laboratory

behavior; in practice, both of these are capable of ending a bacterial infection.

• The bactericidal activity of antibiotics may be growth phase dependent and in most but not all cases action of many bactericidal antibiotics requires ongoing cell activity and cell division for the drugs' killing activity

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a. Static vs. Cidal•The graphs show the growth curves for a

bacterium treated with two drugs.

•The upper curve shows the activity of a

bacteriostatic drug.

•The bacterial growth resumes when the

drug is withdrawn.

•The cidal drug, shown in the lower graph,

kills bacteria from the time of administration

to the culture.

• Penicillins, aminoglycoside, vanconmycin, bacitracin , the polymyxin, and colistin are bactericidal.

• Tetracyline , Fusidic acid , Macrolides, Sulfonamides and sulfones on the other hand, are bacteriostatic.

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Classification of Antibiotics• Bacteriostatic vs. Bactericidal

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Target organism• Another method - according to which

bacterial strains they affect:

• For example;Staphylococcus, Streptococcus, or Escherichia coli,.

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Spectrum of activity

May also be classified based on spectrum of activity;1. Broad spectrum

2. Narrow spectrum

3. Extended spectrum

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Antibiotic Spectrum of Activity

• No antibiotic is effective against all microbes

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b. Based on action against the infecting organism



Classification: Mode of action• Cell wall synthesis inhibitors

– Beta-lactams (penicillins, cephalosporins, aztreonam, imipenem)

– Poly-peptides (bacitracin, vancomycin)

• Protein synthesis inhibitors– Aminoglycosides– Tetracyclins– Macrolides– Chloramphenicol– Clindamycin

• Inhibitors of essential metabolites (folate)

– Sulfonamides– Trimethoprim

• Injury to plasma membrane

-polymyxin B - mystatin

- amphotericin B - miconazole

• Inhibition of njucleic acid replication and transcription- Quinolones- Rifampin

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Inhibitors of Cell Wall Synthesis

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Cell wall synthesis inhibitorsBacterial cell wall:

Three types:• Gram-negative (e.g. E.coli, Salmonella)

– Few peptidoglycan layers (Lipopolysaccheride)• Gram-positive (e.g. Staphylococci, Listeria)

– Many peptidoglycan layers (Lipoteichoic acid)– Stains w/ crystal-violet/iodine

• Acid-fast positive (Mycobacteria)– Cell wall contains waxy substance (Mycolic acid)– Stain w/ acid fast test (heating required)

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GKM/BIO319:Antibiotics/Sem02/2013 1413/02/2013

• There are three types– Gram-negative (e.g. E.coli,

Salmonella)• Have few peptidoglycan layers• (stain safranin or fuchsin)

– Gram-positive (e.g. E.coli, Staphylococci, Listeria

• Have many peptidoglycan layers

• stains with crystal violet• typically lack the outer

membrane found in gram-ve.

– Acid-fast Positive (Mycobacteria)• Cell wall contains waxy

substance (Mycollic acid)• Stains with acid fast (heating

required)


Antibiotics - Cell wall synthesis inhibitors

Beta-lactam antibiotics:1928 - Alexander Fleming

observes the antibacterial effects of Penicillin

1940 - Florey and Chain extract Penicillin

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The beta lactams• The β-lactam group of antibiotics includes an enormous

diversity of natural and semi-synthetic compounds that inhibit several enzymes associated with the final step of peptidoglycan synthesis.

• All of this enormous family are derived from a β -lactam structure: a four-membered ring in which the β -lactam bond resembles a peptide bond.

• The multitude of chemical modifications based on this four-membered ring permits the astonishing array of antibacterial and pharmacological properties within this valuable family of antibiotics.

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The Beta-Lactams


R – variable side chain

Monobactam nucleus


Penicillinase (b Lactamase)

Figure 20.8

More when we discuss antibiotic resistance

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• Penicillium notatum produces the only naturally occurring agent – penicillin G or benzylpenicillin

• Penicillium chrysogenum produces 6-aminopenicillanic acid, raw material for semi-synthetics

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Targets for beta lactams• The targets for β-lactam drugs are the penicillin binding proteins

(PBP's), so called because they bind radioactive penicillin and can be detected by autoradiography of gels on which bacterial proteins have been separated electrophoretically.

• The penicillin binding proteins have transpeptidase or carboxypeptidase activity and they act to regulate cell size and shape.

• They are also involved in septum formation and cell division. Bacteria have several individual penicillin binding proteins, each with a separate function.

• Conventionally these are numbered according to size, with PBP 1 as the largest protein.

• The PBP 1 of one bacterium will not necessarily have the same function as the PBP 1 of a different organism.

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Target• The β-lactam antibiotics also stimulate the activity of

autolysins. • These are enzymes that are responsible for the natural

turnover of cell wall polymers to permit growth of the cells.

• Under normal conditions, these enzymes produce controlled weak points within the peptidoglycan structure to allow for expansion of the cell wall structure.

• This activity is stimulated by β-lactams, causing a breakdown of peptidoglycan and leading to osmotic fragility of the cell and ultimately to cell lysis.

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Penicillins - classification

– Narrow spectrum – penicillinase sensitive– Narrow spectrum – penicillinase resistant– Broad spectrum penicillins– Extended-spectrum penicillins

• Cephalosporines• Carbapenems• Monobactams• Vancomycin, Bacitracin

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Natural Penicillins• Penicillin G, Penicillin V

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Penicllins (general info.)

• Derived from the fungus Penicillium• Attains therapeutic concentration in most

tissues• However has poor CSF penetration• Renal excretion• Side effects: hypersensitivity, nephritis,

neruotoxicity, platelet dysfunction

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PenicillinsInhibit transpeptidase required for cross-linking peptidoglycan

chainsAlso inactivate an inhibitor of an autolytic bacterial enzyme => lysis

Narrow spectrum – penicillinase (= β-lactamase) sensitive• Benzylpenicillin

– Naturally occuring– Poor oral availability (sensitive to stomach acid)=> given by injection– Active against gram-positive bacteria

• Phenoxymethylpenicillin– Better oral availability (acid resistant)

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Narrow spectrum – penicillinase (= β-lactamase) resistant• Methicillin– Semisynthetic– Poor oral availability (only parenteral)– Active against gram-pos bacteria– Mostly used for Staphylococcus aureus

• Oxacillin– Good oral availability

• Cloxacillin• Dicloxacillin

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Broad spectrum – penicillinase (= β-lactamase) sensitive

(Also referred to as Aminopenicillins)

• Ampicillin– Semisynthetic– Good oral availability– Active against gram-pos and gram-neg bacteria– Active against enterobacteria

• Amoxicillin– Excellent oral availability

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Extended spectrum – penicillinase (= β-lactamase) sensitive

(Also called Carboxypenicillins)

• Carbenicillin– Semisynthetic– Poor oral availability– Active against gram-pos and gram-neg bacteria– Active against Pseudomonas aeruginosa, Klebsiella

• Ticarcillin• Mezlocillin• Pipercillin

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Penicillins

Figure 20.613/02/2013


CephalosporinesDerived from Cephalosporium sp.

(same antibiotic mechanism as penicillins)Cross-allergies with penicillins are commonSome CSs antagonize Vitamin K => leads to bleedingSome CSs block alcohol oxidation => disulfiram effect ( read a bit)Classified into generations:• 1-4• Increasing activity against gram-negative bacterial and

anaerobes• Increasing resistance to destruction by beta-lactamases• Increasing ability to reach cerebrospinal fluid

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Cephalosporins• The first cephalosporins - first generation while later, more

extended spectrum cephalosporins were classified as second generation cephalosporins.

• Each newer generation has significantly greater gram-negative antimicrobial properties than the preceding generation,

• Most cases with decreased activity against gram-positive organisms.

• Fourth generation cephalosporins, however, have true broad spectrum activity.

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Cephalosporin structure

• Base molecule is 7-aminocephalosporanic acid produced by a Sardinian sewer mold

• R groups determine spectrum of activity and pharmacological properties

• Mechanism of action/resistance and class pharmacology essentially the same as penicillins

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Will consider:

• Examples of each generation• Effectiveness• Absorption/Excretion• Distribution• Therapeutic uses • Metabolism

Cefaclor structure – Member of The 2nd generation cephalosporins


First Generation Cephalosporins

• Cefazolin, cephalexin, cephadroxil

• Excellent against susceptible staph and strep

• Modest activity against G -ve• Cefazolin given parentally,

others orally• More than half of the drug is

bound to plasma proteins• Excreted by kidneys

unmetabolized• Good for staph and strep skin

and soft tissue infections



Second Generation Cephalosporins• Include;

– Cefaclor,– cefuroxime, – cefprozil

• Modest activity against G+, increased activity against G-, works against anaerobes

• Cefaclor and cefprozil given orally• Absorption and excretion same as first

generation.• Good for treating respiratory tract infections,

intra-abdominal infections, pelvic inflammatory disease, diabetic foot ulcers

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Third Generation Cephalosporins• Ceftaxime, ceftriaxzone,

cefoperazone, cefpodoxime• Broad spectrum killers• Drugs of choice for serious infections• No effect against Listeria and beta-

lactamase producing pneumococci• Cefpodoxime given orally, others

parentally• Most excreted by kidney• Therapeutic uses

– Bacterial meningitis (2 exceptions)– Lyme disease– Life-threatening G -ve sepsis

(infection)



Fourth Generation Cephalosporin

– e.g.Cefepime– Same antimicrobial spectrum as third

generation but resists more beta-lactamases– Given parentally, excellent penetration into

CSF– Good for nosocomial infections

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Toxicity/Contraindications of Cephalosporins

• Hypersensitivity reactions (uncommon) essentially same as for penicillins

• Cross-reaction between 2 classes


lecture 02

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