ANTIBIOTICS AND ANTIMICROBIAL AGENTS

ANTIBIOTICS & ANTIMICROBIAL AGENTS

• TOPICS• General principles• Antimicrobial: • Classification• Mechanisms of Action &

Resistance

ANTIBIOTICS& ANTIMICROBIAL AGENTS

• General principles• 1. Desired properties• -- broad range; bactericidal vs bacteriostatic; good

distribution; selective toxicity• 2. Measurement of antimicrobial activity• -- Diffusion [ Kirby-Bauer}; macro & micro

dilution; determination of MIC & MBC}• 3. Antimicrobial combinations• -- synergism; antagonism; indifference;

indications for combined therapy

ANTIBIOTICS & ANTIMICROBIAL AGENTS

• DEFINITION

• Antibiotics are natural or synthetic compounds that inhibit or kill bacteria

• Factors affecting choice of antibiotics

• A. safety & side effects

• B. The narrowest spectrum

• C. method of administration

• D. Cost of therapy

ANTIBOTICS& ANTIMICROBIAL AGENTS

• Mechanism of action of Antibiotics

• Most antibiotics target procaryotic structures

• 1. Enzymes used in synthesis of peptidoglycan

• 2. Enzymes or machinery used in the production of procaryotic nucleic acid & proteins

Antibiotics/ Mechanisms of action• Antibiotics target essential processes for bacteria

growth and/or division• 1.Effects on cell wall integrity or cell wall

synthesis inhibitors

• 2. Interruption of cell membrane structure and function

• 3. Inhibition of protein synthesis

• 4. Inhibition of essential metabolite

• 5. Interference with nucleic acid metabolism

ANTIMOCROBIAL CLASSIFICATION

• Major Antimicrobial biochemical classes• 1. Beta- lactams• 2. Macrolides• 3. Aminoglycosides• 4. Tetracyclines• 5. Glycopeptides• 6. Sulphonamides• 7. Quinolones

Cell wall synthesis inhibitors

• 1. β lactam antibacterial agents• Intact beta-lactam ring is essential structural feature for active

beta-lactam• E.g penicillins,cephalosporins, carbapenems, monobactams• Beta-lactams agents bind to transpeptidases and inhibit

peptidoglycan formation ; interfering with cell wall synthesis• Trandpeptidases bind penicillins hence called penicillin

binding proteins (PBP)• PBPs are different for G+ & G –ve bacteria• Bacteria may produce enzymes called beta-lactamases, that

hydrolyse the beta-lactam ring and inactivate the antibiotic

Inhibition of cell wall synthesis

• Vancomycin: a glycopeptide• Forms a complex with residues of peptidoglycan

precursors

• Also inhibit biochemical reactions in cell wallcatalysed by transpeptidases & D,D- carboxypeptides

• Beta-lactams and vancomycin (active site; cell wall)can act synergistically with an aminoglycoside antimicrobial against enterococci

• Cell wall active agents puncture hole in well& aminoglycoside enters & goes thro cytoplasm to reach its active site, the ribosomes

Inihibition of Nucleic acid ( DNA) replication

• E.g Quinolone antimicrobial:

• Naladixic acid (old) ; acts on aerobic Gram-negative spp.

• &Fluoroquinolones>ciprofloxicin, norfloxacin and ofloxacin (new) ; have much broader spectrum of activity

• DNA gyrase controls folding of or supercoiling of the DNA during DNA replication

• Enzyme is essential for preventing DNA entanglement during replication of the circular bacterial chromosome

• Antibiotics bind DNA-gyrase complex; inhibit enzyme function; lead to bacteria death

Inhibition of Protein synthesis

• Interference with protein synthesis taking place in ribosomes by agents are able to stop cell division

• Bacterial ribosome 2 subunits:50S & 30S

• Antimicrobials bind to one or both subunits and cause misreading of genetic code or formation of abnormal, nonfunctional protein complexes

• Binding to 30S subunit

• Aminoglycoside (gentamycin, streptomycin): Tetracyclines ( bactericidal – binding to ribosome is transient)

• Binding of 50S subunits: Macrolides( erythromycin): chloramphenicol and clindamycin

Inhibition of Folate Metabolism

• Bacteria usually lack the ability to take up folic acid from the environment and must synthesize it internally

• Trimethoprim & sulphonamide interfere with folate metabolism

• They competitively block the synthesis of tetrahydrofolate

• Trimethoprim and sulphonamides are usually administerd together because trimethoprim potentiates sulphonamides

Biochemical Mechanisms of Bacterial Antimicrobial resistance

• Bacteria resist the killing effects of antimicrobials by:

• 1. Alteration of the antimicrobial’s target receptor molecule in the bacteria

• 2. Decreasing the accessibility of the antimicrobial to the target by altering entry of the drug into the cell or increasing removal of the drug from the cell

• 3.Destruction or inactivation of the drug• 4. Synthesis by bacteria of a new metabolic

pathway that is not inhibited by drug

Bacterial resistance to antimicrobials

• General• Bacterial resistance arises thro’ a multistep

process from low level to to high level • A plasmid may be acquired which contains genes

for full brown resistance• Multiple mechanisms of resistance in a single

bacteria leads to high level of resistance• Mechanisms of resistance are often specific to a

particular drug in relation to a specific bacterial spp.

Bacterial resistance to antibiotics

• Alteration of the Target Receptor

• Drug is unable to bind to altered target receptor.• E.g 1. Altered PBPs in some Strep. Pneumoniae cause

resistance to Pen G• 2. Resistance of some S. aureus to beta-lactamase stable

penicillins• 3. Fluoroquinolones resistance is often associated with

mutation in DNA gyrase and therefore inhibit binding• 4. Macrolide-lincomycin resistance in staphylococci and

streptococci is due to methylation in the 50S ribosomal subunit RNA; which decreases binding

• The gene which causes this change in the ribosomal RNA is plasmid mediated and encoded on transposons

Bacterial resistance to antibiotics

• Decreased accessibility of antibiotic to target site

• A.Decreased uptake :• Membrane characteristics can inhibit the

antimicrobial from crossing the membrane and entering the cell

• B. Or drug can be altered in its passage across membrane; it cannot bind its target

• C. Increased efflux; resistant bacteria can actively remove the drug from the cell

Decreased accessibility of antibiotic to target site

• Examples

• Tetracycline resistance; • due to decrease in levels of drug accumulation

caused by decreased uptake and increased efflux• resistance is usually plasmid-mediated• Plasmid containing tetracycline resistance move

among members of Enterobacteriaceae& also between S. aureus, S. epidermidis, S pyogenes, S. pneumoniae and S. faecalis

Decreased accessibility of antibiotic to target site

• Aminoglycoside resistance:• Largely due to alteration of the drug in periplasmic space

due to • bacterial enzymesthat acetylate, phosphorylate &

adenylate the aminoglycosides• Altered Compound binds to ribosomes with poor uptake

into cell• Genes coding for the drug altering enzymes are often

found on transposons• These genenes have been identified in Enterobacteriaceae,

P. aeruginosa. S. pneumoniae and Gram-positive spp

Destruction or inactivation of the antibiotic

• Examples• Beta-lactamases• In G+ & G-ve bacteria• Bacteria producing beta-

lactamases• Staphylococci,

enterococci• Beta lactamases r genes

chromosomally or plasmid mediated

• Widely distributed

• Chloramphenicol resistance

• Due to Intracellular enzyme a.k.s Chloramphenocol transacetylate

• Enzyme acetylates hydroxyl groups on the drug structure

• Decreased binding of drug to 50S ribosome

Resistance to Beta Lactam Antibiotics

• The beta -lactamase inhibitors• Control the beta- lactamase ( enzymes that that inactivate

the antibiotics)• Strategy is to give a beta-lactamase in combination with

the beta lactam antibiotic( e.g Augmentin)• 3 inhibitors in use; clavulanic acid, sulbactam &

tazobactam• They are suicide inhiibitors of the beta-lactamase• Work against most G-ve beta- lactamases and those in S.

aureus • They do not work against the chromosomally encoded

beta- lactamases of Enterobacter, Citrobacter, Serratia& Pseudomonas

Synthesis of a new metabolic pathway

• --Bacteria can produce a new enzyme that is not inhibited by the antimicrobial

• Trimethoprim-sulphamethoxazole resistance

• Due to bacteria that produce a new dihydrofolate reductase not inhibited by trimethoprim

• Also a new dihydropteroate synthetase not susceptible to sulphonamides