Antimicrobial Agents Mohammad Reza Fazeli, PharmD, PhD Professor of Microbiology Department of Drug and Food Control Faculty of Pharmacy Tehran University of Medical Sciences

Overview of Antibiotics as Therapeutic Agents w Selective Inhibition/Toxicity Due to the differences in structure and metabolic pathways Harm microorganisms, not the host w Four major sites: Cell wall Ribosomes DNA Cell membrane

Spectrum of Activity w Broad Spectrum Antibiotics: Effective against many types Example: Tetracycline w Narrow Spectrum Antibiotics: Effective against very few types Example: Penicillin

Bactericidal v. Bacteriostatic w Bactericidal: Kill bacteria Used when the host defense mechanisms are impaired w Bacteriostatic: Inhibit bacteria Used when the host defense mechanisms are intact

MECHANISMS OF ACTION OF MAJOR GROUPS OF ANTIBIOTICS

STRUCTURE OF  -LACTAM ANTIBIOTICS

PENICILLIN HOME

“Penicillin Home” w Looks like a house with a new room added to the side w Think of the R-group as of an antenna w Changing “antennae” and or finishing the “basement” will create better “homes” (penicillins)

[Penicillin] Home Improvement Project w Adding a new antenna creates broad spectrum penicillins Example: Ampicillin w Adding additional antennae and finishing the basement creates cephalosporins Example: 1st, 2nd, 3rd, & 4th generation cephalosporins

SPECTRUM OF ACTIVITY OF CEPHALOSPORINS

MECHANISM OF ACTION OF  -LACTAM ANTIBIOTICS

β-Lactam antibiotics inhibit the formation of peptidoglycan cross-links in the bacterial cell wall, but have no direct effect on cell wall degradationpeptidoglycan cross-linkscell walldegradation The β-lactam moiety of penicillin binds to the enzyme (transpeptidase) that links the peptidoglycan molecules in bacteria. The enzymes that hydrolyze the peptidoglycan cross- links continue to function, which weakens the cell wall of the bacteriumβ-lactamenzymetranspeptidasehydrolyze Bacteria constantly remodel their peptidoglycan cell walls, simultaneously building and breaking down portions of the cell wall as they grow and dividepeptidoglycan

Gram-positive Gram-positive bacteria are called protoplasts when they lose their cell walls. Gram-negative bacteria do not lose their cell walls completely and are called spheroplasts after treatment with penicillin  -lactam antibiotics are ineffective against protoplasts and spheroplasts: protoplasts Gram-negativespheroplasts

MECHANISMS OF ACTION OF ANTIRIBOSOMAL ANTIBIOTICS

Inhibition of Protein Synthesis w Anti-ribosomal antibiotics impair ribosomes by binding to either 50S or 30S ribosomal subunits w Ribosomes are essential for translation of mRNA into proteins w No translation  No protein synthesis w No protein synthesis  No growth

MECHANISM OF ACTION OF SULFONAMIDES AND TRIMETHOPRIM

Folic acid (also known as folate, vitamin M, vitamin B 9, vitamin B c or folacin are forms of the water-soluble vitamin B 9.formswater-soluble vitamin B 9 Folic acid is itself not biologically active, but its biological importance is due to tetrahydrofolate and other derivatives after its conversion to dihydrofolic acid in the liver.tetrahydrofolatedihydrofolic acidliver

Vitamin B 9 (folic acid and folate) is essential for numerous body functions. The human body needs folate to synthesize DNA, repair DNA, and methylate DNA as well as to act as a cofactor in certain biological reactions. It is especially important in aiding rapid cell division and growth, such as in infancy and pregnancy. Children and adults both require folic acid to produce healthy red blood cells and prevent anemia.essentialbody functionscell divisiongrowth Childrenadults producered blood cells anemia

Mechanisms of Resistance w Genetic Mechanisms w Non-Genetic Mechanisms

Genetic Mechanisms w Chromosome-mediated Due to spontaneous mutation: in the target molecule in the drug uptake system w Plasmid-mediated Common in Gram-negative rods Transferred via conjugation Multidrug resistance w Transposon-mediated

Non-Genetic Mechanism w Inaccessibility to drugs (e.g., abscess, TB lesion) w Stationary phase (insusceptible to inhibitors of cell wall synthesis) w Protoplasts and spheroplasts (insusceptible to inhibitors of cell wall synthesis)

The End Result of Genetically Conferred Resistance w Production of drug-inactivating enzymes w Modification of target structures w Alteration of membrane permeability

Resistance to Beta-Lactams w Gram +  -lactamase (Penicillinase) Alteration of the transpeptidase enzyme w Gram -  -Lactamase (Penicillinase) Alteration of porins

How can we test for susceptibility/resistance?

Antibiotic Susceptibility Testing w Dilution Method w Disc Diffusion Method w E-test w High-Tech Methods

Dilution Method w Prepare two fold [antibiotic] dilutions w Add 1/2 a million bacterial cells per tube w Incubate overnight w Check for turbidity w Establish the MIC: The lowest concentration of the drug that prevented the bacterial growth (no turbidity)

Disc Diffusion Method w Seed agar plates with bacteria in question w Place antibiotic-discs over the seeded plate w Incubate overnight w Measure the inhibition zones w Relate the results to the zones given in the interpretive chart w There is an inverse relationship between the MICs and zone diameters

Therapeutic Index = Max. Safely Achievable Level MIC