Download Chapter 13: Antimicrobial Therapy

Chapter 13: Antimicrobial Therapy
Antimicrobial Agents
•A special group of chemotherapeutic agents used to treat diseases caused by microbes
•Antibiosis:
literally means “against life”
•Antibiotic:
A chemical substance produced by microbes which has the capacity to inhibit or destroy
the growth of bacteria/other microbes
•Synthetic/semi-synthetic drugs
History of Chemotherapy
•1910: Paul Ehrlich used Salvarsan to treat syphilis
•1935: Gerhard Domagk discovered prontosil, a red dye, inhibits growth of many gram-positive bacteria
•1936: Ernest Fourneau discovered that the sulfanilamide portion contained antimicrobial activity
General Properties of Antimicrobial Agents
•Selective Toxicity
•The Spectrum of Activity
•Modes of Action
•Side Effects
•The Resistance of Microorganisms
Selective Toxicity
•The antimicrobial agent must harm the microbes without causing significant damage to the host
•Toxic dosage level:
causes host damage
•Therapeutic dosage level:
successfully eliminates the pathogenic organism if the level is maintained
Spectrum of Activity
•The range of different microbes against which an antimicrobial agent acts
•Broad spectrum:
Agents effective against both gram-positive and gram-negative bacteria
•Narrow spectrum:
Agents effective against a small number of microbes
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Modes of Action
1. Inhibition of cell wall synthesis
2. Disruption of cell-membrane function
3. Inhibition of protein synthesis
4. Inhibition of nucleic acid synthesis
5. Action as anti-metabolites
Inhibition of Cell Wall Synthesis
•Inhibiting cell wall synthesis selectively damages bacterial and fungal cells
•Penicillin and cephalosporin contain a structure called a ß-lactam ring
•The ß-lactam ring attaches to the enzymes that cross-link peptidoglycans and prevent cell wall
synthesis
Disruption of Cell Membrane Function
•Certain polypeptide antibiotics, such as polymyxins
•Act as detergents and distort bacterial cell membranes
•Bind to phospholipids in the membrane
•Especially effective against gram-negative bacteria which have an outer membrane
Disrupters of Cell Membranes
•Polymyxins (A, B, C, D, and E)
•Obtained from soil bacterium Bacillus polymyxa
•Usually applied topically, often with bacitracin, to treat skin infections caused by Pseudomonas
•Internally, can cause numbness in the extremities, serious kidney damage, and respiratory arrest
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Inhibition of Protein Synthesis
•An example of selective toxicity
•Attack bacterial cells without significantly damaging animal cells
•Aminoglycoside antibiotics: Tetracycline, Erythromycin, streptomycin, chloramphenicol
•Bind to the bacterial 30S ribosomal subunit and interfere with the translation of mRNA
Tetracyclines
•Obtained from species of Streptomyces
•Are bacteriostatic, readily absorbed from the digestive tract, and become widely distributed in tissues
and body fluids
•Have the widest spectrum of activity of any antibiotics
•However, they destroy the normal intestinal microbiota and ofter produce severe gastrointestinal
disorders
Inhibition of Nucleic Acid Synthesis
•Differences between the enzymes used by bacterial and animal cells to synthesize nucleic acids
•Antibiotics of the rifamycin family bind to a bacterial RNA polymerase and inhibit RNA synthesis
Action as Anti-metabolites
•Substances that affect the utilization of metabolites and prevent metabolic reactions
•Function in two ways:
1. Competitively inhibiting enzymes
2. Erroneously incorporated into important molecules such as nucleic acids
Kinds of Side Effects
•The side effects of antimicrobial agents fall into three general categories:
1. Toxicity
2. Allergy
3. Disruption of normal microbiota
Acquisition of Resistance
Microorganisms normally acquire antibiotic resistance by genetic changes:
1. Natural Selection
2. Chromosomal Mutation
3. Extrachromosomal Resistance
• R Plasmids or R factors
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Mechanisms of Resistance
1. Alteration of Targets
2. Alteration of Membrane Permeability
3. Development of Enzymes
4. Alteration of an Enzyme
5. Alteration of a metabolic pathway
Alteration of Targets
•This mechanism usually affects bacterial ribosomes
•The mutation alters the DNA such that the protein produced or target is modified
•Antimicrobial agents can no longer bind to the target
•Resistance to erythromycin, rifamycin, and anti-metabolites
Enzyme Development
•B-lactamase
•These enzymes are found in various bacteria
•They catalyze the breaking of the B-lactam ring in penicillins and some cephalosporins
First-Line, Second-Line Third-Line Drugs
•1930’s sulfonamides were found to cure the disease, gonorrhea
•Sulfonamide-resistant strains developed and penicillin was used as a cure
•Penicillin-resistant strains developed and spectinomycin was used
•Now there are spectinomycin-resistant strains of gonorrhea
Effects of Premature Termination of Antibiotic Treatment
Kirby-Bauer Method of Determining Microbial Sensitivities to Various Antibiotics
Minimal Inhibitory Concentration (MIC) Microbial Susceptibility
An Epsilometer (E) Test for Determining Antibiotic Sensitivity
Interferon's and Immunoenhancers
•Cells infected with viruses produce proteins collectively referred to as interferons
•These interferon proteins induce neighboring cells to produce antiviral proteins
•Antiviral proteins prevent these cells from becoming infected
•Interferons are currently being genetically engineered and tested as antiviral agents
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