chapter 20: antimicrobial drugs
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Chapter 20: Antimicrobial Drugs. What are antibiotics?. Definition: chemicals produced by one microorganism to kill or inhibit the growth of other microorganisms Now included the semi-synthetic antibiotics Effective against bacteria Antibacterial drugs – largest group of antibiotics - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 20: Antimicrobial Drugs
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What are antibiotics?Definition: chemicals produced by one
microorganism to kill or inhibit the growth of other microorganisms
Now included the semi-synthetic antibioticsEffective against bacteriaAntibacterial drugs – largest group of
antibioticsGeneral terms used to include other
microbes are antimicrobial drugs or antimicrobials
Difference between disinfectants/antiseptics and antimicrobial drugs
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Antibiotic spectrumBroad vs. narrow spectrum
antibioticsBenefits and problems with each
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Microbes that produce antibiotics:
Bacteria: Bacillus and Streptomyces
Fungi: Cephalosporium and Penicillium
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How do antibiotics work?
1. must kill pathogen and not kill host
2. fewest sides effects in host; highest toxicity in pathogen
3. exploit differences between the pathogen and the host
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Modes of action of the antibiotics:1. Inhibition of cell wall synthesis
◦ cell wall is weakened by these when cell is growing
◦ ex. penicillin(s) and cephalosporins◦ low toxicity to humans
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Modes of action of the antibiotics:2. Inhibition of protein synthesis
◦ binding to 70S ribosome◦ differences between 70S and 80S
ribosomes◦ ex. chloramphenicol, erythromycin,
streptomycin, tetracyclines◦ host mitochondria may also be affected
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Modes of action of the antibiotics:3. Increase permeability of plasma
membrane◦ ex. polymyxin B – used topically◦ can have high toxicity in humans◦
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Modes of action of the antibiotics:4. Inhibit synthesis of RNA/DNA
◦ can be very toxic◦ ex. quinolones – DNA
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Modes of action of the antibiotics:5. Inhibition of metabolic
pathways◦ ex. sulfanilamide – folic acid pathway◦ low toxicity because of the absence of this
pathway in humans
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Antagonism vs. synergism of 2 or more antibiotics
Antagonism:◦Ex. Tetracycline penicillin (Why?)
Synergism:◦Ex. TMP-SMZ (SMX) trimethoprim
and sulfamethoxazole
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Prophylaxis – preventative use of antibiotics
Ex. of their use:1. before surgery – i.e.
appendix removal
2. before dental work in heart patients
3. in AIDS patients to prevent many infections
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Factors affecting antimicrobial activity in vivo –
1. In the environment
A. metabolic state of pathogenEx. Mycobacterium tuberculosis
B. distribution of drug is different in different tissues Ex. blood/brain barrier and necrotic tissue
C. location of pathogen Ex. Chlamydia inside host cells
D. interfering substances Ex. low pH in stomach; binding to proteins in body, etc.
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Factors affecting antimicrobial activity in vivo –
2. Concentration of antibiotics in body
A. absorption, inactivation, excretion
B. distribution of the drug – systemic vs. local vs. topical
C. variability – dosing flutuations, how long antibiotic lasts in the body.
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Antiviral drugs
Nucleotide analogs (look similar to the bases + sugars of DNA) Ex. AZT – looks like thymine; acyclovir – looks like guanine
Enzyme inhibitors Ex. reverse transcriptase inhibitor – important in controlling HIV infection
Interferons – naturally produced by our immune systemsEx. alpha, beta, and gamma; prevents further infection
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Antifungal drugs
Targets for antifungal drugs???Possible side effects???Ex. amphotericin B and
imidazole affect sterol productionGriseofulvin – binds to keratin in
skin cells, treats skin fungal infections
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Antibiotic resistance (also called drug resistance)
What is happening?As microbes are exposed to drugs,
the ones with defenses against them survive
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Microbial defense mechanisms against antibiotics
1. Change receptor for drug
2. Bind to drug and inactivate it, ex. penicillinase (b-lactamase)
3. Change target site, ex. change ribosome structure
4. Change metabolic pathway
5. MDR pump– multi-drug resistance pump
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How does drug resistance develop in different microbes?
Antibiotics overprescribed and not regulatedMany countries do not require a prescriptionImproper dosageAntibacterial soaps/surfaces, etc.Antibiotics in animals (used as our food
source)People insisting on receiving antibiotics,
even for viral infectionsNosocomial infections and the chronically
infected
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Antibiotic sensitivity tests
Kirby-Bauer test – zones of inhibition
Minimal inhibitory concentration test
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The End