Chapter 20 PPT

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
 General terms used to include other microbes
are antimicrobial drugs or antimicrobials
 Difference between disinfectants/antiseptics and
antimicrobial drugs
Antibiotic spectrum
vs. narrow spectrum antibiotics
Benefits and problems with each
Microbes that produce antibiotics:
Bacteria: Bacillus and Streptomyces
Fungi: Cephalosporium and Penicillium
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
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
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,
◦ host mitochondria may also be affected
Modes of action of the antibiotics:
3. Increase permeability of plasma
◦ ex. polymyxin B – used topically
◦ can have high toxicity in humans
Modes of action of the antibiotics:
4. Inhibit synthesis of RNA/DNA
◦ can be very toxic
◦ ex. quinolones – DNA
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
Antagonism vs. synergism of 2 or
more antibiotics
◦ Ex. Tetracycline  penicillin (Why?)
◦ Ex. TMP-SMZ (SMX) trimethoprim and
Prophylaxis – preventative use of
Ex. of their use:
1. before surgery – i.e. appendix
2. before dental work in heart
3. in AIDS patients to prevent many
Factors affecting antimicrobial activity
in vivo –
1. In the environment
A. metabolic state of pathogen
Ex. 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.
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.
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
Antifungal drugs
Targets for antifungal drugs???
 Possible side effects???
 Ex. amphotericin B and imidazole affect
sterol production
 Griseofulvin – binds to keratin in skin
cells, treats skin fungal infections
Antibiotic resistance
(also called drug resistance)
What is happening?
As microbes are exposed to drugs, the
ones with defenses against them survive
Microbial defense mechanisms against antibiotics
Change receptor for drug
Bind to drug and inactivate it, ex. penicillinase (blactamase)
Change target site, ex. change ribosome structure
Change metabolic pathway
MDR pump– multi-drug resistance pump
How does drug resistance develop in
different microbes?
Antibiotics overprescribed and not regulated
Many countries do not require a prescription
Improper dosage
Antibacterial soaps/surfaces, etc.
Antibiotics in animals (used as our food
People insisting on receiving antibiotics, even
for viral infections
Nosocomial infections and the chronically
Antibiotic sensitivity tests
Kirby-Bauer test – zones of inhibition
 Minimal inhibitory concentration test
The End

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