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39 Cards in this Set

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  • Back
who came up the principle of selective toxicity. exlain the principle?
- it is the key to chemotherapy against microbes
-it says that an effective antimicrobial agent must be more toxic to a pathogen than to the pathogen's host.
- it is possible b.c of the difference of structure of the host and the pathogen.
what is an antibiotic?
-it is a biologically product agent, produced to gain competitive advantage when nutrients are scarce.
-they are 2ndary metabolites, produced at the end of log phase or during stationary phase
-competition for nutrients leads to the production of compounds that prohibit growth of other microbes
how are antibiotics made?
they are produced naturally by the organism.
how do antibiotics work?
They prevent the NAG/NAM peptide link, produces strings, not mesh and plasma membrane bulges thru holes in cell wall causes it to burst due to osmotic pressure
-so basically they prevent cross-linkage of NAM subunits, this leads to incorrectly formed peptidoglycan-->cell wall is weakened--> bacteria is less resistant to effects of osmotic pressure-->the cell wall bulges--> causes to cell lyses
how do you measure the effectiveness of antibiotics?
1. u can use the Kirby-Bauer aka susceptibility test--> measure zone of inhibition (clear, area surrounding the disk where microbes do not grow)--) measure the diameter in mm--> size of zone of inhibition can be compared to standard table for that particular antibiotic--) it will let you know the effectiveness of the antibiotic by the size of the zone
what is zone of inhibition ?
in the kirby-bauer the zone of inhibition is characterized by the clear, area surrounding the disk where microbes do not grow
What are the mechanisms by which antibiotics work?
they inhibit:
1.cell wall synthesis
2. pathogen's attachment to host or recognition of host
3. DNA/ RNA synthesis
4. protein synthesis
5. disrupt cytoplasmic membrane
how antibiotics interact?
1. synergism: b/w two antimicrobial agents --) combining of an antimicrobial drug and a chemical cane enhance the effect of a drug by deactivating B-Lactamase.
2. antagonistic--) interfer w/ each other --) drugs that slow bacterial growth are antagonistics ot the action of penicillin
Why are some antibiotic chemically modified?
-by altering them scientists can produce synthetics--)drugs that are more effective, longer lasting, or easier to adminster compared to that of the naturally occuring antibiotics
what are synthetics?
antimicribials that are completely synthesized in a lab
how do antibiotics affect growth?
When an antibiotic is present the majority of cells (which are sensitive to the antibiotic) are inhibited or die but resistant cells continue to grow + multiply, often more readily because they face less competition
minimum bactericidal concentration?
the lowest concentration of drug for whic no growth occurs in the subculture.
spectrum of action?
the number of different kinds of pathogens a drug acts against
narrow-spectrum drugs?
drugs that only work against a few kinds of pathogens
Bacteriostatic concentration?
any drug concentration at which growth occurs in subculture
a substance capable of killing bacteria
a toxic produced by bacteria that kills other bacteria
a treatment capable of inhibiting bacterial growth
Describe antimicrobial drugs that are bactericidial?
the kill microbes directly
Describe antimicrobial drugs that are bacteriostatic?
they prevent microbes form growing
broad-spectrum antibiotics?
antibiotics that affect a broad range of gram-positive and gram-negative bacteria.
explain how Paul ehrlich can up with the basis of chemotheraphy?
While attempting to stain bacteria witout staining the surrounding tissue, he speculated about some "magic bullet" that would selectively find and destroy pathogens but not harm the host-->selective toxicity
synthetic drugs?
drugs that have been synthesized by chemical procedures in the laboratory
drugs produce by bacteria and fungi
the treatment of infectious diseases using chemical substanaces
penicilin? explain the structure of pencillin?
a group of over 50 chemical related to antibiotics.
- they all have a common stucture containging a B-lactam ring called the nucleus.
- pencillin molecules ar differentiated by the chemical side chains attached to their nuclei.
-they are produced either naturally of semisythetically
what are the important variables in the experiment that shows how antibiotics effect growth?
variable 1= gram negative vs. gram positive bacteria-->
variable 2= in the inhibition oc cell wall sunthesis you need to recall that peptidoglycan is only found in bacterial cell walls so penicillin wil prevent the synthesis of intact peptidogylcan.
-variable 3= teh difference in ribosomal structure accounts for the selective toxicity of antibiotics that affect protein synthesis.
-proteins affecting 70S ribosome will affect the host b.c the mitochondria contains 70S ribosomes
-MIC= minimmal inhibitory concentration is determined using the E Test, which enables a lab techniciam to estimate the MIC
- u determine the MIC by making a sequence of decreasing concentrations of the drug in a broth, which is then inoculated with the test bacteria--)the wells that do not show growth(higher concentration than the MIC) can be cultured in broth free of the drug. if growth occurs in this broth, the drug was not bactercidal
-so basically broth dilution test is used in which a standardized amt of bacterial is added to serial dilutions of antibiotic--) after incubation --)measure turbidity (cloudiness) to indicate growth
Minimal inhibitory concentration= the lowest antibiotic concentration that prevents visible bacterial growthe
why do some antibiotic work against some species and not others?
different species have different structures and components that are not affected by some antibiotics.
for example if a drug such of bacitracin is used it would only be effective against gram positve bacteria because this antimicrobial inhibits the synthesis of cell walls so if a species do not have a cell wall if will not be an effective drug
--some antibiotics will only affect the 30S ribosome so if it is not a prokaryote then it would not be affective by this drug (prokarytotes=70S (30S+50S)
how do some organisms become resistant to antibiotics?
a microbe for example could become resistant to an antibiotic such as trimethoprim by synthesizing very large amts of the enzyme that the drug/antibiotic is targeted
Mechanisms by which antibiotics become resistanct?
1. destruction or inactivation of the drug
2. prevention of penetration to the target site within the microbe
3. alteration of the drug's target sites
antibiotic interactions?
– synergism
 action of one antimicrobial agent enhancing the other’s activity
– antagonistic action
 action of one interferes with the other
– additive
 neither synergistic or antagonistic
therapeutic index?
 measurement of the toxicity of a drug is called the therapeutic index
what causes Resistance to Antimicrobial Drugs?
 overuse and misuse of antimicrobial drugs
– resistance is generally a result of
 inactivation by microbial enzyme
 prevention of reaching target
 alteration of target
What are R-plasmids?
they are extra pieces of DNA that they may cause a cell to become resistant
- some micro-orgos produce these enzymes and this enzyme will deactivate the drug by inducing changes in the cell membrane that prevent entry of the drug, by altering the drug's receptor to prevent its binding
cross resistance?
when resistance to one chemotherapeutic agent confers resistance to similar drugs
what does the Beta-lactams do?
- they have lactam ring and it prevents bacteria from cross-linking NAM subunits of peptidoglycan in the bacterial cell wall during growth