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antimicrobial agent
natural or synthetic chemical that kills or inhibits growth of microorganisms
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*bacteriostatic
growth is inhibited, but cells are not killed
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*bacteriocidal
cells are killed, but not lysed
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*bacteriolytic
cells are killed and lysed
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*minimal inhibitory concentration (MIC)
lowest concentration of an agent that completely inhibits growth
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*antibiotic assay by tube dilution
- a series of decreasing concentrations of antibiotic is prepared in the culture medium
- each tube is inoculated and incubation is allowed to proceed
- growth occurs in tubes with concentrations below the MIC
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*Kirby-Bauer disk susceptibility test
- add culture inoculated in liquid over nutrient agar plate
- place antibiotic discs on surface
- incubate
- test organism shows sensitivity to some antibiotics, indicated by inhibition of bacterial growth around discs after incubation
- compare zone of inhibition to standard chart
- routinely used for testing antibiotic sensitivity in pathogens
- generated antibiograms - indicated sensitivity of clinical ioslates
- track emergence of antibiotic resistance strains of pathogens
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chemotherapeutic agents
- antimicrobial agents used in vivo
- can be synthetic, naturally occuring, or semi-synthetic
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growth factor
- specific chemical substance required in the medium for growth of organism
- organic compounds required in very small amounts and only by some cells
- includes vitamins, amino acids, purines and pyrimidines
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growth factor analogs
substance that is structurally similar to growth factor, but blocks utilization of the growth factor
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*selective toxicity
inhibit bacteria or other pathogenic agents without adversely affecting host
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antimicrobials structure
- diverse structures have anti-microbial activity
- structural classification class can correlate with mode of action
- very similar structures tend to have same mode of action, but may not
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antimicrobial spectrum of action
- very specific spectrum of action - isoniazid
- broad spectrum of action - tetracycline
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antimicrobials modes of action
- bacteriostatic, -cidal, -lytic
- targets: cell wall, cell membrane, DNA synthesis, RNA synthesis, protein synthesis, metabolism
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penicillin
structure: b-lactam ring
- mode of action:
- inhibits transglycosylase and trans peptidase
- bacteriolytic
spectrum: most effective against gram positive bacteria
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gramicidin
structure: cyclic peptide
- mode of action:
- forms cation channel that disrupts membrane polarity
- bacteriolytic
used topically on skin
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quinolone
structure: quinolone
- mode of action:
- interact with bacterial DNA gyrase
- prevent gyrase from supercoiling DNA and result in DNA with breaks
- supercoiling is required for packaging of DNA in bacterial cell
- DNA repair mechanisms are induced in uncoordinated fashion
- irreversible damage to DNA and death
- bacteriocidal
- spectrum of action:
- gram negative and positive bacteria
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sulfanilamide
structure: PABA analog
- mode of action:
- growth factor analog (PABA)
- blocks synthesis of DHF and folate
- binds reversibly
- bacteriostatic
- spectrum of actvivity:
- gram positive and negative bacteria
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tetracycline
structure: napthacene ring system
- mode of action:
- reversibly binds to site on 30S ribosomal subunit
- aminoacyl-tRNAs do not bind productively to the A site on the ribosome
- inhibits protein synthesis
- bacteriostatic
spectrum of activity: broad
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