Microbial Growth Control II - Antibiotics

natural or synthetic chemical that kills or inhibits growth of microorganisms

growth is inhibited, but cells are not killed

cells are killed, but not lysed

cells are killed and lysed

lowest concentration of an agent that completely inhibits growth

• 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

• 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

• antimicrobial agents used in vivo
• can be synthetic, naturally occuring, or semi-synthetic

• 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

substance that is structurally similar to growth factor, but blocks utilization of the growth factor

inhibit bacteria or other pathogenic agents without adversely affecting host

• 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

• very specific spectrum of action - isoniazid
• broad spectrum of action - tetracycline

• bacteriostatic, -cidal, -lytic
• targets: cell wall, cell membrane, DNA synthesis, RNA synthesis, protein synthesis, metabolism

structure: b-lactam ring

• mode of action:
• inhibits transglycosylase and trans peptidase
• bacteriolytic

spectrum: most effective against gram positive bacteria

structure: cyclic peptide

• mode of action:
• forms cation channel that disrupts membrane polarity
• bacteriolytic

used topically on skin

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

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

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