Experiment 19: Antibiotic Sensitivity Testing

what are antibiotics produced by and to do what?

produced by microorganisms

inhibit or kill other microbes and have selective toxicitiy

more toxic to microbes than humans/human cells

what are the 2 classifications of antibiotics?

1. broad spectrum

2. narrow spectrum

what are broad spectrum antibiotieics?

target wide range of bact including big groups (like gram + AND -)

use when pathogen is unknown

what are narrow spectrum antibiotics?

target more specific group of bact (like Gram + OR - not both)

use for known pathogens

how do many antibiotics stop protein synthesis in bact cells?

by binding to 70S prok ribosome

but have low affinity for bigger 80S ribosomes in euk cells so less impact on us

sensitive bacterial strain?

drug is effecvtive in treating the infection caused by this strain.

resistant bacterial strain?

not killed or inhibited by the drug.

what do clear zones indicate?

where conc of antibiotic was too high for cell to grow (static) or killed (cidal)

exp 19 learning objectives

1. demonstate sterilty and aseptic technqiue

2. interpret clea cones and assess bact sensitivity or resistance

3. diff between broad and narrow spectrum drugs

exp 19 materials

1. E. coli

2. S. aureus lvl 2

3. TSA plates

4. Antibiotics

   1. Tetrtacycline

   2. Penicillin G

   3. Erythomycin

   4. Polymyxin B

Exp 19 Method

1. mark 4 spots for each antibiotic on plate.

2. dip forceps in alc and drip xs, flame.

3. pick up antibioic and put on plate and press lightly w forceps.

4. repeat. incubate at 37deg for 48hr

Results - Tetracycline

prevents binding of tRNA to 70S ribo

only in prok bc euk have 80S ribo

BROAD spectrum

Results - Erythromycin

attached to bact ribosomes, affects translation

gram -ve most effective - limited use

Results - Penicilin G

inhibits synthesis of peptidoglycan

Gram + most effective bc GRam - protcted by LPS

some derivatives are broad spectrium

Results - Polymyxin B

alterns membrane permeability, binds to phospholipids

topical treatment for gram -ve infections