Antibiotics Lab

Antibiotic

Substance produced by a microorganism that inhibits growth of another microorganism

Antibiosis

Interactions between microbes where at least 1 is harmed

Kirby-Bauer Method

Determines if microbe is sensitive or resistant to an antibiotic

Zone of inhibition

Clearing, indicates bacteria’s growth was impacted

Diameter of zone of inhibition is compared to interpretive standard for each antibiotic

Might have to measure radius if too close to edge or not visible and times by 2

E test

Determines the lowest effective concentration antibiotic inhibits bacteria growth

minimum inhibitory concentration (MIC)

Uses a plastic strip w/ 1 antibiotic in different concentrations

Vitek susceptibility test

Determine if microbe is resistant or sensitive to an antibiotic via automated method

Can test lots of antibiotics

A card contains wells each containing growth medication and a different antibiotic

Wells are inoculated w/ a sample and observed by measuring turbidity

Incubated and read via computer

Turbid = microbe grew = microbe resistant to antibiotic

Clear = microbe didn’t grow = microbe sensitive to antibiotic

Broad spectrum

Effective against a wide variety of bacteria

Gram + and Gram - bacteria

Narrow spectrum

Only effective against certain bacteria

Either Gram + or Gram - bacteria, not both

Antibiotic mechanisms

1. Inhibition of cell wall synthesis

2. Inhibition of protein synthesis

3. Inhibition of nucleic acid replication

4. Injury to plasma membrane

5. Inhibition of essential metabolite synthesis

Target: bacterial cell wall

Inhibit peptidoglycan synthesis

Affect Gram + bacteria

Affect Gram - bacteria if it can get through porins

Penicillin (P)

Targets G+

Does not fit well through porins

Ampicillin (Am)

Targets Gram + and some Gram -

Does fit well though porins

Target: cell membrane

Makes membrane leakier

Affects Gram - bacteria more than Gram + bacteria

Polymyxin B (PB)

Positively charged

Attracted to negatively charged lipoproteins

Broad spectrum

Most fit through porins

Protein synthesis process is conserved

Examples:

• Chloramphenicol (C)

• Streptomycin (S)

• Tetracycline (Te)

• Gentamicin (Gm)

• Erythromycin (E)

Steps of isolation & identification

1. Streak plate from a clinical sample

2. Isolate colony

3. Gram stain to start ID (shape, arrangement, gram reaction, size)

4. Metabolic test to complete ID (BAP, McConkey, MSA)

5. Antibiotic sensitivity tests (informs treatment options)

What must be considered when deciding which antibiotic to prescribe for a patient?

• Pathogen identity

• Gram + or Gram -

• Sensitivity and resistance to antibiotics

Which antibiotics are protein synthesis inhibitors?

• Chloramphenicol

• Erythromycin

• Gentamamicin

• Tetracycline

• Streptomycin

Which antibiotics are peptidoglycan synthesis inhibitors?

• Penicillin (P)

• Ampicillin (Am)

Which antibiotic causes injury to the plasma membrane of bacteria?

Polymyxin B

Which antibiotics only work on Gram + bacteria?

• Penicillin

• Erythromycin

Which antibiotics only work on Gram - bacteria?

Polymyxin B

Which antibiotics are broad spectrum and work on Gram + and Gram - bacteria?

• Chloramphenicol

• Streptomycin

• Tetracycline

• Erythromycin (mostly works on Gram - as it does not fit well through porins

What are the best antibiotics for Staphylococcus aureus?

• Penicillin

• Ampicillin

What are the best antibiotics for E. coli?

• Gentramycin

• Streptomycin

• Polymyxin B

Why are bacteria becoming resistant to many antibiotics?

Mutations and high multiplication