38 - Antimicrobials and Resistance

What is PRSA?

Penicillin Resistant Staphylococcus aureus

What allows for antibiotic resistance?

1. Over perscription

2. Prescribing below the optimum dose

True or False: we induce antibiotic resistant bacteria

False → bacteria are constantly mutation and selectively encourage growth of bacteria that is resistant due to mutations

What is constitutive resistance? Examples?

• Bacteria are reistance because that don’t have the systems to uptake ABX or don’t have the targets the ABX is looking for → their structure ain’t right!

  • Mycoplasma + B-lactam ABX → lack of cell wall

  • Penicillin G + Enterobacteriaceae → tiny channels and lipid layer won’t allow PG to enter

  • Obligate anaerobes are resistant to aminoglycosides (active transport requires oxygen → anaerobes don’t have that system)

What type of resistance is related to previous ABX exposure?

Acquired

What is acquired resistance? Examples?

Bacteria become resistant to ABX via mutations in uptake system and targets

• Staphylococci + Penicillin G → make enzymes that inactivate ABX (penicillinase/beta-lactamase)

What is the mechanism of acquired resitance?

1. Alter drug target

• Modify target site (Ex. methylation of rRNA changes macrolide binding site on 50S ribosomal subunit)

• Reduce target site → alternative pathways so ABX doens’t change their function (ex. alternative folic acid synthesis pathway)

2. Alter drug uptake

• Inhibit uptake → decrease pore size so ABX is too big

• Increase excretion of ABX (tetracyclines)

3. Inactivate Drug

• Enzymes (ex. beta-lactamases, make into anti-binding form)

How is bacteria resistant to multiple drugs?

1. Multi-ABX resistance

• Different resistance mechanisms = different classes

• Ex. Resistance to aminoglycosides (altered ribosomal protein) and penicillin (via beta-lactamase production)

2. Cross-resistance

• Resistance due to the same mechanism = same class

What does it mean that cross-resistance is unidirectional?

Resistance in older class drug means that the bacteria could be sensitive or resistant to new class drug BUT

Resistance in newer class drug means that the bacteria MUST be resistant to older class drug

How to bacteria acquire resistance?

1. DNA Mutation

2. Acquisition of new DNA with mutations

What type of mutations results in antibiotic resistance and what is required?

Selective Advantage Mutations

• Must have ABX present → “Selective Pressure”

Describe the different kinds of mutations that can cause bacteria to be resistant (or not) to ABX

• Most mutations are lethal → bacteria dies

• Mutations can give them

  • disadvantages (die out over time)

  • no advantage

  • selective advantage (become dominant type → ABX resistance)

Describe how bacteria acquires mutations from other bacteria

• Genetic transfer → between strains → genetic diversity

1. Conjugation → plasmid transfer via sex pilli

2. Transduction → transfer through virus, bacteriophage delivers viral genes which are integrated into chromosome

3. Transformation → free genes in environment from dying bacteria (least effective)

What type of mechanism is responsible for multi-antibiotic resistance?

Genetic Transfer between bacteria

Besides antibiotic resistant genes, what can bacteriophages also introduce into the bacterial chromosome?

Virulence factors

What mechanism allows multiple antibiotic resistance genes to be introduced to bacteria?

Conjugation

What mechanism is more common in gram negative bacteria? Why?

Conjugation → gram + doesn’t have sex pillus and has a thick peptidoglycan layer

What should we consider when selecting an antibiotic?

1. Gram + or -?

2. Route

3. Historical data

4. In vitro sensitivity

5. ABX sensitiivty testing

What ABX are used for gram - vs. gram + bacteria?

Positive = Penicilin

Negative = Gentamicin

What ABX are used for rickettsia?

Tetracyclines

What ABX are used for Streps?

Penicillin

How does Kirby Bauer determine ABX sensitivity?

• Spread bacteria on plate

• Put a disk of ABX on there → not able to grow by ABX = not resistant

How does E Test determine ABX sensitivity?

• Spread bacteria on plate

• Strips have different levels of ABX in them

• Able to see minimum concentration inhibiting bacteria

How does Broth Dilution Method determine ABX sensitivity?

• Dilute ABX across a bunch of tubes

• Add same amount of bacteria to each one

• See what dilution the bacteria doesn’t grow

• Able to see minimum concentration inhibiting bacteria