Mechs of Antibiotic Resistance - Pilch 4/26/16

Question 1

role of drugs in drug-resistance

Answer 1

rovides strong selective pressure in favor of resistant cell by preventing growth of wild-type, drug-sensitive cells

bacteriostatic drugs may have mutagenic props → non-specifically increase probabilities of many kinds of mutation

Question 2

how do bacteriostatic drugs actually serve antibiotic function if they only inhibit further growth?

Answer 2

immune system takes care of the rest!

however, bc they leave more bacteria alive and around, their shelf life is less than bacteriocidal drugs

• more bacteria left alive → more opportunities for resistance-devpt

Question 3

antibiotic resistance

where/how does it come about?

Answer 3

resistant strains most often pop up in hospitals, ppl who have recevied prior antimicrobial tx

within hospitals, highest rates of antibiotic use → highest rates of resistant bacteria

among patient population, longer duration of exposure to antibiotics → higher chance of being colonized by resistant organisms

Question 4

acquired drug resistance

genetic basis for resistance: 4 mechs

Answer 4

genetic basis for resistance: gene transfer

1. transformation : DNA containing resistance genes picked up from the environment and incorporated into genome and is expressed
2. transduction : resistance genes transferred from one bacterium to another via bacteriophage
3. conjugation: DNA containing resistance genes transferred in a plasmid via a direct connection through a pilus
   * no carrier required
4. transposition: allows resistance genes to move between plasmid and genomic chromosomal DNA
   * requires special enzymes (which bacteria have)

Question 5

acquired drug resistance

biochemical basis for resistance

Answer 5

biochemical basis for resistance

1. decreased intracellular drug level

• decreased drug entry
• increased drug efflux

ex. Gram- bacteria with porin mutations → decreases passive drug diffusion
ex. tetracycline resistance via acquisition of genes that encode active efflux system
* low specificity, high promiscuity pumps! don’t bind v strongly, but pump out many drugs
2. increased drug inactivation
* bacterial enzymes that make drugs biologically inert
ex. Gram- beta-lactamases → cut beta-lactam ring of penicillins and cephalosporins
* augmentin : beta-lactamase inhibitors, can be combined with drug to overcome resistance
ex. enzymatic modification of aminoglycosides
* overcome with nextgen drugs that have modified active sites
3. decreased conversion of drug to more active compound

• most often with drugs that act as antimetabolites
• arises via mutation/selection

ex. resistance to antifungal 5-flucytosine via decreased activity of UMP pyrophosphorylase
4. increased conc of a metabolite that antagonizes drug action
* increased production of the normal metabolite competes for drug, leading to resistance
ex. resistance to sulfonamides via increased PABA levels
5. altered amount of target enzyme/receptor

• resistance plasmids containing multiple copies of genes encoding target or that confer inducible expression of gene
• mutation in a regulatory gene that controls expression of the gene encoding the target

why? you’d need a higher dose of drugto get the same effect! = resistance

6. decreased affinity of receptor for drug
   ex. trimethoprim resistance via production of DHFR with reduced affinity for drug
   ex. streptomycin resistance via mutation of 12S protein on 30S ribosomal subunit
7. decreased activity of an enzyme required to express drug effect
   ex. mutants deficient in autolytic enzymes (resp for making new wall, critical for cell division) which break down sections of proteoglycan matrix during growth → turns drugs that would be bacteriocidal into bacteriostatic drugs!

Question 6

Gram+ vancomycin resistance

(vs. array of Gram- mechs for resistance)

Answer 6

vancomycin targets Gram+ via thick peptidoglycan wall → binds D-Ala-D-Ala moiety, interferes with crosslinking and formation of PG matrix

resistance: VanHAX operon: 3 genes coding unique ends that don’t bind vancomycin and allow for different (still strong) crosslinking

• alter target of drug

enterococci expressing the VanHAX operon are vancomycin-resistant!

• most common causo of endocarditis

Question 7

principles of antimicrobial chemotherapy to combat drug resistance

Answer 7

1. avoid administration of sub-inhibitory doses

• sub inhib doses promote selection of resistant subpops

2. administer combo of 2 drugs with diff mechs of action and diff biochem pathways of resistance

• never combine drugs that work the same way!

3. take full course