Micro: Antimicrobial Resistance

Question 1

Name/define the two forms of antimicrobial resistance.

Answer 1

Innate - existence before exposure to the drug.Acquired - antibiotic use-driven selection and accumulation of resistant strains

Question 2

Name four examples of innate resistance. How does these resistances work?

Answer 2

G+ bacteria resistant to polymyxins; lack of lipopolysaccharide layer (LPS) G- bacteria resistance to vancomycin; vancomycin too large to cross thick outer membrane Intracellular bacteria to b-lactams; b-lactams poorly absorbed by animal cells Chlamydia and mycoplasma to b-lactams; both bacteria lack peptidoglycan cell walls

Question 3

Name the 5 basic principles of antibiotic resistance.

Answer 3

1. Resistance will likely emerge given sufficient time/drug use 2. Antibiotic resistance is progressive. 3. Bacteria resistant to one drug are most likely to become resistant to other drugs. 4. If resistance is present, it’ll decline slowly if at all. 5. Anti-biotic use affects those in immediate and extended healthcare environment.

Question 4

What is a bactericidal agent? What situations would be use bactericidal agents?

Answer 4

Bactericidal = causes cell death Used during chronic infections and infections known to create biofilms.

Question 5

What is a bacteriostatic agent?

Answer 5

Bacteriostatic = slows cell growth

Question 6

How do bacteriostatic agents clear infections if they only slow bacterial growth?

Answer 6

They don’t clear infections. They slow infections to control them until the immune response can clear them.

Question 7

What general class of antibiotics are bacteriostatic? What is one member of this class that is bactericidal?

Answer 7

All protein synthesis inhibitors are bactiostatic EXCEPT aminoglycosides, which are bactericidal.

Question 8

Define MIC.

Answer 8

MIC = Minimum inhibitory concentration. It is the lowest concentration that a drug must achieve to inhibit growth of a certain bacterium.

Question 9

Define Breakpoint.

Answer 9

Breakpoint = the MIC that defines whether a bacterium is susceptible or resistant to a drug.

Question 10

Use MIC and breakpoint to define if a bacterium is resistant, susceptible, or intermediate to a drug.

Answer 10

Resistant = If the breakpoint of a drug for a bacterium is below or equal to its MIC. Susceptible = if the breakpoint is above the MIC Intermediate = if the breakpoint is resistant for some measurements and susceptible for others.

Question 11

Define MBC.

Answer 11

MBC = minimum bactericidal concentration. Its the lowest concentration that a drugs much achieve to kill a certain bacterium.

Question 12

Name/Define the 6 indications to use multi-drug therapy

Answer 12

1. Probable synergy – the two drugs we’re prescribing have shown to work together for a stronger effect 2. Polymicrobial infection – there’s more than one microbe and have to treat with more than one antibiotic 3. Uncertain diagnosis – we’re not entirely sure what the bacterium is, so let’s hit it from a few angles 4. Reduction in emergence of resistance – a bacterium will struggle to adapt to two or more drugs at the same time, so we can reduce the chance of resistance emerging 5. Reduction of therapeutic levels of toxic drug – a drug that is toxic at therapeutic levels by itself can be made therapeutic and non-toxic at lower levels in conjunction with another drug (not entirely sure on this one, edit later) 6. Organ coverage (eg, access to CSF) - not entirely sure on this one, edit later

Question 13

Name/define the three general mechanisms of antibiotic resistance.

Answer 13

1. Exclusion - resistance born from excluding the antibiotic from the bacterium’s cytoplasm 2. Altered target - somehow changing the drug’s target to inhibit binding 3. Enzymatic inactivation - producing an enzyme to inactivate a drug.

Question 14

What is the most robust mechanism of resistance?

Answer 14

Enzymatic inactivation is the most robust form of resistance because no sacrifice in activity must be made.

Question 15

What are the two types of exclusion?

Answer 15

Innate and Acquired exclusion

Question 16

Give three types of acquired exclusion resistance mechanisms.

Answer 16

1. Altered membrane lipids - reduce drug binding/diffusion 2. Altered porins or transport properties - make it more difficult for antibiotics to enter the bacterium. 3. Increase efflux pump expression - increasing the ability to actively pump out antibiotics. Seen in tetracycline resistance and often seen in multi drug resistance. Induced by drug presence

Question 17

What are the two types of altered target resistance?

Answer 17

1. Amino acid substitutions to reduce affinity for antimicrobial agents. 2. Acquisition of new activity to altered the drug’s target.

Question 18

What limits the range of mutation that can occur in altered target resistance?

Answer 18

Functionality. The altered binding sites have to still function for the bacterium for it to thrive, so it can only change so much.

Question 19

How are newer antibiotics designed to combat altered target resistance?

Answer 19

New drugs are designed to bind to multiple sites so if one site is altered, it can still be efficacious at another site.

Question 20

(left) Which antibiotics are this bacterium resistant? Susceptible? (right) What is the plate on the right testing?

Answer 20

(left) This Kirby-Bauer test shows it’s definitely resisting A and C. Its resistant to D and F to a some degree. Its susceptible to B, E, and G. (right) This E-strip is testing different concentrations of an antibiotic to see what concentration is efficacious.

Question 21

Name the two types of enzymatic inactivtion; give examples of each.

Answer 21

Antimicrobial destruction - cleavage or some other method of destroying the enzyme to inactivate it.

Ex) b-lactamases

Antimicrobial modification of hydroxyl or amino groups

Ex) acetylation, adenylation, or phosphorylation

Question 22

Which mechanisms are used in resisitance of antimicrobials that target the cell wall?

Answer 22

All three (exclusion, altered target, inactivation)

Question 23

How to Gram- bacteria use exlcusion to combat cell antimicrobial agents? Are they innate or acquired?

Answer 23

Penecillins - combination of poor uptake and efflux pump expression

Vancymycin - outer membrane porins are not large enough to accomodate this drug’s entry

Thse bacteria may also modify their porins to acquire b-lactam exclusion.

Question 24

How does Staph aureus (G+) use exclusion to combat vancomycin?

Answer 24

Can increase the thickness of its cell wall to exclude vancymycin from the cell wall precursors that it attaches to and trap vancomycin in the cell wall.

Question 25

What are considered the most important drug targets altered by bacteria? How are they altered?

Answer 25

Penicillin-binding proteins (PBPs). These proteins are the transpeptidases involved in crosslinking the peptidoglycan cell wall of Gram+ bacteria and are the site of b-lactam action. Bacteria have many types of transpeptidases, so resistance can form through upregulating transpeptidases with reduced or no b-lactam affinity.

Question 26

How does MRSA use PBP alteration for resistance to b-lactams?

Answer 26

It has the altered mecA gene that encodes an altered PBP that resists ALL b-lactam action.

Question 27

How do *streptococcus *and neisseria used use PBP alteration for resistance?

Answer 27

Some acquire altered PBPs that are mosaics from other specie that can confir resistance.

Question 28

Describe target alteration mediated Vancomycin resistance from the vanA gene. Why is vancomycin resistance such a big deal?

Answer 28

vanA encodes an altered D-ala D-lactate (normally D-ala D-ala), which is not a PBP and cannot be recognized by vancomycin. Thus, cell wall cross-linking can continue.

This is a big deal because vancomycin is the drug of choice for MRSA infection. Linezolid can be used, but its not good to lose 50% of your MRSA treatable drugs.

Question 29

How does b-lactamase secretion differ in Gram- vs Gram+ bacteria?

Answer 29

Gram- have more complex secretion systems, realeasing b-lactamase into their perplasm. This allows for greater b-lactamase concentration, thus a more prominent role in resistance.

Gram+ are less complex. They simply excrete b-lactamase into their immediate environment. Thus it takes a lot of gram+ bacteria together to have a strong effect.

Question 30

What are Extended spectrum b-lactamases (ESBLs)? What are the three most common types?

Answer 30

While normal b-lactamses only work on penicillins, ESBLs work on penicillins and cephalosporins.

1. TEMs
2. SHVs
3. CTX-Ms

Question 31

What antibiotic do ESBLs generally stay sensitive to?

Answer 31

Carbapenem

Question 32

What bacterium can encode for a cabapenemase?

Answer 32

*Klebsiella pneumoniae *(KPC). Its often multidrug resistant and super difficult to purge from hospitals.

Question 33

What bacteria encode for a cephalosporinase? Why are most penicillin resistant bacteria often treatable with cephalosporinases?

Answer 33

Most do, but they’re low level and can only fend off poorly absorbed b-lactams.

Only high level expression of cephalosporinase can fend off cephalosporins, so most will be susceptible.

Question 34

How has the development of b-lactams altered their effectiveness?

Answer 34

As newer generations of b-lactams have been developed, they have become more effective going form only treating some gram+ bacteria to now having effects on gram- and resistant bacteria.

Question 35

What are the three antimicrobials that target protein translation and what resistance mechanisms are used?

Answer 35

1. Tetracycines - efflux pumps/ribosomal masking
2. Macrolides - efflux pumps
3. Chloramphenicol - acytlation

Question 36

Why were are floroqinolones thought not to be naturall resisted by bacteria? Are they resisted by any bacteria?

Answer 36

Floroquinilones are completely synthestic antibiotics, so there shouldn’t be “natural” resistances to them.

They are plasma-encoded resisted some multidrug resistant enteric bacteria and through less specific mechanisms like efflux pumps.

Question 37

What are the 5 keys to control of antimicrobial resistance?

Answer 37

1. Develop new antibiotics (but currently little industry activity)
2. Prudent use of existing antibiotics: No antibiotics for viral infections, even though the patient wants treatmennt and pain killers for self-limiting infections (many ear aches)
3. Develop more vaccines to prevent infections in the first place
4. Improve infection control in hospitals
5. Reduce antibiotic use and animal-to-human infection in agriculture and husbandry