Use and resistance to antibiotics

Question 1

Describe the difference between intrinsic and acquired antibiotic resistance

Answer 1

Intrinsic: an innate ability to resist activity of a particular antimicrobial agent through inherent structural or functional characteristics which allow tolerance of the drug (may lack target or have an altered target)
Acquired: when a microorganism obtains the ability to resist the activity of a drug (alteration of target, provides bypass mechanism, efflux systems)

Question 2

How can bacteria change their genetic make up to result in resistance?

Answer 2

• New genes (insertions/ inheritance on plasmids)
• Loss of genes (deletions)
• Modification of function of genes by the above or point mutations
• Modification of promotors leading to altered expression (point mutations)

Question 3

What is transferable acquired resistance?

Answer 3

Tends to be based on genetic transfer through plasmids, some transposons have transfer mechanisms but most move via the plasmid. Transposons can hop between chromosomes and plasmids.

Question 4

Give some examples of intrinsic resistance

Answer 4

Lack target: gram +ve bacteria are resistant to LPS targeting antibiotics because they don’t have the outer membrane.
Target is different to other Genera: Enterococci are generally resistant to cephalosporins as they have a low affinity for enterococcal PBP

Question 5

Describe acquired resistance via mutation

Answer 5

• A common mutation for resistance
• Loss of genes (deletions)
• Modification of existing genes
• Modification of expression levels
• e.g. mis sense mutation would result in an Aa in binding site altering to a different Aa so drug couldn’t bind
• e.g. DNA gyrase mutates so that it can tolerate flouroquinolones

Question 6

Describe acquired resistance by new genes

Answer 6

• Acquisition of resistance genes such as:
  Beta lactamases (break structure)
  Aminoglycoside modifying enzymes/ Target site modifying enzymes (both add modifications)
  Efflux systems (new gene or upreg of existing gene)

Question 7

Describe MRSA resistance

Answer 7

Methicillin resistant Staph aureus (MRSA)

• MRSA encodes for an alternate PBP= PBP2a
• So although PBP1 is bound by the antibiotic the presence of PBP2a means that crosslinking still occurs so it can tolerate the present of the antibiotic.

Question 8

Describe acquired resistance modification of cell wall

Answer 8

In Enterococcus mechanism of resistance to vancomycin involved the terminal Aa which van binds to being altered

• This mod can be found on a transposon
• The transposon can hop about within the chromosome onto plasmids aiding its transfer

Question 9

Describe resistance by efflux

Answer 9

These transport groups of compounds with similar chemical properties

• Internalised drug is transported out to below effective conc
• 5 different families of transporters which all move similar compounds so you can get selection for resistance pumps by range of agents leading to CROSS RESISTANCE

Question 10

What do the abbreviations MDR, XDR and PDR stand for?

Answer 10

MDR: Multi drug resistant- Acquired non susceptibility to at least one agent in three or more categories
XDR- Extensively Drug resistant: acquired non susceptibility to at least one agent in all but two or fewer antimicrobial category
PDR- Pan drug resistant: Acquired non susceptibility to all agents in all antimicrobial categories

Question 11

Describe gene transfer for acquired resistance

Answer 11

Plasmids allow: transfer of resistance between closely related bacteria/ transfers of complex collections of genes to express resistances
Cassettes and transposable elements allow: increased mobility of complex collections of genes to express resistances/ ability to hop in chromosomes and plasmids and between

Question 12

What are ther other acquired resistance gene transfer routes than plasmids?

Answer 12

Transduction: bacteriophage mediated
Transformation: uptake of naked DNA from environment

Question 13

List the different susceptibility tests used

Answer 13

1) Dilution test (MIC)
2) Disc diffusion
3) E-strip diffusion (rapid simple MIC)

Question 14

Briefly describe the procedure for a disc diffusion test

Answer 14

1) Drop bacteria onto plate using a sterile spreader to spread onto surface of non selective agar
2) Use sterile forceps to place discs on agar label side up
3) Incubate under specified conditions and view. Normally 37C for 24hrs
* *Agar must be the right thickness or diffusion circle is inaccurate, also right amount of bacteria must be used or it skews the results and may make it look like the drug is less effective than in reality.

Question 15

What is a break point assay?

Answer 15

It differs from a sensitivity test, it relies on a distinct separation of R vs S, antibiotics in plates, number of strains tested on plates, with a control plate with no antibiotics

Question 16

Who governs the use of antibiotics?

Answer 16

• Recommendations in Europe, UK and US

- Require effective surveillance of resistance at local, national and international levels

Question 17

What are the pros/ cons to prophylactic use of AB?

Answer 17

-Pros: minimal if therapy commences later then 3 hours post contamination
-Cons:
Toxicity
Drug resistance
Residues in food animals
Cost
(Prophylactic treatment should only be used if there is evidence that usage will reduce mortality/ morbidity and the dosage should be the same as the therapeutic dose)

Question 18

Give examples of prophylactic AB use

Answer 18

• Dry cow therapy
• Feedlot pneumonia on calves
• Strangles in horses

Question 19

Which AB are Dobermans hypersensitive to?

Answer 19

Sulphonamides