Antimicrobial Resistance

Question 1

Minimum inhibitory concentration

Answer 1

Lowest concentration of a drug that will completely inhibit growth of a bacterial strain
- a bacterial strain is resistant to an antibiotic when its MIC is higher than the normally achievable and tolerated concentration of the drug attained in tissues with maximum dosage

Question 2

Antibiotics act by inactivating a _____

Answer 2

Specific bacterial target

- ex: cell wall, ribosome, DNA gyrase

Question 3

Beta-lactams

Answer 3

Alteration of the target protein (penicillin-binding protein, PBP) so that the antibiotic no longer binds to it

• ex: penicillin and cephalosporin
• PBPs are bacterial enzymes (transpeptidases) that mediate crosslinking of a peptidoglycan in formation of the cell wall
• this type of resistance is a common mechanism that causes gram-pos bacterial beta-lactam resistance
• PBP 2a causes resistance to beta-lactam antibiotics, including methicillin in Staph aureus –> remains active at beta-lactam concentrations that inhibit most PBP enzymes

Question 4

Macrolides and lincosamides

Answer 4

Bacterial enzyme that methylates an adenine on 23S rRNA (plasmid mediated)

Question 5

Aminoglycosides

Answer 5

Mutations that alter a specific ribosomal protein

- passive form of modification

Question 6

Tetracyclines

Answer 6

Ribosome protection by a bacterial cytoplasmic protein

Question 7

Quinolones

Answer 7

Mutations that alter the affinity of bacterial DNA gyrase for the antibiotic

Question 8

Beta-lactamase

Answer 8

Bacterial enzyme that cleaves the beta-lactam ring

• secreted in the periplasmic space by gram-neg, secreted into extracellular fluid by gram-pos
• beta-lactamases are usually active against only a subset of beta-lactam antibiotics
• beta-lactamase inhibitors (clavulanic acid) can prevent inactivation by beta-lactamases

Question 9

Chloramphenicol acetyltransferase (CAT)

Answer 9

Bacterial enzyme that mediates inactivation of chloramphenicol by covalent attachement of an acetyl group (plasmid mediated)

Question 10

Aminoglycosides

Answer 10

Bacterial enzymes inactivate the antibiotic by attaching group (phosphoryl, adenyl, acetyl) that reduce its transport into the cell and interfere with binding to the ribosome

Question 11

Impermeability

Answer 11

• gram-neg outer membrane limits antibiotic access to the cytoplasmic membrane because antibitotics must first diffuse through the pores in outer membrane (porins)
• thought to be the reason for E. coli innate resistance to macrolides
• mutations in porins can limit diffusion of antibiotics, a single porin mutation can confer resistance to more than one antibiotic type

Question 12

Active efflux

Answer 12

• tetracycline: bacterial cytoplasmic membrane proteins that catalyze energy-dependent transport of tetracycline out of the cytoplasm (plasmid mediated)
• prevents sufficient antibiotic concentrations in the cytoplasm to inhibit protein synthesis
• active efflux system also exists for quinolones

Question 13

Genetic mechanisms to antibiotic resistance

Answer 13

• genetic resistance is likely to be detected by susceptibility testing –> usually detected at the onset of therapy
• unlikely to appear during a course of antibiotic therapy
• genetic resistance can arise by gaining the ability to resist one or more antibiotics by acquisition of genes or by resistance from random mutation

Question 14

Antibiotic resistance acquired on plasmids

Answer 14

Horizontal gene exchange

• almost all antibacterial drugs used medically have a corresponding resistance gene on at least one type of R plasmid
• resistance (R) plasmids often mediate resistance to more than one antibiotic
• plasmids can carry transposons
• broad host range plasmids can be transferred across bacterial species
• 60-90% of resistance genes in gram-neg are carried on plasmids

Question 15

Gene acquisition results in rapid acquisition of _________

Answer 15

High-level resistance

Question 16

____ and _____ both require acquisition of a gene for the function to be acquired

Answer 16

Enzymatic antibiotic inactivation and active efflux

Question 17

Acquired genes have a tendency to be stable in a strain because ______

Answer 17

They generally cause relatively little disadvantage to the bacteria

Question 18

Resistance from random mutation

Answer 18

• random mutations are typically point mutations (change of one base pair into another)
• random mutations can revert back to the original gene sequence
• single base pair change can sometimes result in high level resistance, but often multiple mutations have to be acquired, so development of resistance can be gradual

Question 19

How do random mutations result in resistance?

Answer 19

Either reduce permeability or alter target affinity

Question 20

Do random mutations cause growth impairment, or reduction in virulence?

Answer 20

Yes

• fitness cost: often mutation causing resistance also has a cost due to decreased function of the altered target
• in absence of antibiotic, the sensitive strain has the selective advantage

Question 21

Does random mutation or horizontal gene transfer have a higher fitness cost?

Answer 21

Random mutation

Question 22

Alteration of target

Answer 22

• gene acquisition

- random mutation

Question 23

Inactivation of antibiotic

Answer 23

• gene acquisition

Question 24

Impermeability

Answer 24

• random mutation

Question 25

Active efflux

Answer 25

• gene acquisition

Question 26

What are you testing for in a susceptibility test?

Answer 26

Genetic resistance

Question 27

Phenotypic variant

Answer 27

A small fraction of the original bacterial population can survive antibiotic therapy
- phenotypic variant is not due to any genetic changes
- is typically not detected on antibiotic sensitivity testing
= transient resistance: results in slow therapeutic response, therapeutic failure, or recurring infection
- will only be seen during therapy due to selective pressure for parental “wild-type” phenotype in absence of antibiotic
- cost of resistance is high: on removal from the host or from antibiotic selection, bacteria revert rapidly to full susceptibiity

Question 28

Bacterial persisters

Answer 28

Non or slow growing reversible phenotypic variants of the wild type, tolerant to bactericidal antibiotics

• tolerance of antibiotics due to inhibition of essential cell functions during antibiotic stress, resulting in inactivity of the antibiotic target
• persistance requires coordinated metabolic changes, entry and exit from persister state is regulated by signal molecules

Question 29

Small colony variants

Answer 29

Bacteria have slow growth and colonies have small size and atypical morphology

• SCVs have reduced electron transport, which leads to decreased ATP synthesis
• cause of reduced electron transport can be reduced hemin or menadione biosynthesis or defective thymidine deficiency
• result is decreased metabolism and decreased uptake of antibiotic, causing temporary antibiotic resistance
• SCVs have decreased virulence and decreased virulence factor production

Question 30

L-forms

Answer 30

Occurs when bacteria lose their cell wall, they are temporarily more resistant to beta-lactam antibiotics

• usually in gram-neg bacteria, but recently found in Listeria and Mycobacterium
• only works when bacteria are in an environment where they are not susceptible to osmotic lysis

Question 31

Biofilm

Answer 31

A structured community of bacteria enclosed in a self-produced polymeric matrix and adherent to an inert or living surface
- bacteria in a biofilm can convert into a regular planktonic state

Question 32

Resistance in biofilm due to

Answer 32

• slower growth rates of bacteria within biofilms
• decreased diffusion of antibiotics through the biofilm (protective matrix)
• accumulation of enzymes that contribute to resistance
• activation of stress response in bacteria in biofilms

Question 33

Weaning and backgrounding strategies reduce the need for antibiotics

Answer 33

• low stress weaning
• pre stress vaccination
• easy adaptation to new diets

Question 34

____, ____, and _____ improve milk quality, prevent lameness, and reduces the need for antibiotics

Answer 34

Better diets, comfort, and sanitation