20 - Antibiotic Resistance and Genetics

Question 1

4 main bacterial antibiotic resistance mechanisms

Answer 1

1. Altered antibiotic target
2. Inactivate antibiotic (Degrading the antibiotic or Chemically modify antibiotic)
3. Reduced antibiotic accumulation
4. alternate pathway to bypass the inhibited one

Question 2

Altering the target of the antibiotic

Answer 2

• Target protection
• Target modification
• Insensitive functional target

Question 3

Target protection

Answer 3

Resistance protein blocks access of antibiotic target

Question 4

Target modification

Answer 4

Resistance enzyme modifies antibiotic target

Question 5

Vancomycin resistance

Answer 5

arises when bacteria change the terminal D-ala in the pentapeptide of peptidoglycan into D-serine or D-Lac, so vancomycin cannot bind

Question 6

Insensitive functional target

Answer 6

Resistance protein is an alternative enzyme that is insensitive to antibiotic

Question 7

Antibiotic degradation

Answer 7

Hydrolysis of the β-lactam ring of penicillins and cephalosporins by penicillinase (β-lactamase) enzymes

Question 8

Chemically altering antibiotic

Answer 8

Acetyltransferase adds an acetyl group to aminoglycosides or
chloramphenicol, inactivating them

Question 9

Reduced antibiotic accumulation

Answer 9

Antibiotic efflux

Question 10

Antibiotic efflux

Answer 10

• Rapid extrusion of antibiotic via efflux pump
• Extrusion is energy dependent
• Bacteria can also reduce uptake

Question 11

Target bypass

Answer 11

Enterococci are intrinsically resistant to sulphonamides because they are able to use pre-formed folic acid from their environment (unlike other bacteria)

Question 12

Vertical gene transfer

Answer 12

Transfer of genes from parent to progeny

Question 13

Horizontal gene transfer

Answer 13

• Transfer of genes from one independent, mature organism to another
• Important in bacterial evolution, including the gain of antibiotic-resistance genes

Question 14

Four possible fates of HGT donor DNA in the recipient cell

Answer 14

• Integration via recombination with recipient’s genome
• Self-replication of the donor DNA (if plasmid)
• No replication and eventual loss of donor DNA
• Degradation of donor DNA

Question 15

Mobile genetic elements

Answer 15

DNA segments that can move to different locations on genomes and/or move from cell to cell.

Question 16

Examples of mobile genetic elements

Answer 16

• Plasmid
• Transposable element
• Integron

Question 17

Plasmid

Answer 17

Double-stranded DNA molecule, usually circular, that can exist and replicate independently of the chromosome

Question 18

Resistance (R) plasmid

Answer 18

• A plasmid bearing one or more antibiotic-resistance genes
• Gene cassette encoding antibiotic resistance inserts into integron which inserts into transposon which in turn inserts into a plasmid

Question 19

Transposable element

Answer 19

Small linear DNA molecule that carries genes for transposition and thus can move around the genome, or from one genome to
another (e.g. from plasmid to chromosome) via recombination

Question 20

Transposon

Answer 20

• Also carries other genes such as antibiotic resistance genes
• Is found as part of a bacterial chromosome or a plasmid, cannot self-replicate

Question 21

Integron

Answer 21

• Very small linear DNA molecule which can capture even smaller “gene cassettes” encoding antibiotic resistance
• Is found as part of a transposon or a plasmid, cannot self-replicate

Question 22

Three main mechanisms of horizontal gene transfer in bacteria

Answer 22

• Conjugation
• Transduction
• Transformation

Question 23

Conjugation

Answer 23

Gene transfer mediated by plasmids that requires contact between cells

Question 24

Transduction

Answer 24

Gene transfer mediated by bacteriophages

Question 25

Transformation

Answer 25

• The uptake of a piece of free DNA by a cell
• Both chromosomal DNA and plasmid DNA can be transformed

Question 26

Process of conjugation

Answer 26

1. F plasmid encodes proteins to build the sex pilus and the protein
   systems that perform DNA transfer
2. Sex pilus tip binds to recipient cell. Pilus retracts, pulling donor and recipient cells together to make contact and form a mating pore
3. Plasmid is nicked (one strand is cut), rolling circle replication of plasmid DNA starts
4. Both cells recircularize their plasmids, synthesise second strands to form double-stranded plasmids, and form pili; both cells are now viable donors in conjugation

Question 27

Relaxosome

Answer 27

Protein complex including a relaxase protein that nicks and unwinds the DNA, ready for transfer of a single DNA strand through the pore

Question 28

Transferosome / T4SS

Answer 28

Pushes the DNA/relaxase complex into the recipient cell

Question 29

Virulent phage

Answer 29

Only have a lytic cycle

Question 30

Temperate phages

Answer 30

Have a lytic cycle, or a lysogenic cycle

Question 31

Lytic cycle

Answer 31

• Phage injects its DNA into cytoplasm
• Phage DNA directs the synthesis of many new phages
• Cell lyses and releases the new phages
• New phages can bind to bacterial cells

Question 32

Lysogenic cycle

Answer 32

• Phage DNA integrates into bacterial host’s chromosome to form a lysogen.
• Phage DNA is inactive (prophage) and replicated along with chromosome
• Host cell does not lyse and grows normally.

Question 33

Prophage induction

Answer 33

Exposure to stress such as UV light triggers activation of the prophage and its excision from the host chromosome – lytic cycle
resumes

Question 34

Generalised tranduction

Answer 34

• Transfer of any genes from donor
• Occurs during the lytic cycle of virulent and temperate phages
• Bacterial genes are packaged by mistake into a phage head (capsid). Phage carries only bacterial genes (i.e. no phage genes).
• Any bacterial gene can be transduced, including antibiotic resistance genes

Question 35

Specialised tranduction

Answer 35

• Transfer of only genes adjacent to prophage
• Occurs during the lysogenic cycle of temperate phages only
• Transducing phage carries mainly phage genes plus some bacterial genes, which it will inject into its next host cell to form a transductant
• Only certain bacterial genes will be in the right location to be picked up and transduced by the temperate phage (i.e. they were right next to the
  prophage)

Question 36

Misuse of antibiotics

Answer 36

• Use as growth enhancers in animals
• Non-completion of antibiotic courses
• Unregulated antibiotic sales
• Use of wrong or suboptimal antibiotic doses