iii. Microbial antibiotic action and mechanisms of bacterial resistance

Question 1

Mechanism of action of antibiotic agents: (8)

Answer 1

Question 2

Inhibit cell wall synthesis:

• Β-lactams: (2)

Answer 2

o Β-lactam antibiotics contain a β lactam ring in their molecular structures

o Work by inhibiting peptidoglycan cross-linkage
Peptidoglycan is an important component of the cell wall of GP and GN organisms. When this cross-linkage does not occur, there is disruption of the cell wall.

Question 3

• Β-lactams include: (5)

Answer 3

▪ Penicillins
▪ Β-lactam inhibitor combinations (amoxicillin, clavulanate)
▪ Cephalosporins (1-5)
▪ Monobactams
▪ Carbapenems

Question 4

Inhibit cell wall synthesis:
- Glycopeptides: (2)

Answer 4

o Inhibit peptidoglycan cross-linking in gram positive organisms

o Example: Vancomycin, teicoplanin

Question 5

Inhibit cell wall synthesis:
- Fosfomycin:

Answer 5

o Inhibits the enzyme pyruvyl transferase during the first step of peptidoglycan synthesis.

Question 6

Disrupt cell membrane:
- Lipopeptides: (3)

Answer 6

o Binds irreversibly to the cytoplasm membrane of bacteria resulting in disruption of cellular ion concentration gradients. This causes cell death.
o Activity against Gram-positive bacteria.
o Example: Daptomycin

Question 7

Disrupt cell membrane:
- Polypeptides: (2)

Answer 7

o Detergent-cationic property: Disrupts the lipid component of the cytoplasmic membrane of gram-negative bacteria leading to cell death.

o Example: Polymixin B, Polymixin E (Colistin: Antibiotic used for salvage therapy in extensively drug-resistant gram-negative infections)

Question 8

Inhibit DNA:
- Inhibit DNA replication:

How does this occur? (2)

Answer 8

o Quinolones
o Inhibit bacterial DNA gyrase and topoisemerase required for DNA uncoiling leading to inhibition of DNA replication and eventually cell death.

Question 9

• Inhibit DNA replication:
  o Different classes of quinolones: (4)

Answer 9

Question 10

How is DNA inhibited? (4)

Answer 10

• Inhibit DNA replication
• Inhibit DNA transcription
• Inhibit DNA function
• Inhibit DNA synthesis

Question 10

• Inhibit DNA transcription:
  o Rifamycins:

Answer 10

Acts by binding to bacterial DNA dependant RNA polymerase, resulting in inhibition of RNA synthesis.
Example: Rifampicin

Question 11

• Inhibit DNA function:
  o Metronidazole:

Answer 11

Receives electrons under anaerobic conditions and forms toxic metabolites that damage bacterial DNA.

Question 12

• Inhibit DNA synthesis:
  o Sulphonamides and trimethoprim/bactrim:

Answer 12

Act by inhibiting the synthesis of tetrahydrofolate which is necessary for bacterial DNA synthesis.

Question 13

What are the types of
Antibiotic Resistance? (2)

Answer 13

• Intrinsic resistance
• Acquired resistance

Question 14

What is - Intrinsic resistance? (3)

Answer 14

Question 15

What is acquired resistance? (2)

Answer 15

o Changes in usual genetic makeup of a microorganism. Bacteria acquires this resistance with repeated exposure to antibiotics – Often seen
in the hospital setting.

o The resistance is unpredictable

Question 16

Mechanisms of acquiring resistance genes in bacteria: (3)

Answer 16

• Transformation
• Conjugation
• Transduction

Question 17

Mechanisms of acquiring resistance genes in bacteria:
- Transformation: (3)

Answer 17

o Bacteria acquire new genetic material from the environment
o Resulting in an altered gene with altered affinity for antibiotic
o Example: Streptococcus pneumoniae can acquire short fragments of penicillin binding protein (PBP) genes from closely related species to form a new PBP that has less affinity for penicillin and therefore resistance develops.

Question 18

Mechanisms of acquiring resistance genes in bacteria:
- Conjugation: (6)

Answer 18

Question 19

Mechanisms of acquiring resistance genes in bacteria:
- Transduction: (4)

Answer 19

o Transfer of DNA from one bacterium into another via bacteriophages (A virus that infects bacteria). Virus carries mechanism of resistance and takes it into a bacterial cell, thereby bacteria can also develop the mechanism of resistance.

o When a bacteriophage infects a bacterium, it takes over the bacteria’s genetic processes to produce more phage.

o Bacterial DNA may be incorporated into the new phage DNA during this process.

o Once the bacteria die these new phage go on to infect other bacteria with genes from the previously infected bacterium. When virus moves from one bacterial organism to another, it can acquire additional genes (some which may have mechanisms of resistance) – Mode of transmission of resistance from one bacterial organism to another.

Question 20

Mechanisms of acquired resistance:
- Genetic material can also be transferred by _______ & ________ (jumping genes) Both are mobile genetic material

Answer 20

INTEGRONS & TRANSPOSONS

Question 21

Mechanisms of acquired resistance:
- INTEGRON:

Answer 21

• INTEGRON: “Gene cassette”. Mobile egentic material that can enter into bacterial organisms but need to attach to specific sites on the DNA of the bacterial organism. Advantage of an integron – Also has its own mechanisms to express that particular gene (positive regular, negative regulator for gene). Therefore, it can express the gene without additional genetic support from the host bacterial DNA.

Question 22

Mechanisms of acquired resistance:
- TRANSPOSONS:

Answer 22

• TRANSPOSONS: Mobile genetic material. Can move from one bacterial organism to another. They do not have any specific site of integration into the host’s DNA (can combine at any point). Cause modification to the host-bacterial DNA.

Question 23

Mechanisms of acquired resistance:
- Resistance can also occur with time – A normal mutation could occur based on the exposure of the organism to various ______ (Naturally occurring chromosomal mutation that would occur over time following exposure to antibiotics).

Answer 23

antibiotics

Question 24

Mechanisms of acquired resistance:
- More common is _____,_____ and ______.
- Antimicrobial resistance is growing at such an alarming rate - The mobile DNA acquires resistance mechanisms from bacteria as it move from 1 species of bacteria to another.

Answer 24

Transformation, Conjugation and Transduction

Question 25

Mechanisms of antibiotic resistance: (3)

Answer 25

• Enzymatic inactivation or alteration (A)
• Reduced availability of antibiotic (B)
• Modification of the antibiotic target site (C)

Question 26

Mechanisms of antibiotic resistance:
- Enzymatic inactivation or alteration (A) =

Answer 26

Enzymes bind to antibiotic molecule & either destroy antibiotic molecule (hydrolyse) or prevent it from binding to target sites.

Question 27

Mechanisms of antibiotic resistance:
- Enzymatic inactivation or alteration (A)
o Enzyme inactivation, examples: (2)

Answer 27

▪ Example: β-lactamases (hydrolyse B-lactam agents) produced by some bacteria cleave β-lactam ring of β-lactam antibiotics which results in inactivation of the antibiotic.

▪ Example: Staphylococcus aureus, Neisseria gonorrhoea, Haemophilus influenzae (Gram positive and gram negative)

Question 28

Mechanisms of antibiotic resistance:
o Enzyme modifying the antibiotic:

Answer 28

Some bacteria produce enzymes that add an inactivating chemical group to the antibiotic causing the antibiotic to be inactivated.

Question 29

Mechanisms of antibiotic resistance:
o Enzyme modifying the antibiotic:

▪ Acetyl, phospho & adenylyl tranferases can modify the ________ antibiotic in Gram positive & Gram negative bacteria resulting in poorer affinity of the antibiotic to ribosomes. Can phosphorylate or methylate the antibiotic – Modified antibiotic cannot attach to ______ at site of action.
▪ Example:

Answer 29

aminoglycoside
ribosome
▪ Example: Staphylococcus aureus, Pseudomonas spp.

Question 30

Mechanisms of antibiotic resistance:
- Reduced availability of antibiotic (B)
o Impermeability:

Answer 30

Certain bacteria are intrinsically resistant to an antibiotic due to impermeability to that particular antibiotic.

Question 31

Mechanisms of antibiotic resistance:
- Reduced availability of antibiotic (B)
o Impermeability:

▪ _____ _____ & β-lactams (Penicillin G, aminopenicillins & aminopenicillin β lactamase inhibitor combinations ,first, second & third generation cephalosporins, ertapenem ).
Small _____ that do not allow these agents to permeate into the cell.
▪ Aminoglycosides enter bacteria by an_____ ______ transport mechanism and therefore have little effect against anaerobic bacteria.

Answer 31

Pseudomonas spp
porins
oxygen dependent

Question 32

• Reduced availability of antibiotic (B)
  o Porin loss:
  Examples…

Answer 32

▪ Example: Escherichia coli & β-lactams (Not intrinsically resistant to beta-lactam agents however, porin loss can occur conferring the resistance.)

Question 33

• Reduced availability of antibiotic (B)
  o Efflux pumps:

Answer 33

Antibiotics pumped out of cell so rapidly that they cannot accumulate in the cytoplasm.

Question 34

• Reduced availability of antibiotic (B)
  o Efflux pumps:
  Examples…

Answer 34

Escherichia coli and streptococci resistance to macrolides tetracyclines, quinolones

Question 35

Mechanisms of antibiotic resistance:
- Modification of the antibiotic target site (C) = (2)

Answer 35

o Antibiotics normally bind to specific binding proteins on the bacterial cell surface
o Changed site = Blocked binding

Question 36

Mechanisms of antibiotic resistance:
- Modification of the antibiotic target site (C)

Examples…(4)

Answer 36

▪ Ribosomal point mutation: Tetracyclines macrolides, clindamycin, aminoglycosides
▪ Altered DNA gyrase: Quinolones
▪ Modified penicillin-binding proteins: Penicillins
▪ Mutation in DNA dependant RNA polymerase: Rifampicin