1.14. Antibiotic action & mechanisms of bacterial resistance

Question 1

list the mechanisms of action of antibiotic agents

Answer 1

1. inhibit cell wall synthesis
2. disrupt cell membrane
3. inhibit protein synthesis
4. inhibit DNA

Question 2

which antibiotic agents inhibit cell wall synthesis?

Answer 2

1. B-lactams
2. glycopeptides
3. Fosfomycin

Question 3

which antibiotic agents disrupt the cell membrane?

Answer 3

1. Lipopeptides

2. Polypeptides

Question 4

what do antibiotic agents act on to inhibit protein synthesis?

Answer 4

1. inhibit 30s ribosome (AT)

2. inhibit 50s ribosome (mol)

Question 5

which antibiotic agents inhibit DNA?

Answer 5

1. Quinolones (inhibit DNA replication)
2. Rifamycins (inhibit DNA transcription)
3. Metronidazole (inhibit DNA function)
4. Sulphonamides & trimethoprim (inhibit DNA synthesis)

Question 6

(1) β-lactams

Answer 6

•β-lactam antibiotics contain a β-lactam ring in their molecular structures
•Work by inhibiting peptidoglycan cross-linkage
•Include: 
1. Penicillins
2. β-lactam-inhibitor combinations
3. Cephalosporins
4. Monobactams
5. Carbapenems

Question 7

(2) Glycopeptides

Answer 7

• Inhibit peptidoglycan cross-linking in gram positive organisms
• E.g. vancomycin, teicoplanin

Question 8

(3) Fosfomycin

Answer 8

•Inhibits the enzyme pyruvyl transferase during the first step of peptidoglycan synthesis

Question 9

(1) Lipopeptides

Answer 9

• Binds irreversibly to the cytoplasm membrane of bacteria resulting in disruption of cellular ion concentration gradients. This causes cell death.
• Activity against Gram positive bacteria E.g. Daptomycin

Question 10

2)Polypeptides

Answer 10

•Disrupts the lipid component of the outer membrane of gram negative bacteria leading to cell death.
E.g. polymixinB , polymixinE (colistin)

Question 11

Inhibit 30S ribosome

Answer 11

Act on 30S ribosomal subunit. Prevent translation of mRNA to tRNAfor peptide chain synthesis
•1) Aminoglycosides
•E.g. streptomycin, gentamycin, amikacin, tobramycin
2) Tetracyclines
•E.g. doxycycline, minocycline

Question 12

Inhibit 50S ribosome

Answer 12

Act by binding to 50S ribosomal subunit thereby interfering with protein chain synthesis
1)Macrolides
E.g erythromycin, clarithromycin, azithromycin
2) Lincosamides
E.g. clindamycin
3) Oxazolidones
E.g. Linezolid

Question 13

(1) Inhibit DNA replication

Answer 13

• Quinolones
• Inhibit bacterial DNA gyrase & Topoisemeraserequired for DNA uncoiling leading to inhibition of DNA replication and eventually cell death
• Narrow spectrum/first generation=Nalidixicacid
• Broad spectrum /second generation= Ciprofloxacin, levofloxacin, ofloxacin, •Expanded spectrum / third generation=Sparfloxacin
• Extended spectrum /fourth generation=Gatifloxacin, moxifloxacin

Question 14

(2) Inhibit DNA transcription

Answer 14

•Rifamycins
•Acts by binding to bacterial DNA-dependant RNA polymerase, resulting in inhibition of RNA synthesis.
E.g. rifampicin

Question 15

(3)Inhibit DNA function

Answer 15

Metronidazole

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

Question 16

(4)Inhibit DNA synthesis

Answer 16

Sulphonamides and trimethoprim

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

Question 17

Intrinsic resistance

Answer 17

• Innate ability of a bacterial species to resist the activity of a particular antimicrobial agent
• The organism possesses inherited structural or functional characteristics
• This kind of resistance is predictable

Question 18

Acquired resistance

Answer 18

• Changes in usual genetic makeup of a microorganism

* The resistance is unpredictable

Question 19

list the mechanisms of acquiring resistance genes in bacteria

Answer 19

1. transformation
2. conjugation
3. transduction

Question 20

Transformation

Answer 20

• Bacteria acquire new genetic material from the environment
• Resulting in an altered gene with altered affinity for antibiotic
• E.g. 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 21

Conjugation

Answer 21

• Bacteria contain plasmids which are circular DNA structures that are found in the cytoplasm
• Genes carried by plasmids include those that code for metabolic enzymes, virulence and antibiotic resistance
• Plasmids can pass from one bacterium to another, through pili, allowing resistance genes to spread
• Requires cell-to-cell contact
• E.g. Enterobacteriacea & resistance to β-lactams-extended spectrum β-lactamases are found on plasmids

Question 22

Transduction

Answer 22

• Transfer of DNA from one bacterium into another via bacteriophages (a virus that infects bacteria)
• When a bacteriophage infects a bacterium, it takes over the bacteria’s genetic processes to produce more phage.
• Bacterial DNA may be incorporated into the new phage DNA during this process .
• Once the bacteria dies these new phage go on to infect other bacteria with genes from the previously infected bacterium

Question 23

list the mechanisms of antibiotic resistance

Answer 23

1. enzymatic inactivation or alteration
2. reduced availability of antibiotic
3. modification of the antibiotic target site

Question 24

Enzyme inactivation

Answer 24

•Enzymes bind to antibiotic molecule & either destroy antibiotic molecule or prevent it from binding to target sites.
•Example:
•β-lactamases produced by some bacteria cleave β-lactam ring of β -lactam antibiotics which results in inactivation of the antibiotic
(Staphylococcus aureus, Neisseria gonorrhoea, Haemophilus influenzae)

Question 25

Enzyme modifying the antibiotic:

Answer 25

some bacteria produce enzymes that add an inactivating chemical group to the antibiotic causing the antibiotic to be inactivated •Example •acetyl, phospho & adenylyl tranferasescan modify the aminoglycoside antibiotic in Gram positive & Gram negative bacteria resulting in poorer affinity of the antibiotic to ribosomes (Staphylococcus aureus, Pseudomonas spp)

Question 26

name 3 mechanisms which lead to reduced availability of antibiotic

Answer 26

1. impermeability 2. porin loss 3. efflux pumps

Question 27

(1) Impermeability

Answer 27

bacteria are intrinsically resistant to antibiotic due to being impermeable to that antibiotic •Example: Pseudomonas spp & β-lactams ( Penicillin G, aminopenicillins & aminopenicillin –β-lactamase inhibitor combinations ,first, second & third generation cephalosporins, ertapenem) –(small porinsthat do not allow these agents to permeate) •Aminoglycosides enter bacteria by an oxygen dependent transport mechanism and therefore have little effect against anaerobic bacteria

Question 28

(2) Porin loss

Answer 28

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

Question 29

(3)Efflux pumps

Answer 29

•Antibiotics pumped out of cell so rapidly that they cannot accumulate in the cytoplasm •Example:Escherichia coli and streptococci resistance to macrolides, tetracyclines, quinolones

Question 30

3) Modification of the antibiotic target site

Answer 30

•Antibiotics normally bind to specific binding proteins on the bacterial cell surface. •Changed site= blocked binding Examples •Ribosomal point mutation: tetracyclines, macrolides, clindamycin, aminoglycosides •Altered DNA gyrase: Quinolones •Modified penicillin binding proteins: Penicillins •Mutation in DNA dependant RNA polymerase: Rifampicin