Antimicrobial Agents and Resistance

Question 1

Why does antibiotic resistance exist?

Answer 1

As bacteria produce antibiotics themselves, so they themselves have to be resistant to their own antibiotics

Question 2

How is antibiotic resistance passed on?

Answer 2

Plasmids containing resistant genes can be released into the environment and taken up by neighbouring bacteria.

Question 3

What are a) Antiseptics b) Disinfectants c) Antibiotics

Answer 3

a) Used on skin
>Too toxic for internal use

b) inanimate surfaces (usually stronger than antiseptics, kill microorganisms but not spores)
>Too toxic for internal use

c) Low molecular mass compounds that kill or inhibit the growth of bacteria and can be ingested or injected into the human body with minimal side effects.

Question 4

What are 5 requirements that make a good antibiotic?

Answer 4

1. Killing/ inhibition
   >Bacteriostatic compounds for intact immune system as immune system will kill bacteria.
   >Bactericidal compounds needed for defective immune system
2. Selective toxicity
   >Target structures only found in pathogens or is differently structured in humans.
3. Pharmacokinetics
   >Distribution of compound in the body so that the correct concentration reaches the target areas.
4. Broad spectrum
   >Symptoms can be caused by a range of bacteria, so need antibiotics to effect multiple strains.
5. Few side effects

Question 5

What are a) bacteriostatic b) bactericidal compounds?

Answer 5

a) bacteriostatic are antimicrobial agents that inhibit the growth of bacteria without directly killing them.

b) Bactericidal compounds are antimicrobial agents that directly kill bacteria.

Question 6

What damage can antibiotics do to our body?

Answer 6

May destroy natural flora allowing other organisms in human (e.g. yeast vaginitis)

Question 7

What are 3 antibiotics which inhibit the process of cell wall biosynthesis in bacteria?

Answer 7

Cycloserine, Phosphomycin, and Bacitracin (against gram-positive bacteria as have outer peptidoglycan layer)

Question 8

How does a) Clycoserine b) Fosphomycin c) Bacitracin inhibit cell wall biosynthesis in gram-positive bacteria?

Answer 8

a) Cycloserine is a structural analogue of D-alanine, it binds and inhibits the enzyme D-alanine racemase, which is involved in the synthesis of D-alanine. leads to a reduction in the amount of D-alanine available for the formation of the peptidoglycan layer, resulting in weakened cell walls and bacterial death.

b) Fosphomycin binds and inhibits the enzyme MurA. This prevents the production of the peptidoglycan precursor UDP-N-acetylglucosamine.

c) Bacitracin interferes with the dephosphorylation of the lipid carrier molecule, undecaprenyl pyrophosphate, preventing its reuse and leading to a depletion of peptidoglycan precursors in the cell.

Question 9

What are 3 examples of β-lactams?

Answer 9

penicillin, methicillin, cephalosporins.

Question 10

How do β-lactam (like penicillin, methicillin, cephalosporins) work?

Answer 10

The β-lactam ring is similar in structure to the D-alanyl-D-alanine component of the bacterial cell wall. They irreversibly bind to Penicillin binding proteins (PBPs) inhibiting them so they can’t catalyse transpeptidation and transglucosylase reactions. So the peptidoglycan layer weakens leading to cell lysis.

Question 11

What are a) transpeptidation b) transglucosylase reactions

Answer 11

a) Process by which glycan strands are synthesized and extended to produce peptidoglycan

b) Process by which peptide cross-links are formed between adjacent peptidoglycan strands making the peptidoglycan layer rigid.

Question 12

What is the difference between gram-positive and gram-negative bacteria?

Answer 12

1. Gram-positive bacteria have a thick outer layer of peptidoglycan
2. Gram-negative bacteria have a thinner layer of peptidoglycan in their cell wall, and an additional outer membrane composed of lipopolysaccharides (LPS)

Question 13

What type of bacteria do β-lactam work better against and why?

Answer 13

> Gram-positive

> As gram-negative bacteria have an outer membrane of LPS creating a barrier to reach the PNPs.

Question 14

What are the 4 main targets of antibiotics?

Answer 14

1. Cell wall biosynthesis
2. Nucleic acid synthesis
3. Protein synthesis
4. Cytoplasmic membrane function

Question 15

What are 4 antibiotics which target nucleic acid synthesis in bacteria?

Answer 15

Sulfonamides, trimethoprim, Quinolones, and Rifampin

Question 16

How do Sulfonamides and Trimerthoprim inhibit nucleic acid synthesis in bacteria?

Answer 16

Sulfonamides and trimethoprim inhibit tetrahydrofolic acid production which is required for nucleic acid synthesis.

Question 17

How do Quinolones inhibit DNA replication in bacteria?

Answer 17

Quinolones bind to and inhibit DNA gyrase, so negative supercoiling can no longer occur so DNA replication cannot occur.

Question 18

How does Rifampin inhibit RNA synthesis in bacteria?

Answer 18

Rifampin inhibits RNA polymerase.

Question 19

What are 5 antibiotics which target protein synthesis in bacteria?

Answer 19

Macrolides, lincosamides, chloramphenicol, Aminoglycosides, and Tetracyclines

Question 20

How do Macrolides, Linosamides, and Chloramphenicol inhibit protein synthesis in bacteria?

Answer 20

1. Macrolides bind to the 50S subunit of the bacterial ribosome and prevent the elongation of the polypeptide chain during translation by blocking the exit tunnel of the ribosome
2. Linosamides and Chloramphenicol inhibit the peptidyl transferase activity of the ribosome, which is responsible for the formation of peptide bonds between amino acids during protein synthesis

Question 21

How do Aminoglycosides inhibit protein synthesis in bacteria?

Answer 21

Bind to 30S subunit and prevent the 50S subunit from binding, leading to the formation of non-functional ribosomes that are unable to carry out protein synthesis.

Question 22

How do Tetracyclines inhibit protein synthesis in bacteria?

Answer 22

Tetracyclines bind to 30S subunit and prevent decoding of mRNA at ribosomes. Aminoacylate tRNA can not align with mRNA so polypeptides form incorrectly.

Question 23

What are two reasons for why it is easier for bacteria to develop resistance to antibiotics that target ribosomes?

Answer 23

1. As ribosomes are essential for life any mutation or modification that reduces the effectiveness of the antibiotic without affecting ribosome function may give the bacteria a selective advantage.
2. As ribosomes are complex structures composed of many protein and RNA molecules, there are many potential target sites for antibiotics to bind. Complexity makes it easier for bacteria to evolve resistance mechanisms that allow them to evade the effects of antibiotics.

Question 24

What are 3 antibiotics which inhibit cytoplasmic membrane function in bacteria?

Answer 24

Gramicidin, Polymyxins, and Daptomycin

Question 25

How does Gramicidin disrupt cytoplasmic membrane function in bacteria?

Answer 25

Gramicidin forms cation channels spanning the membrane. Causes H+ to leak from cell, so cannot keep proton motive force (PMF) so ETC cannot create as much ATP. (gram- and + as cytoplasmic membrane found on both).

Question 26

How does Polymyxins disrupt cytoplasmic membrane function in bacteria?

Answer 26

Polymyxins bind to and disrupt the outer lipopolysaccharide layer of gram-negative bacteria leading to leakage of cellular contents.

Question 27

How do Daptomycin disrupt bacterial cell membranes?

Answer 27

Ca2+ dependent insertion into cell membrane. Forms transmembrane channel leading to K+ leaving the bacterium leading to cell death (not lysis).

Question 28

What is the definition of an organism resistant to antbiotics?

Answer 28

A resistant organism is one that will not be inhibited or killed by an antibacterial agent at concentrations of the drug achievable in the body after normal dosage.

Question 29

What is a Persister?

Answer 29

As antibiotics aren’t sterilising, not 100% of the organisms are killed, those that survive are called persisters.

Question 30

What are the 4 main sources of antibiotic resistance in the Western World?

Answer 30

1. Over-prescription of antibiotics
2. Accessibility of antibiotics (can buy over the counter in some countries- too easy)
3. Improper use by patients
   >Failure to complete antibiotic courses may leave some bacteria with resistance alive and without competition so can divide.
4. Over-use of antibiotics in agriculture to promote growth and prevent infections in livestock. Development of antibiotic-resistant bacteria in animals, which can then be transmitted to humans through food consumption.

Question 31

Why are antibiotics used in agriculture?

Answer 31

1. Keep animals from getting ill
2. Promote growth so can be sold quicker:
   a. May clean out intestine of animal, so the bacteria that take away from the nutrients are gone
   b. Removal of harmful bacteria in intestine, that interfere with ability of animal to grow

Question 32

What are the two main types of antibiotic resistance in bacteria?

Answer 32

1. Innate (intrinsic) resistance
   >natural, inherent resistance of a bacterial species to a specific antibiotic.
2. Acquired resistance
   > resistance that a bacterium acquires through genetic changes.

Question 33

Describe 2 types of innate antibiotic resistance

Answer 33

1. Lack of target
   >Some bacteria are naturally resistant to certain classes of antibiotics because they lack the specific target site that the antibiotic is designed to bind to.
   >E.g. Mycoplasma have no peptidoglycan so are resistant to penicillins.
2. Impermeable to antibiotics
   >Gram-negative bacteria have an outer membrane that can restrict the entry of some antibiotics

Question 34

Describe the 4 types of Acquired antibiotic resistance?

Answer 34

1. Reduced uptake
   >Single mutation in outer membrane (in gram -) porins (non-specific channels) disrupting the passive entrance of molecules so antibiotics can’t enter.
2. Efflux
   >Pump antibiotics out of the cell, reducing the concentration of the antibiotic inside. Gram negative bacteria as require LPS membrane
   >E.g. Multidrug resistance (MDR) exporters use ATP.
3. Inactivation of drug via enzymatic cleavage or modification
   >Bacteria produce enzymes that can degrade or modify the antibiotic e.g. b-lactamase hydrolyses lactam ring of penicillin
4. Alteration of target
   >Bacteria modify the target site of the antibiotic so that it is no longer recognized or bound by the antibiotic e.g. alterations in penicillin binding proteins (PBPs) causes low affinity for b-lactams.

Question 35

What is a last ditch effort to gain antibiotic mutations?

Answer 35

Some cells become hypermutational in stressful conditions, have less regulation in DNA replication so introduces more mutations to increase the chance of a helpful mutation.

Question 36

What are the 3 mechanisms used for bacteria to acquire genetic resistance to an antibiotic?

Answer 36

1. Transformation
   >The take up of DNA from the environment (surrounding bacteria)
2. Transduction
   >Genetic material is transferred between bacteria by a bacteriophage
3. Conjugation
   >Direct cell-cell contact allows transfer of plasmids between bacterial cells

All are transformation and transduction are examples of horizontal gene transfer, where genetic material is transferred between bacteria that are not related by reproduction.

Question 37

What are two examples of structures which carry antibiotic resistance genes?

Answer 37

Plasmids and Transposons

Question 38

What are 3 ways to combat the misuse of antibiotics?

Answer 38

1. Find new drugs
   >Screen environmental samples, alter existing drugs, use crystal structure of target molecule.
   >Costs $1billion per drug/ takes 10-12 years
2. Vaccination
   >More immunity against bacteria means less need for antibiotics in first place, less viral infection means less miss-diagnosis (giving antibiotics for virus harming human flora).
3. Diagnostics
   >Next generation sequencing tells us what species, strain, resistance will be encountered. So we don’t waste the wrong antibiotics; reducing the wrong antibiotics being given.

Question 39

What are the resistance mechanisms against a) Tetracyclines b) Macrolides, streptogramins and lincosamide c) Quinolones and rifampin

Answer 39

a) Many ways to stop antibiotic binding to ribosomes, e.g. point mutations, proteins blocking binding.

b) Due to RNA methylase. Methylates adenine residue within binding site for all the drugs.

c) due to point mutations in DNA gyrase and RNA polymerase