Antibiotics

Question 1

What is a bacteriostatic antibiotic?

Answer 1

One that causes cessation of growth, but the organism stays viable
- the body can then kill off by itself, because when the bacteria can’t multiply the balance is tipped towards the host

Question 2

What is a bactericidal antibiotic?

Answer 2

One that kills the bacteria
- if the patient is immunocompromised need to use this because they don’t have any leucocytes to be able to do it themselves

Question 3

What are the properties of penicillin G?

Answer 3

Has no toxic effects (unless allergic), but has to be injected and is painful (not orally active)
- useful on GPC, GPR, GNC
Mimics the D-ala-D-ala bond –> locks all the bacterial enzymes (DD-transpeptidase) up so the bacteria can’t make a cell wall

Question 4

What are the properties of penicillin V?

Answer 4

Has an added side chain so it is more acid stable –> can be taken orally

Question 5

What are the properties of methicillin?

Answer 5

Also needs to be injected, one of the more toxic penicillins but can be used to treat penicillin resistant staph
** now have MRSA though

Question 6

What are the properties of ampicillin?

Answer 6

Has a broader spectrum - effective against GPC, GPR, GNC, GNR

Question 7

What is the structure of peptidoglycan?

Answer 7

Repeating disaccharides in one layer and cross linking peptides in another which make a unique N-acetyl muramic acid

Question 8

What is the biosynthesis of peptidoglycan?

Answer 8

Precursors are synthesised from intermediates in the cytoplasm which become immobilised on the inner aspect of the plasma membrane
- then using energy the blocks are transferred to the outside

Question 9

What is the action of vancomycin?

Answer 9

It is a glycopeptide used to combat MRSA, binds directly to the D-ala-D-ala

• doesn’t act on gram -ve bacteria at all because it is a very big molecule and highly charged
• however can get VISA - intermediate staph aureus, which are furry on the outside and have made more cell wall material to mop up the vancomycin

Question 10

What is a beta lactamase?

Answer 10

Enzymes produced by bacteria which recognise the same PBP as beta lactams and break open the ring structure, rendering them ineffective

Question 11

What is clavulanic acid and how does it work?

Answer 11

A beta-lactamse inhibitor
- doesn’t have great antimicrobial action so cannot be used to treat infection on its own, but given with amoxicillin or ampicillin and they act synergistically

Question 12

What is the mechanism of action of aminoglycosides?

Answer 12

Eg Gentamicin, acts on the recognition stage of protein synthesis

• interfere with recognition by binding to the site where the amino acyl transferase will bind, distort the site so it doesn’t look the same
• quite highly charged, so at first only get across the membrane in low concentrations, but as the membrane is weakened can get across in larger concentrations

Question 13

What are the mechanisms of resistance to aminoglycosides?

Answer 13

• enzymatic modification of the drug to make it inactive
• modified outer membrane leading to reduced entry of the drug
• efflux - ATP powered pump which pumps the drug out of the cell before it combines with the ribosomes
• ribosomal mutation - which doesn’t let the drug bind to the ribosome

Question 14

What is the mechanism of action of metronidazole?

Answer 14

A synthetic drug, needs to be reduced to become active

• nitroreductase is needed, we don’t have it but bacteria that live in anaerobic environments do
• drug is active against anaerobic bacteria and very effective against protozoa

Question 15

What are the 3 mechanisms of transfer of genes between bacteria?

Answer 15

Transformation
Phage-mediated transduction
Plasmid-mediated conjugation

Question 16

Describe the process of transformation?

Answer 16

ONLY OCCURS BETWEEN RELATED BACTERIA

- DNA is taken up by competent cells, homologous recombination occurs

Question 17

What is the lysogenic cycle? What is the lytic cycle?

Answer 17

Lysogenic cycle
- temperate phage, just sitting there, but can be encouraged to change into a virulent phage when the phage senses that there is something wrong with the bacteria
Lytic cycle
- phage replicating, becomes packaged, virus then kills the bacteria by lysis from within and hundreds of copies of the virus come out

Question 18

How is shiga-toxin related to phages?

Answer 18

Shiga-toxin is encoded by a phage, within E coli
- if take antibiotics for an infection the phage inside becomes aware that it is under threat, makes lots of copies of itself and its toxins and makes the infection much worse

Question 19

What is lysogenic conversion?

Answer 19

When a bacterium becomes infected with a phage and expresses a new phenotype as a result

Question 20

Describe the process of transduction

Answer 20

Donor bacteria is infected by a bacteriophage, normal and abnormal phages are formed (encapsulates part of the host DNA)
- abnormal phage then infects the recipient bacteria and get a transfer of DNA

Question 21

What is phage typing?

Answer 21

Phage can only infect certain bacteria, so you can determine which bacteria you have by the phage

Question 22

Describe plasmid-mediated conjugation

Answer 22

Can occur between entirely unrelated species

• some plasmids can move between bacteria –> encode a cytoplasmic bridge and link
• the plasmid makes a copy of itself while moving through the bridge so when it separates have 2 cells with the plasmid
• • requires direct contact between the donor and recipient

Question 23

What is a multiresistant plasmid?

Answer 23

A plasmid that has incorporated genes encoding resistance –> build up a whole lot of resistant genes
- can insert themselves right next to each other and can be driven by a single promoter and will get expression of all of the genes

Question 24

What is indifference in antibiotic combinations?

Answer 24

• could just use drug A or drug B and get the same effect

- not much use using them together unless one is toxic

Question 25

What is synergy in antibiotic combinations?

Answer 25

When A + B = better than just A or just B

• can be by blocking sequential steps of a metabolic pathway
• by inhibiting enzymatic degradation
• enhancing microbial uptake by bacterial cells

Question 26

What is antagonism in antibiotic combinations?

Answer 26

When A + B = worse than just A or just B

• inhibition of bactericidal activity by a bacteriostatic agent
• induction of enzymatic degradation
• competition for binding to the same target
• inhibition of target