MI: Antimicrobials 1 Pt.3

Question 1

Recall one side effect of rifampicin

Answer 1

Turns secretions orange

Question 2

Recall one condition of rifampicin prescription

Answer 2

Should never be prescribed alone as resistance develops very quickly

*Resistance is caused by a single amino acid change*

Question 3

Colistin is very toxic. Why is it coming back into use?

Answer 3

It is active against certain multi-drug resistant bacteria

Question 4

What is daptomycin licensed for the treatment of?

Answer 4

MRSA

VRE

Question 5

Recall the 2 classes of antibiotic that inhibit folate synthesis

Answer 5

Sulphonamides

Diaminopyrimidines (e.g. trimethoprim)

Question 6

Give an example of a sulphonamide

Answer 6

Sulfamethazole

*Sulfonamides aren’t used on their own - should be in combination with trimethoprim (co-trimoxazole)

Question 7

What is the main use of trimethoprim

Answer 7

Uncomplicated UTI

Question 8

Which antibiotic is best for treating pneumocystis jirovecii?

Answer 8

Co-trimoxazole

Question 9

Give 2 examples of cell membrane toxins

Answer 9

Daptomycin (lipopeptide with limited activity to gram positives - potential alternative to linezolid and synercid for MRSA and VRE infections)

Colistin (old antibiotic which is very nephrotoxic but it is active against gram negative organisms like pseudomonas)

Question 10

What are the 4 main mechanisms of resistance

Answer 10

1. Inactivation of the antibiotic (eg beta lactamases)
2. Altered target - so antibiotic no longer binds

• E.g. penicillin resistant pneumococci or MRSA where bacteria change the penicillin-binding protein
• E.g. protein-synthesis inhibitors where the binding of the ribosome subunit is prevented

3. Reduced accumulation (most important in gram negs - either due to enhanced efflux or to reduced uptake)
4. Bacteria bypasses antibiotic sensitive step (particularly important for folate inhibitors - bacteria can change the enzyme they use)

Question 11

How is MRSA resistant to all beta lactams?

Answer 11

mecA gene encodes novel penicillin binding protein (2A) / novel PBP 2a

Low affinity for binding beta lactams

Substitutes for essential functions of high affinity PBPs at otherwise lethal concentrations of antibiotics

Question 12

How does Strep pneumoniae develop beta lactam resistance?

Answer 12

Penicillin resistance is the result of acquisition of stepwise mutations in PBP genes

*Lower level resistance can be overcome by increasing dose of penicillin used*

Question 13

How do bacteria become resistant to macrolides?

Answer 13

Adenine-N6 methyltransferase modifies 23S rRNA -> reduces binding of MLS antibiotics and results in resistance

Encoded by erm gene (erythromycin ribosome methylation)

*If bacteria is resistant to erythromycin in this manner but still sensitive to clindamycin, only use clindamycin with caution - sometimes the in-vitro tests aren’t reliable*

Question 14

Which bacteria typically forms “gram pos cocci in clusters”?

Answer 14

Staphylococcus

“Staff” work together in groups/clusters

Question 15

Which bacteria typically forms “gram pos cocci in chains”?

Answer 15

Streptococcus

Strep sounds like ‘stripe’ = chain

Question 16

What gram stain status are enterococci?

Answer 16

Positive

(“Enter-o-coccus” = like letting someone in, positive thing to do)

Question 17

Is streptococci gram pos or neg?

Answer 17

Gram pos

Question 18

Is pseudomonas gram pos or gram neg?

Answer 18

Gram neg

(Pseudo”moan”as - ‘moan’ = negative)

Question 19

Is neisseria meningitis gram pos or gram neg?

Answer 19

Gram neg

(Neisseria starts with N = negative)

Question 20

Is haemophilus gram pos or neg?

Answer 20

Gram neg

Ha”emo”philus - emo = negative

Question 21

Is listeria gram pos or neg?

Answer 21

Positive

Lister = good man = positive