New Developments In FluoroQuinolone Resistance Flashcards
What did fluoroquinolones initially have activity against
Gram -ves
What modifications were made to improve fluoroquinolone spectrume of activity
Fluorine added to 6 carbon –> improved penetration in gram -ve and potency in gram +ve
Additon of cyclic substituent –> increase activity against aerobic gram -ve (e.g. P. aeurginosa)
name the first to 4th generation fluoroquinolones
1: Nalidixic Acid - gram -ve activity only
2: Ciprofloxacin
3: Levifloxacin - improved gram +ve activity particularly strep
4: Gemifloxacin - gram +ve activity increased further and started to be used clinically to treat anaerboic infections
What are fluorquinolones often used to treat
UTIs STIs --> ciprofloxain used to first line for gonorrhhoea BActerial Diarrhoeas Penumonia Limited vs. anaerboes TB - reserve treatment
What do the quinolones target
The bacteria type IIA topoisomerases;
DNA Gyrase
DNA Topo IV
What does DNA gyrase do
Drives DNA towards -ve supercoils
Removes topological streses of DNA
WHat does DNA TopoIV do
Relaxes negative and positive supercoils
Can also untangle linked open circle chromosomes foremd after replicatoin !! –> plays a particular role at the septum between dividing daughter cells and helps with the decateniation process
What does DNA replication tend to do to DNA
Introduce positive super coils –> this is a topological stress which if left unchecked can damage the DNA throguh mechanicam shearing
What is the structure of the the topoisomerases
They have a heterotetrameric structure made of 2 copies of 2 subunits:
GyrA - DNA Binding Subunit
GyrB - ATP Binding subunit
How do topoisomerases work
Subunits come together around central DNA –> dna can also be caught by a secondary site on the top of Gyr B
Confmormational changes driven by binding and hydrolysis of ATP –> introduces a transient DSB in teh DNA in the region between teh A and B subunit, known as the gate region
DNA is driven throguh this segment and then religated –> this has the action of twisting or untwisting DNA
How do fluoroquinoloens work
They Inhibit the DNA topoisomerases
They exploit the fact DSB are created –> create turnary complexes where DSBs are produced but NO religation!
Where do quinolones interact with DNA topoisomerases
Sit between the DNa and charger ends interact with protein reisidues on both subints
What chromosomal mechanisms contribute to fluoroquinolone resitatnce
1) Target site modifications
2) Overproduction/upregulation of efflux
What chromosomal mutations in the target site convey quinolone resistanct
Mutations at Ser83 and Asp 87 –> synergistic mutations
Gram +ve mutations tend to be Topo IV
Gram - ve mutations tend to be in Gyrase
What chromsomal way can convey quinolone resistance through overproduction or pump effluex
Chromosmal Effluex Pumps: Lactase Permease –> MF Transported
These are proton antiporters –> NorA is part of this group
RND Family of H+ antiporters –> lots in gram -ves
What plasmid mediated mechanisms convey quinolone resistance
Qnr Plasmid (MfpA is a Qnr related plasmid often seen in gram +ves_ Also aac gene
How does Qnr work
It associates directly with DNA gyrase and topoisomerase IV forming a physical interaction and complex
Relieves the inhibitory action of fluoroquinolones but DOESNT inhibit the dna topoisomerases supercoiling unctions
Discuss the structure of Qnr
It is a pentapeptide repeat protein - a degnerate repeat of 5 amino acids
It is characterised by cahrged amino acids in the 2nd position and hydrophobic amino acid in the 3rd posotion
It forms quadrilateral beta helices with hydrophobic resideus in the core and charged amino acids on the exterio
How do Qnr Proteins work
May mimic the DNA –> particularly the quadrilateral beta helices may mimic the sugar phosphate backbone of DNA
What is the difference between Qnr and MfpA
Wnr has structural projections that are essential to protection
How do Qnr Projections convey resistance
Large loop projection is essential to DNA gyrase protection
Large loop associates with the tower domain of Gyr A subunit
Small loop associates with the toprim domain of Gyr B!
These basically relieve the inhibitory effect of fluoroquinolone antibiotics on the DNA topoisomerases but do not inhibit the supercoiling activitfy
Gyr A and Gyr B are the subunits in DNa gyrase, what are the subunits in Topo IV
Par C and Par E
Where is the Qnr allelle often found
On complex plasmids conveying resistance to multiple antibiotics
Where is the Qnr gene though to originiate
Shewanella
What other gene do Qnr Plasmids often carry
Aac –> normally gives rise to aminoglycoside resistance
How does the aac gene help contribute to fluoroquinolone resistance
Aac variants at 102 and 179 changes the active stie and can create a hydrophobic interaction with the ring sstem in the quinlones back bones–> these can aceylate ciprofloxacin !! However doesn’t work on all quinolones e.g. no levofloxacin actibity as the acetylation site has a methyl group attached to the piperzinlyl ring
Why does the aac gene have no activity against levofloxacin
As levofloxacn has a methyl group attached to the piperzinlyl rind which is the site of avetylation targeted by it
Is Qnr enough to convey resistance
NO –> Qnr isnt enough for high level resistance
BUT Qnr may help some bacteria survive even at high levels of abx (these arent resistant just tolerant) –> these bacteria may then be able to develop further mutatons i.e. change production in efflux pumps and convey true resitance!
