Pseudomons aeruginsa strain has a VIM gene positive in the PCR product and sequencing but showed a susceptibility for meropenem and imipenem when its MIC was tested especially that it was resistant to carbapenems (disks) in the first time, how could this be explained?
Disk diffusion methods isn't perfect way of assaying.
Recheck your strain for MIC.
If it shows susceptibility, keep digging for what's going on with this gene from sequence to expression.
There's similar situation about Staphylococcus Aureus, strains has mec gene but show susceptibility. Some studies show any mutation in this gene cassette ends to dysfunction.
Regards
Pseudomonas aeruginosa which express metallo beta-lactamases usually have very high MICs to carbapenems.
I have had false-positive results for VIM in Pseudomonas before when a high DNA amount was used, you could try using less DNA and/or other primers. It could also be a contamination issue if you had other VIM positive strains that were also tested.
To check expression of VIM phenotypically, you could perform a combination disk test with imipenem and EDTA - in case of a true VIM it should be positive.
If it is really positive or a second PCR is positive, check for expression as Alireza suggested.
the thing is that strain was resistant to carbapenems, showed a doublediscsynergey positive test and present a VIM gene on pcr but after a while it becomes susceptible to carbapenems but the pcr product had a VIM gene sequence.
Axel Hamprecht Alireza Mordadi
Did susceptibility to other beta lactams also change?
Did that happen after repeated subcultures? Possibly expression changed or the strain lost a plasmid with blaVIM. I have seen isolates with 2 or more copies of blaVIM, one in the chromosome and another one on a plasmid.
If blaVIM is still present, MICs might go up if you culture the isolate on a agar with a carbapenem (should work with also with ceftazidime or other beta lactams).
Do you have the original isolate with high carbapenem MICs still in the freezer?
If so, you could compare the susceptible and the resistant one (e.g. expression of VIM, localization of blaVIM etc.) if you want to find out what happened.
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