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Lack of Agreement Between Fosfomycin Susceptibility Testing Methods Against Pseudomonas

Elizabeth Hirsch, PharmD, and Elizabeth Smith, PharmD candidate, discuss fosfomycin susceptibility testing.

Segment Description: Elizabeth Hirsch, PharmD, assistant professor at the University of Minnesota College of Pharmacy, and Elizabeth Smith, PharmD candidate at the University of Minnesota College of Pharmacy, discuss fosfomycin susceptibility testing.

Interview transcript (modified slightly for readability):

Dr. Hirsch: There has been a lot of discussion recently at CLSI, the Clinical and Laboratory Standards Institute, about the appropriateness of the current susceptibility breakpoints for fosfomycin, and the fact that glucose 6-phosphate is used during in vitro susceptibility testing. However, most likely glucose 6-phosphate is not actually found in the urine. So there are some concerns whether our in vitro testing actually reflects what is happening in vivo in a patient. Currently, there are no susceptibility breakpoints from CLSI specific to Pseudomonas aeruginosa for fosfomycin. In the clinic what is happening is if this drug is tested, the Escherichia coli breakpoints points are being extrapolated to other organisms that we don't have breakpoints for. So we thought it was important to look at activity of fosfomycin against Pseudomonas and extrapolate those breakpoints to see exactly how active the drug looks, according to the E coli breakpoints.

Ms. Smith: The main takeaways from our research were...we were comparing the 4 different susceptibility testing methods: the reference method, which Betsy stated, was the agar dilution, and then we also looked at disk diffusion, Etest and then broth microdilution. The clinical isolates that that we used were slightly skewed to be more resistant—about 50% of our isolates were multidrug-resistant and about 20% were pan-resistant isolates. When we compared the susceptibilities across all 4 methods, they were about 50% susceptible, but the MICs varied widely across all 4 methods ranging from less than 2 all the way up to greater than 1024. Betsy mentioned that current recommendations per CLSI are to ignore the inner colonies that potentially can be frequently seen in disk diffusion and Etest. And so we kind of wanted to look and see how those MICs compared to the parent strains. And so we actually subcultured those out and then ran the same MIC tests using broth microdilution and compared the parent MICs to the daughter MICs and found in each case that the daughter strains were a lot higher in terms of MICs when compared to the parents strains.

Dr. Hirsch: Future research avenues for this project are to continue building on the isolate set that we've been testing. We have isolates from the US and we also have some Australian collaborators. So we're now adding into our isolate set so that we have an international set of isolates to continue looking at activity of fosfomycin against Pseudomonas. And then, as Elizabeth mentioned, we are looking at the frequency of inner colonies that are seen there. And so for the isolates that we have subcultured, the daughter mutant isolates, we are now going to look at those with whole genome sequencing and assess the resistance mechanisms of both the parents strains and the daughter isolates, so kind of to try to understand what are the resistance mechanisms, leading to either non-susceptibility or resistance for Pseudomonas.