We got some clinical and environmental Pseudomonas isolates which are resistant to Meropenem but susceptible to Imipenem by disk diffusion test as per CLSI-2014 guideline. Even they are negative to Modified Hodge Test
Simply because the resistance mechanisms for Imipenem is different from Meropenem mechanisms. oprD gene is responsible for Imipenem resistance while carbapenemase and some other regulator genes responsible for Meropenem resistance. Please take a look at my thesis.
Thesis: Role of oprD Gene in Biofilm Formation and Imipenem Resistance in Pseudomoanas aeruginosa.
The problem with drug resistant Pseudomonas is that the number of plasmids that are getting expressed either separately or with the host DNA. This is because this bacteria has very little or no endonuclease for getting rid of foreign DNA.