I am trying to determine MICs for several clinical isolates for Linezolid, Amikacin sulfate, Ciprofloxaxin HCl, Cefixime trihydrate, and Chloramphenicol. But I am having problem preparing the stock antibiotic solution.
I am following the CLSI M100-24-2014 document, according to which I should dissolve Linezolid in water (page 177), Cefixime in Phosphate buffer of pH 7, and Ciprofloxaxin in water to prepare stock solution of 5120mcg/mL. But they don't dissolve at all. Ciprofloxacin HCl only dissolves in pH 5.1 buffer or below. Cefixime trihydrate only dissolves in pH 6 buffer or below.
I obtained certified sample of Cefixime, Linezolid, and Ciprofloxaxin from either pharmaceutical companies or Reagent suppliers. I have good confidence in the quality of the antibiotic.
So, what could be the problem?
The solubility of each antibiotic should be considered to select a solvent/a buffer for preparing antibiotic stock solutions. The solvent/buffer should be free from any components that interfers with antibiotics activity assays.
In your case, you need to make sure that the pH of your buffers is the correct one. Generally, weak acidic drugs and weak basic drugs are more soluble in basic and acidic solutions, respectively.
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