Effect of divalent cations in bacteriological media on the susceptibility of Xanthonwnas maltophilia to imipenem, with special reference to zinc ions

Abstract

The susceptibility of Xanthomonas maltophilia strains to β-lactams is known to vary according to the type of growth media used. We have assayed the divalent cation content of various susceptibility testing media and correlated this with the suscep tibility to imipenem of 30 strains of X. maltophilia, by calculating the IC₅₀. No correlation was found with Ca⁺⁺ and Mg⁺⁺ content (r = 0·005, P = 0·99), but a highly significant correlation with the Zn⁺⁺ content (7·7–42·7 μmol/L) of the medium was found (r = 0·93, P = 0·003). The effect of Zn⁺⁺ on the susceptibility of X. maltophilia to imipenem was further investigated by adding varying amounts of zinc sulphate to Oxoid Mueller-Hinton agar which has a low Zn⁺⁺ content (l4·2 μmol/L). A highly significant correlation between the Zn⁺⁺ content and the IC₅₀ was observed (r = 0·95, P = 0·001). Some variability was seen from one series of IC₅₀ determinations to another and samples of ultra-pure water were processed in exactly the same fashion as the agar media and then assayed for cation content. No significant increase in Ca⁺⁺ or Mg⁺⁺ content of the water were observed but water autoclaved in universal containers with metal caps and rubber washers acquired up to 40 μmol/L of Zn⁺⁺. Studies of the correlation between in-vitro sensitivity tests and the clinical performance of β-lactams used against X. maltophilia will need to take account of the Zn⁺⁺ content of the susceptibility testing media used.