A chemical and biological study of gentamicin-resistance in Pseudomonas aeruginosa

Abstract

Microelectrophoresis was used to study the surface properties of cells of strains of P. aeruginosa. Cells of gentamicin-resistant strains (not carrying R-factors) were distinguished from cells of gentamicin-sensitive strains by their characteristic shaped pH-mobility curves. The resistant strains had significant amounts of surface lipid, in contrast to negligible amounts on sensitive strains. There was no relationship between divalent cation content and gentamicin sensitivity/resistance of cells.

Cells carrying R-factors mediating gentamicin-resistance generally exhibited pH-mobility curves characteristic of gentamicin-sensitive strains; they also had negligible amounts of surface lipid. Unlike other resistant strains, these strains could be cured and could transfer resistance to acceptor strains. The transconjugant strains had identical surface properties to those of the parent acceptor strains.

The results are discussed in terms of two distinct 'natural' resistance mechanisms, (a) R-factor mediated and (b) another mechanism, which may be exclusion, a mutation at ribosome level or enzyme inactivation controlled by chromosomal genes. The resistance of one of the R-factor strains was partly due to R-factor mediation and partly to another mechanism.

Calcium and magnesium ions in the growth medium increase the resistance of cells to gentamicin. Cells acquiring resistance in this way displayed gentamicin-resistant type pH-mobility curves but had less surface lipid and higher maximum negative mobilityvalues than 'naturally' resistant strains. Cells becoming sensitive on growth in calcium-deficient medium displayed the properties of gentamicin-sensitive cells. Cells grown in highly calcium-supplemented medium possessed greater amounts of polysaccharide and surface divalent cations than did cells grown in calcium-deficient medium. These results are discussed in terms of a barrier mechanism of resistance. A weak 1:1 calcium-gentamicin complex, detected in microcalorimetric studies, may contribute to the antagonistic effect of calcium against gentamicin.