Mechanisms and spectrum of streptomycin resistance in a natural population of Pseudomonas aeruginosa

A survey of 200 strains of Pseudomonas aeruginosa isolated from clinical specimens was made in an attempt to correlate the spectrum of their streptomycin resistance and the mechanism of resistance. The strains can be classified into three groups, according to their level of resistance to streptomycin: susceptible, low-level resistant, and high-level resistant strains. The mechanism of resistance of high-level resistant strains is either an R factor-mediated inactivation of streptomycin by phosphorylation or streptomycin-resistant ribosomes. However, such high-level resistant strains comprised less than 10% of the total strains isolated; the majority of the strains resistant to streptomycin were of low-level resistance. The latter are associated with a diminished uptake of streptomycin, and no evidence of streptomycin inactivation, resistant ribosomes, or R factors could be detected. The most probable explanation of low-level resistance is reduced permeability to streptomycin. Modification of the growth medium used in uptake studies simultaneously affected strongly both streptomycin incorporation and the minimal inhibitory concentration of streptomycin.