Effect of antibiotics on mucoid strains of Pseudomonas aeruginosa.

A study of the susceptibility to 18 antibiotics has been performed on 47 mucoid strains comparatively to 71 fried-egg strains of Pseudomonas aeruginosa. The minimal inhibitory concentration was determinated by agar dilution method. On the whole, the mucoid strains were more sensitive to antibiotics than the fried-egg strains. The more effective antibiotics were carbenicillins, aminosids, colistin and tetracyclines. However, this drug susceptibility of mucoid strains were heterogeneous. The results of a statistical analysis demonstrated that the strains of P. aeruginosa were distributed into two classes: a first class clustered very sensitive strains to most antibiotics and a second class clustered more resistant strains. The mucoid strains were included almost equally into the two classes (respectively 45% and 55%). The fried-egg strains were more homogeneous, since 89% of these strains were in the resistant class. The antibiotics which distinguished the best between the two classes were: tobramycin, netilmicin, amikacin, colistin and tetracyclines. A study of the phenotypes and associations of resistances has been equally performed.