Abstract
Transmissibility of dihydrostreptomycin (DHSM) resistance in Pseudomonas lachrymans was studied. Transmission experiments were conducted by mixing-culture method, and it was found that the resistance of 13 P. lachrymans isolates out of 109 DHSM-resistant ones was distinctly transmitted to the recipient isolate of rifampicin-resistant P. lachrymans. Transmission frequency remarkably increased with the lapse of time of mixing-culture, and it was 10-1 at after 24hr. DHSM resistance was not transmitted when culture filtrate of one of the 13 DHSM-resistant isolates was used as donor, suggesting the presence of resistance (R) plasmid in the donor isolate. Several curing agents such as acriflavine, acridine orange etc. were tested against the donor resistant isolate, but the results were negative. From the resistant isolate and R+ transconjugant obtained from it, plasmid DNA was extracted with cleared lysate method and analyzed by agarose gel electrophoresis. From these experiments, it was clarified that the resistant isolate carries at least 2 kinds of plasmid DNA, molecular weights of which were 86×106 and 58×106 daltons. These plasmid DNAs could not be found in the transconjugant. Moreover, transformation experiments by these plasmid DNA samples on DHSM-susceptible P. lachrymans, P. aeruginosa and Escherichia coli were not successful. It seems, therefore, that these 2 kinds of plasmid DNA are unrelated to DHSM resistance, but interpreted as the so called cryptic plasmids.
