A STUDY ON THE ISOLATIONS OF R PLASMIDS FROM RESISTANT STRAINS OF PSEUDOMONAS AERUGINOSA AND THE TRANSFER OF THEIR RESISTANCE

At two hospitals where gentamicin (abbreviated as GM) resistant strains of Pseudomonas aeruginosa had been isolated with high incidences, organisms of Pseudomonas aeruginosa which are resistant to aminoglycosides were collected in order to isolate their R plasmids carrying resistance to aminoglycosides including GM and examine their genetic properties. Furthermore, each of these organisms was phage typed and analyzed for their relationship to the source of GM resistant R plasmids.<br>A total of 141 strains of P. aeruginosa, consisting of 41 strains isolated at hospital A and 100 strains isolated at hospital B, were examined. Using the agar plate dilution method, strains having resistance to GM, dibekacin (DKB), tobramycin (TOB) and amikacin (AMK) were selected and used as the donor organism for an experiment on transconjugants. For preparing transconjugants, two methods were applied: one was to transfer resistance in a broth conjugally and the other on a membrane filter. As the recipient organism for conjugated transference, P. aeruginosa, ML 4262 (trp^-his^-ilv^-met^-rif^r) and ML 4600 (trp^- his^-) were used.<br>Of all the isolates, GM resistant strains amounted to 56% for hospital A and 38% for hospital B, with high incidences for both hospitals. Most of these strains showed cross-resistance to DKB and TOB. On the other hand, the results of the experiment on the conjugated transference revealed an obvious difference in the incidence of isolation of resistance transferable organisms between the two hospitals. At hospital A, resistance transferable strains were isolated with higher incidences, with all the isolated R plasmids having a resistance marker based on the resistance to GM, DKB and TOB. Regarding the phage types of these organisms having R plasmids, there were some strains classified into different phage types despite the fact that they possessed the same R plasmids. These results indicated a possibility of the spread and establishment of aminoglycosides resistance at the level of R plasmids in addition to a dissemination of GM resistant P. aeruginosa at hospital A.<br>At hospital B, where GM resistance transferring R plasmids were not isolated, aminoglycosides resistant strains with the same pattern of resistance to GM, DKB and TOB amounted to 31 of 37 strains isolated at this hospital. Thirty of these 31 strains exhibited a similar phage type. This fact suggests that there was a dissemination of GM resistant organisms themselves at hospital B.<br>These results showed that the spread of GM resistant organisms is related to abuse of antimicrobial agents and intrahorpital infections and that studies from two sides, namely that at plasmids level and phage typing, are required in order to obtain a countermeasure against the development of resistance.