[In vitro induction of chlorhexidine- and benzalkonium-resistance in clinically isolated Pseudomonas aeruginosa].

A study was made on the MIC distributions of chlorhexidine and benzalkonium chloride against clinically isolated 178 strains of Pseudomonas aeruginosa to find out the existence of strains resistant to those disinfectants and also on the in vitro induction of resistance to both drugs. The MIC of chlorhexidine gluconate was found to be distributed from 78 to 625 micrograms/ml with a single peak at 312 micrograms/ml. All 178 strains of clinical isolates were sensitive to chlorhexidine and none could be induced to become chlorhexidine resistant in vitro, suggesting that P. aeruginosa can not easily acquire chlorhexidine resistance. On the other hand, the MIC of benzalkonium chloride was distributed in two peaks; one peak was benzalkonium sensitive at 625 micrograms/ml (150 strains/178 strains: 84.3%) and the another peak was benzalkonium resistant at 5,000 micrograms/ml (28 strains/178 strains; 15.7%). Six (4.0%) of the 150 benzalkonium sensitive strains acquired benzalkonium resistance by in vitro induction of resistance; the MIC of 5 strains increased from 625 micrograms/ml to 2,500 micrograms/ml and that of the residual 1 strain increased from 312 micrograms/ml to 1,250 micrograms/ml. However, no change of MIC was observed in 28 benzalkonium-resistant strains of clinically isolated P. aeruginosa by in vitro resistance induction. Strains with MIC more than 5,000 micrograms/ml could not be obtained at all. The results suggest that the benzalkonium resistance can be introduced in P. aeruginosa whereas the resistance-acquiring rate is low. These results suggest that chlorhexidine gluconate is the first choice for prevention of Pseudomonas infection in the hospital and benzalkonium is also useful in 0.5% solution is used.