Reliability of the colistin disk test in identification of Serratia marcescens and Serratia liquefaciens.

Resistance to polymycin B or E (colistin) in the disk test, which is used as a means of identification, was tested in 43 strains of Serratia marcescens and 26 of Serratia liquefaciens. While all strains had MIC values greater than 32 mg/l for colistin, false susceptibility to both polymyxins was encountered in the 24 h disk test in 9% of Serratia marcescens and 12-96% of Serratia liquefaciens strains, depending on the kind of polymyxin, temperature and duration of incubation. Using colistin disks and incubation at 25 degrees C for 48 h, the percentage of false susceptible results could be minimized.