Plasmid curing with trimethoprim from salt-tolerant pseudomonads

Curing of plasmid from two moderately halophilic Pseudomonas strains AZ-2 and AZ-3 was accomplished with trimethoprim (TMP). With high temperature (43 degrees C), sodium dodecyl sulphate (SDS) and ethidium bromide (EB), loss of plasmid was not achieved. With 20 mu g ml(-1) of TMP treatment for 24 hours, 61% and 86% salt sensitive colonies were scored for AZ-2 and AZ-3, respectively. Complete loss of plasmid from 50 and 58% salt sensitive colonies (accordingly from AZ-2 and AZ-3) was ascertained on gel by electrophoresis of total cell lysate. Salt-tolerance property of both bacterial strains was lost with curing of plasmid. The loss of plasmid did not affect the. biochemical nature of the strains, while physiological behaviour was altered. Plasmid free derivatives of AZ-2 (AZ-2.1) became sensitive to kanamycin, while plasmid free derivative of AZ-3 (AZ-3.1) was sensitive to kanamycin as well as ampicillin. Sensitivity of cured strains was differently increased to NaHCO3, Na2SO4, MgSO4 and KNO3.