Ampicillin-Resistant R Factors Derived from Shigella Strains

Abstract

Sixteen Shigella strains resistant to ampicillin(APC) were isolated from 3760 strains examined in 1965. Five strains among these isolates were resistant to tetracycline(TC), chloramphenicol(CM), streptomycin(SM) and sulfanilamide(SA) in addition to APC, and transferred their APC resistances by conjugation together with the aforementioned four drugs. The 5-drug resistance was transferred to an Escherichia coli strain and the resistance was analyzed by both conjugation and transduction. Three out of the five Shigella strains transferred a single type of R (TC. CM. SM. SA. APC) factor, but the linkage relationship between the resistant markers were different from each other. The ampicillin gene, however, was always linked close to the genes governing both transfer and replication. The 5-drug resistance was transferred to an E. coli strain from the remaining two Shigella strains. According to the genetic studies of E. coli strains which had acquired the 5-drug resistance, it was concluded that the two parent Shigella strains carried two types of R factors in a cell, i. e., fi⁺R (TC. CM. SM. SA) and fi^- (SM. APC). These two R factors in a cell sometimes recombined and formed a recombinant R factor possessing the 5-drug resistance in which the ampicillin gene was linked close to the transfer gene. The enzymological properties of the five penicillinases coded by R factors possessed by the five Shigella strains were all similar to each other and of the type I penicillinase, which was demonstrated from Klebsiella pneumoniae and mediated by most of the R factors.