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 1 penicillinase, which was demonstrated from Klebsiella pneumoniae and mediated by most of the R factors.

Genetic Structure of an R Factor Conferring Ampicillin Resistance

“…Transfer of R factor. When the R factor was transferred between Shigella and E. coli strains, the procedures were basically the same as those reported previously [25].…”

“…These results suggest that the resistant genes are arranged on the same segment of an R factor. On the other hand, Tanaka et al [25] reported that the linkage order between amp and other resistant genes varied in different R factors but the amp gene has never been deleted alone leaving other resistant genes on an R factor, showing the close linkage between the amp and tra regions. Epidemiological surveys in Proteus, Klebsiella, and E. coli strains have disclosed that the R factors carrying only APC resistance are isolated frequently, indicating the close linkage between the amp and tra genes [20].…”

“…L agar, L soft agar, and L broth were used for transduction [13]. These media are the same as those reported elsewhere [25]. SS agar (Nihon Eiyokagaku, Tokyo) was used as a selective media when R factors were transferred from E. coli to Salmonella strains.…”

“…The Shigella flexneri var. X strain, JS576, is a source of the APCresistant R factor RM201, as reported in a previous paper [25]. Two substrains of E. coli K12, W3630(ma0 and ML1410(met), and two Salmonella strains, S. typhimurium LT2 and S. chittagong S224, were used as the recipients of conjugal transfer and transduction of R factors.…”

Drug Resistance and Distribution of R Factors among Escherichia coli Strains

“…SM.SA)-resistant strains transferred only the (SM.SA)-resistance markers. Majority of multiple-resistant strains were found to carry two types of R factors in a cell, that is, in a hetero-R state [12,13].…”