Sensitivity of Escherichia coli to atabrine conferred by R factor and its potential clinical significance

Screening method of agents against the R factor by the use of an Hfr made by integrative suppression with an R factor

By the use of an Hfr strain made by integrative suppression of a derepressed R factor, R100-1 in a dnaA-ts mutant of Escherichia coli and several other appropriate control strains with and without the R factor, a screening system was established which can be used to detect agents which are either inhibitory to the replication of the R factor and lead to its "curing" or are preferentially inhibitory to the growth of the R-carrying bacteria. The validity of this system was shown with several agents already known to be experimentally effective.

Antibiotic resistance from two perspectives: man and microbe

Despite much effort, antibiotic resistance continues to increase. Looking back, it is clear that this was an inevitable consequence of antibiotic use. From a bacterial viewpoint, the introduction of antibiotics was a tremendous stimulus to evolution. As a survival reaction to stress (selection pressure) bacteria, by means of their extreme biochemical and genetic versatility, have adapted to 21st Century conditions. Resistance can be to some extent contained by less and better use of antibiotics, but ultimately novel approaches to the treatment and prevention of infectious diseases will have to be forthcoming. This will only be achieved if best use is made of alternative resources presently available and most importantly, man's ingenuity must be fully engaged.

The susceptibility of conjugative resistance transfer in gram-negative bacteria to physicochemical and biochemical agents

Over thirty years of studies have established that conjugative transfer of plasmid-encoded resistance to drugs and heavy metals can take place at high frequency between various organisms under laboratory conditions. The detected transfer frequencies in soil, in aquatic environments, and in the urogenital and respiratory tracts of healthy animals and man have generally been low. However, the conversion of bacteria from susceptible to resistant to antibiotics has been observed often during antimicrobial therapy. This has formed a challenge for the antibacterial treatment of pathogenic bacteria and called for the evaluation of the extent of conjugative transfer in various environments. Several biochemical and physicochemical factors inhibit conjugation, show preferential toxicity against plasmid-bearing cells, or stimulate plasmid curing. These factors include various agents such as detergents, anesthetics, mutagens and antibiotics which affect membrane potential, membrane permeability, protein synthesis and the processing of DNA. The application of the data on these agents, summarized in this review, might be helpful in preventing drug multi-resistance from spreading. Also these data might be valuable in studies which use conjugation as a tool or which treat the molecular mechanisms involved in conjugation.

A kinetic model for plasmid curing

A simple mathematical model of drug-induced plasmid elimination (curing) considering density-dependent growth rates and plasmid transfers is presented. It describes nonlinear population dynamics of conjugative plasmids during in vitro curing experiments in batch culture. The model was tested on kinetics of acridine orange curing of F'lac plasmid. Effects of density dependence, plasmid elimination, selection for plasmidless segregants, conjugation, initial and maximal population density, and postsegregational killing on curing kinetics are simulated and discussed.

Development of high-level streptomycin resistance affected by a plasmid in lactic streptococci

Some lactose-negative (Lac-) mutants of Streptococcus lactis C2 and ML3 exhibited development of very high level streptomycin resistance after incubation with subinhibitory concentrations of the drug for 18 to 22 h. These drug-resistant mutants showed no loss of resistance even after 6 months of subculturing in broth without any drug. The parental Lac+ strains did not show mutation to high-level streptomycin resistance. The Lac+ characteristic of the parental strain was conjugally transferred to Lac- derivatives of C2 and ML3, showing the ability to mutate to high-level resistance. When transconjugants were analyzed for this characteristic, they showed both mutable and nonmutable Lac+ types. The results suggested that genetic information for mutation to high-level streptomycin resistance in lactic streptococci resides on the chromosome, and its expression is affected by a plasmid. The plasmid profiles of strains C2, ML3, C2 Lac-, ML3 Lac-, and two kinds of transconjugants confirmed the presence of a plasmid of approximately 5.5 megadaltons in strains showing no mutation to high-level streptomycin resistance, while strains missing such a plasmid exhibited high-level streptomycin resistance after incubation with subinhibitory concentrations of the drug.

Inhibition by paromomycin of R-factor transfer of tetracycline resistance between Escherichia coli and Salmonella pullorum

In vitro assays showed that low concentrations (5--10 and 20 microgram/ml) of the antimicrobial paromomycin sulfate are able to block or diminish significantly the transfer of the tetracycline resistance R-factor between Escherichia coli and Salmonella pullorum. This observation is important because it offers the possibility of preventing the formation of tetracycline-resistant pathogens, a limiting factor of tetracycline use in both human and veterinary medicine.

Susceptibility to kasugamycin of Escherichia coli carrying conjugative and nonconjugative R plasmids

Ten conjugative and two nonconjugative R plasmids, all of which were naturally isolated, were tested for their ability to confer epistatic susceptibility to kasugamycin upon the host Escherichia coli. All of the conjugative plasmids, which belonged to various incompatibility groups, conferred a plasmid-determined epistatic susceptibility to kasugamycin upon the host cells, whereas the nonconjugative plasmids did not.

Plasmid-determined epistatic susceptibility to kasugamycin

The effect of two representative plasmids, R100 and F8-gal, on the susceptibility of Escherichia coli to kasugamycin was studied. R(+) and F(+) cells were found to be more susceptible to this antibiotic than R(-) and F(-) cells, respectively. Retransfer and curing experiments of these plasmids show that this increased susceptibility of host cells to kasugamycin was conferred by either of the plasmids. At the early stage of growth of R100(+) cells in the presence of kasugamycin, R(-) segregants overgrew the population and then they were replaced by kasugamycin-resistant mutants of the R(+) cells which became the majority cell line of the population. The former phenomenon is assumed to be due to the increased susceptibility of R100(+) cells to kasugamycin, and the latter is probably related to the finding that R100 enhances the spontaneous mutation of host cells to resistance to kasugamycin. The practical and experimental significance of these findings are discussed.

Specific role of sex pili in the effective eliminatory action of sodium dodecyl sulfate on sex and drug resistance factors in Escherichia coli

Evidence is presented for the specific role of sex pili in the eliminatory action of sodium dodecyl sulfate (SDS) on sex (F) and drug resistance (R) factors in Escherichia coli K-12 strains leading to their loss. SDS at 0.03% concentration lysed JE3100 F(8) (+) (F-gal)/gal(-)fla(-)pil(-) in Penassay broth after they had grown exponentially and reached maximum growth to the extent that the agent at concentrations higher than 1% did. However, the agent was only effective in eliminating sex factors from JE3100 in high frequencies at concentrations higher than 1%. Increase of osmotic pressure of the culture with SDS at concentrations as low as 0.03 to 0.1% by addition of sucrose led to the substantial increase of elimination efficiency. Reconstruction experiments between F(8) (+) and F(-) cells in the SDS culture revealed the selective growth of F(-) cells as well as a delay of maximum growth of F(-) variants derived from F(8) (+) cells, compared with those of F(8) (+) cells, as well as F(-) cells originally added to the culture. The agent was not very effective in eliminating sex factors from JE3427 F(8)m(+)5/fla(-)pil(-) cells which lack the function of production of F pili. F(8)m(+)5 cells showed a sensitivity toward SDS intermediate between those of F(8) (+) and F(-) cells. SDS was further effective in eliminating R factors from KE132 R(100-1) (+)/fla(-)pil(-) cells in high efficiency; however, the action was not efficient with KE133 F(100) (+) cells possibly with fewer sex pili than R(100-1) (+). Action of acridine orange on these F(+) or R(+) strains was found to be different in some aspects from that of SDS.

Drug sensitivity and mutability to drug resistance associated with the presence of an R factor

Studies on the growth kinetics of R+and R−cultures ofEscherichia coliin the presence of nalidixic acid (NA), acriflavine (AF) and kanamycin (Kan) showed that each drug caused a decline in viability of both R+and R−cells for several hours. During further incubation the viability rose rapidly for the R+cultures, but either rose less rapidly (AF and Kan) or continued to decline (NA) for R−cultures. Distribution curves of the resistances of individual clones of R+and R−bacteria to atabrine, NA, AF and Kan suggested that the presence of an R factor in the host bacterium increased its mutation rate to resistance to these drugs: this would account for the more rapid growth rate of R+cells during the latter stages of incubation in their presence. The mutations causing increased resistance to NA and to Kan were located in the bacterial chromosome and not in theRfactor.

Selective enrichment of R- segregants as the main mechanism of "curing" of the R factor by acridine dyes

By the use of appropriate strains ofEscherichia coli, Shigella flexneriandSalmonella typhimuriumwith and without an R factor, R100, the mechanism of ‘curing’ of R factor by acridine dyes was examined. This R factor was shown to confer increased sensitivity to acriflavine upon the host cells.E. colistrain W-3630, once infected with R100, has never been observed to segregate R−cells. When mixtures of R+and R−cells of this strain were grown in acriflavine broth, the proportion of R−cells increased and was also correlated with the proportion in the initial inoculum. Other bacterial strains carrying R100segregate R~ cells spontaneously. Growth tests starting with varying proportion of R+and R−cells of these strains in acriflavine broth also gave a marked correlation between the initial and final proportions of R−cells, and indicated that the main cause of ‘curing’ the R factor was the selective enrichment of R−segregants present in the initial inocula or arising spontaneously during growth of the R+culture. These results suggest that the mechanisms underlying the ‘curing’ of F and R factors are different. Tests with several acridine dyes gave results similar to those with acriflavine.

Recombination between a thermosensitive kanamycin resistance factor and a nonthermosensitive multiple-drug resistant factor

The thermosensitive kanamycin (KM) resistance factor, R(KM)(t), and a nonthermosensitive multiple-drug resistance factor, R(100), were simultaneously introduced into Escherichia coli and Salmonella typhimurium. The temperature sensitivity of both R factors remained unchanged as long as they replicated independently. Under certain conditions, however, a new thermosensitive R factor harboring resistance markers for kanamycin, streptomycin (SM), and sulfanilamide (SA) was obtained by recombination between the R(KM)(t) and R(100) factors. R factors carrying resistance markers for KM and SA, or for SM and SA, were obtained from the recombinant R(KM SA SM)(t) by spontaneous segregation. Though the R(100) factor has been known as an fi(+) (positive for F-mediated fertility inhibition of its host) type and it does not restrict any coexisting phages, the thermosensitive recombinants of R(100) with R(KM)(t) and their segregants were found to be fi(-) and to restrict the replication of all T-even phages, as does the R(KM)(t) factor. Double infection immunity was not observed between the R(KM)(t) and R(100) factors.