GENETIC RECOMBINATION BETWEEN THE RESISTANCE TRANSFER FACTOR AND THE CHROMOSOME OF ESCHIERICHIA COLI

GENETIC INSTABILITY IN ESCHERICHIA COLI

Schwartz, Norman M. (Yale University, New Haven, Conn.). Genetic instability in Escherichia coli. J. Bacteriol. 89:712-717. 1965.-Results obtained from a transductional analysis demonstrated that an extremely unstable reversion of lac(-) is a suppressor mutation. This mutation suppresses 2 of 22 different lac(-) alleles tested. Five different Hfr strains transferred the suppressor soon after mating and at low frequency. Acridine orange enhanced the segregation of lac(-) clones from the unstable suppressed mutant. Stable suppressed lac(-) clones were produced by the unstable suppressed mutant. Two such stable suppressors were transferred at high frequency and mapped at different chromosomal sites on the basis of time of entry data. One of the stable suppressed lac(-) mutants reverted to the unstable suppressed lac(-) state. It is proposed that the suppressor is carried on an extrachromosomal element. The differences in stability observed may reflect differences in the state of the suppressor-bearing element.

Trimethoprim resistance controlled by a combination of plasmid and chromosomal genes

R388s is a mutant of the R factor, R388. Unlike its parent R388s does not confer trimethoprim resistance, though it does confer sulphonamide resistance and the ability to transfer. It is shown that when bacteria harbour R388s mutations to trimethoprim resistance occurred at a high frequency (3·5 × 10−4), whereas such mutations never occurred when the host lacked the R factor. Bacteria harbouring R388s did not mutate to resistance to any other antibiotics at a detectable frequency.

The expression of trimethoprim resistance in bacteria possessing R388s appears to require two genes, one located on the R factor and the other situated presumably on the chromosome. The mechanism of trimethoprim resistance in these bacteria is probably due to reduced permeability and is thus quite unlike the insusceptible target site mechanism of the parent R factor, R388.

Tetracycline resistance in Escherichia coli isolates from hospital patients

Hospital isolates of Escherichia coli resistant to tetracycline (TC) were studied to identify mechanisms which regulate TC resistance levels and ability to transfer TC resistance. Antibiotic resistance patterns, resistance levels to TC, and ability to transfer TC resistance were determined for the isolates. Similar data were obtained for the transferable plasmids after transfer to several new host strains of E. coli. Of the 110 isolates, 50% were able to transfer TC resistance by conjugation. There was a nearly linear relationship between the minimum inhibitory concentration (MIC) of TC for the hospital strains and the percentage of strains at a given MIC that could transfer TC resistance. The strains that were simultaneously resistant to tetracycline, streptomycin, and ampicillin had relatively high MICs of TC and high ability to transfer TC resistance. These results and surveys of TC-resistant E. coli by others suggest that TC resistance levels and transmissibility may be influenced by other resistance markers. The isolates which did not transfer TC resistance by conjugation were tested for the presence of TC resistance plasmids by mobilization or by transformation with deoxyribonucleic acid from the isolates. Evidence for plasmid-mediated TC resistance was found in 92 (84%) of the 110 hospital strains.

Extrachromosomal inheritance in bacteria

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Plasmid-linked resistance to inorganic salts in Staphylococcus aureus

The penicillinase plasmids, a series of extrachromosomal resistance factors in Staphylococcus aureus, were found to carry determinants of resistance to a series of inorganic ions as well as resistance to penicillin and, in some cases, erythromycin. Most of the ions involved were inhibitory but not lethal to the bacteria; the resistance markers conferred an increase in resistance by comparison with susceptible organisms of between 3- and 100-fold, depending on the ion involved. Separate genetic loci for resistance to arsenate, arsenite, lead, cadmium, mercuric, and bismuth ions were demonstrated. Resistance to antimony and resistance to zinc were also found but were not separated genetically from resistance to arsenite and cadmium, respectively. The ion resistance markers appeared to form a cluster on the plasmid, with no other known marker within it. Naturally occurring plasmids were observed that lacked one or more of these ion resistance markers, as well as penicillinase-negative strains that were resistant to one or more of the ions. The patterns of markers carried by these various strains may provide some understanding of the evolution of a plasmid linkage group.

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

A comparative study of the inhibitory effect of Atabrine on R(-) and R(+) strains of Escherichia coli showed that R(+) cells were killed when grown in the presence of Atabrine, whereas R(-) cells were not. It would appear, therefore, that R factor confers sensitivity to Atabrine on the host cells. The "curing" of R factor from R(+) cells by the ultraviolet light-acridine orange method rendered the "cured" cells more resistant than even the parent R(-) cells. The "cured" cells reinfected by R factor were more sensitive than the "cured" cells but less sensitive than the original R(+) cells. After growth once in Atabrine, and even after subcultures in drug-free medium, the growth of R(+) cells in the presence of Atabrine was more rapid than that of the R(-) cells. R(-) cells made resistant by growing them repeatedly in streptomycin, chloramphenicol, tetracycline, and sulfathiazole in succession also showed a higher degree of sensitivity to Atabrine than the original R(-) cells. When mixtures of R(-) and R(+) cells were grown in 120 mug/ml of Atabrine, R(+) cells were killed and the culture consisted predominantly of R(-) cells. A mixture of R(-) and R(+) cells (1:10,000) inoculated into the Atabrine-containing medium and treated 24 hr later with chloramphenicol was completely killed.

Some characteristics of the resistance transfer factor (RTF) episome as determined by inactivation with tritium, P32, and gamma radiation

The resistance transfer factor (RTF) episome was studied by measuring its inactivation by Co(60) gamma radiation, by incorporated P(32), and by tritium incorporated as tritium-labeled thymine. The D(37) for Co(60) irradiation was 7 to 9 x 10(4) rad. Growth of the bacteria harboring the RTF in BUdR (bromouracil deoxyriboside) increased the sensitivity of the RTF to the gamma radiation. The RTF was markedly inactivated by tritium after growth of the host (thymine requiring) bacteria in tritium-labeled thymine, thus further establishing the presence of thymine in the genome of the RTF. Assuming the efficiency of inactivation by P(32) to be 10%, the phosphorus content of the RTF was estimated to be about 2 x 10(5) P atoms/episome. The data suggest the RTF contains double stranded DNA with a molecular weight of the order of 3 to 8 x 10(7).