Host cell growth in the presence of the thermosensitive drug resistance factor, Rts1

We have confirmed and extended the observation of Terawaki et al. that the R factor, Rts1, alters the growth of its host at 42 C. In all media tested there was a period during which total cell numbers increased linearly, while viable counts remained constant. During this period the rate of precursor incorporation per cell particle into deoxyribonucleic acid, ribonucleic acid, and protein declined steadily. These patterns were a consequence of the accumulation of increasing numbers of cells which had lost colony-forming ability. A temperature shiftdown experiment showed that the colony formers could, after a lag, go on to divide normally, whereas most of the noncolony formers could not undergo even a limited number of divisions after shiftdown. The number of normal divisions which occurred after shiftup of Rts1 cells to 42 C was medium dependent. In rich medium there were, on the average, two or three doublings; in glucose medium, one; and in glycerol medium, only a fraction of a doubling. Even in glucose medium, however, no increase in viable counts was observed during growth at 42 C if the cells were first starved for glucose for 1 h at 42 C. A temperature shiftdown from 42 C to 27 C during glucose starvation reversed the effect of starvation at 42 C alone. These results are consistent with the hypothesis that the thermosensitive Rts1 component(s) responsible for the host effects is present at permissive temperature, but can undergo a reversible temperature-induced alteration which then interferes with some essential host function. The detrimental effects of this R factor on its host were also reflected in a heightened sensitivity to kanamycin and actinomycin D at 42 C. Electron microscope observations revealed changes in the appearance of the cell membrane. Membranous invaginations were noted at discrete sites in the cell.

Strand-specific nick in open circular R-factor deoxyribonucleic acid: attachment of the linear strand to a proteinaceous cellular component

The fraction of covalently closed circular R-factor R12 deoxyribonucleic acid (DNA) isolated from Proteus mirabilis was found to depend upon the pH of the buffer used to suspend the cells at the time of lysis. Examination of the open circular DNA revealed that there is one strand-specific nick per R-factor molecule. This same DNA strand also appeared to be bound to a proteinaceous particulate fraction of the cell.

Elevated temperature technique for the isolation of salmonellas from sewage and human faeces

Modified Moore's swabs, placed in sewers for 5 days, were used to concentrate salmonellas from sewage. Duplicate cultures of swab strips in selenite broth were incubated at 41 and 37 degrees C. respectively. Salmonella organisms were recovered consistently from the swabs when the enrichment broths were incubated at 41 degrees C. However, when equal portions of the same swabs were incubated at 37 degrees C., only 22% of them yielded Salmonella organisms. These results indicate an advantage in incubating the selenite broths at 41 degrees C. rather than 37 degrees C. in attempting to isolate salmonellas from sewage.One hundred and fifty faecal samples were examined for salmonellas by culture in selenite broths incubated at 41 and 37 degrees C. Twelve (8%) samples were positive at 41 degrees C. compared to only 10 (6.7%) positive samples at 37 degrees C. This difference is not statistically significant to indicate an advantage of the elevated-temperature of incubation over the conventional temperature in attempting to isolate salmonellas from human faeces. Moreover, results of the recovery rates of S. paratyphi B, S. typhi, and S. typhimurium indicate that an incubation temperature of 37 degrees C. is more appropriate for recovering salmonellas from artificially infected faecal samples than an incubation temperature of 41 degrees C. This stresses the inability of laboratory studies to mimic conditions in nature.

Characteristics of some co-trimoxazole-resistant Enterobacteriaceae from infected patients

Two isolates of Proteus mirabilis and four of Esch. coli which would not grow on commonly used commercial sensitivity test media are reported. These organisms appear to be resistant to co-trimoxazole, are exacting towards thymidine, and five of six were isolated from patients who had been treated with co-trimoxazole. A method for the detection of these strains is given and their significance in the laboratory and clinical practice discussed.

Effect of thymine limitation of chromosomal deoxyribonucleic acid synthesis in Proteus mirabilis

The effects of thymine limitation on the rates of growth, deoxyribonucleic acid (DNA) synthesis, and increase in viable cell number for a thymine auxotroph of Proteus mirabilis were investigated. At thymine concentrations of 1.0 mug/ml and below, these rates were markedly decreased. After a reduction in thymine concentration from 10 mug/ml to 0.2 mug/ml, mass synthesis continued at the preshift rate for several hours. In contrast, the rate of DNA synthesis immediately decreased, resulting in a decrease in the DNA to mass ratio to about one-half of its normal level. Viable counts remained constant for several hours after the reduction in thymine concentration, and enlarged cells and multicellular "snakes" were formed. The rate of DNA synthesis was reduced at thymine concentrations below approximately 1.7 mug/ml. The addition of thymine to cultures which had been completely starved for thymine increased the rate of DNA synthesis to at least twice its normal value; this suggests that extra rounds of chromosome replication can be induced in P. mirabilis as previously observed in Escherichia coli.