SUMMARY: A strain of from the sheep rumen showed spontaneous loss of urease activity when subcultured at the normal rumen temperature of 38 °C, although in mixed cultures or loss of urease was not apparent. The rate of loss of urease in pure cultures was increased at incubation temperatures above 38 °C, but loss was never complete. However, at temperatures below 38 °C loss was greater, and at 22 or 18 °C the urease was completely eliminated. Incubation with sodium dodecyl sulphate (0.002 %) or ethidium bromide (2.5 x 10 M) caused complete loss of urease activity. The urease activity was also eliminated when the streptococcus was grown aerobically, and this loss of activity was irreversible.

It is suggested that the urease activity is controlled by a plasmid gene and that aeration, low growth temperature and chemical agents ‘cure’ the streptococcus of the plasmid. Attempts to demonstrate the presence of covalently closed circular extrachromosomal DNA by caesium chloride-ethidium bromide equilibrium density-gradient centrifugation were unsuccessful.


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