The Elimination of Urease Activity in Streptococcus faecium as Evidence for Plasmid-coded Urease

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Abstract
A strain of S. faecium from the sheep rumen showed spontaneous loss of urease activity when subcultured at the normal rumen temperature of 38.degree. C, although in mixed cultures in vivo or in vitro loss of urease was not apparent. The rate of loss of urease in pure cultures was increased at incubation temperatures > 38.degree. C, but loss was never complete. At temperatures < 38.degree. C loss was greater, and at 22.degree. or 18.degree. C, the urease was completely eliminated. Incubation with sodium dodecyl sulfate (0.002%) or ethidium bromide (2.5 .times. 10-5 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. The urease activity is probably controlled by a plasmid gene and 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 CsCl-ethidium bromide equilibrium density-gradient centrifugation were unsuccessful.

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