Use of the Escherichia coli SSB gene to prevent bioreactor takeover by plasmidless cells

Reactor takeover by plasmidless cells is a major problem encountered when producing proteins from plasmid-borne genes in genetically engineered bacteria. We have approached this problem by deleting the essential ssb gene from the Escherichia coli chromosome and placing it on a plasmid. Plasmidless cells do not accumulate even after growing such strains under non-selective continuous culture conditions for extended periods of time. Other ssb-containing plasmids can be readily introduced into this E. coli strain by a plasmid-displacement technique. Using this system, we have achieved very high levels of β-lactamase production in continuous culture without selective pressure.