TABLE 3.

Stability of plasmid-based form I Ps expression in S. enterica serovar Typhi Ty21a^a

Plasmid	Vector	Insert size (kb)	% Form I Ps-positive colonies at:
			12 h	24 h
pXK68	pBR325	15.7	12.5	2.5
pXK66	pBR325	13.6	80	45.5
pXK67	pGB-2	15.7	78	69
pXK65	pGB-2	13.6	100	98.5
pXK45	pGB-2	13.3	100	97

^aA form I-positive colony of each strain was inoculated in LB broth and grown for 12 h (approximately 25 generations) before dilution and regrowth in fresh LB broth for an additional 12 h. Samples taken at 12 or 24 h were plated on LB agar, and the resulting colonies were assayed for form I Ps by colony immunoblotting.