TABLE 2.

Heat stability of STh chimeras^a

Toxic sample (2 MU)	Enterotoxicityb
	Unheated control	Heated for 15 min
		90°C	95°C	100°C	105°C
STp (control)c	0.151 (100)	0.144 (100)	0.067 (25)	0.081 (25)	NT
pEHSTN24	0.141 (100)	0.105 (62)	0.094 (50)	0.079 (25)	0.061 (0)
pEHSTC22	0.127 (100)	0.099 (57)	0.089 (50)	0.078 (25)	0.067 (0)
pEHProSTC28	0.154 (100)	0.107 (62)	0.095 (50)	0.076 (20)	NT
pSTC17 + pDSPH524	0.134 (100)	NT	0.143 (100)	0.102 (100)	0.104 (66)

^aThe solid culture supernatant fractions of E. coli DH5α strains with the indicated plasmids were prepared, their titers were adjusted to 2 MU, and they were tested in the suckling mouse assay (5 to 10 mice per sample). 

^bAs described in Table 1, footnote a. Values are G/C ratios; the values in parentheses are scores expressed as percentages. NT, not tested. 

^cPure native STp toxin (Calbiochem) was adjusted in titer to 2 MU and used as a positive reference control.