TABLE 3.

Proportion of fimH⁺ E. coli strains displaying the switch element only in the OFF position after three passages in static broth^c

Strain source	Relative frequency of switch-blocked strains (%)
	Resident	Transient
Control individuals	6	15a
IgA-deficient individuals	30b	25

^aP = 0.60, in comparison with resident strains from control individuals.

^bP = 0.03, in comparison with resident strains from control individuals.

^cThe position of the fim switch element was examined by PCR and restriction cleavage (40) in 44 strains from control individuals (31 resident and 13 transient) and 35 strains from Ig-A deficient individuals (23 resident and 12 transient). Before the position of the switch element was assessed, all strains were passaged three times in static broth in order to induce switch to the ON position. The table shows the proportion of strains in which the bacterial mixture yielded only a PCR product representing the promoter in the OFF position, which was interpreted as a phase switch block. Proportions were compared using Fisher's exact test.