Adherence of B. mallei and B. pseudomallei strains to human respiratory epithelial cells. The effect of a bpaC mutation on the adherence of B. pseudomallei (Bp) DD503 and B. mallei (Bm) ATCC 23344 to monolayers of A549 (panels A and D) and HEp-2 (panels B and E) cells and cultures of NHBE (panels C and F) was measured in duplicate on at least 3 separate occasions. Strains were incubated with epithelial cells for 3-hr. Cells were then washed to remove unbound bacteria, lysed, diluted and spread onto agar plates to enumerate bound bacteria. The results are expressed as the mean percentage (±standard error) of inoculated bacteria adhering to epithelial cells. Asterisks indicate that the difference between the adherence of the bpaC KO mutant and that of the parent strain is statistically significant (P value shown in parentheses).