Acid phosphatase secretion by and accumulation within L. pneumophila wild-type and pilD mutant bacteria. Wild-type 130b (black columns) and mutant NU243 (white columns) were grown in BYE broth, and then, at late log phase, culture supernatants (left) and cell lysates (right) were examined for phosphatase activity at pH 5. The values presented are the mean and standard deviations from triplicate cultures. For both the supernatant and lysate comparisons, the differences in strain activity were significant (P < 0.001, Student's t test). Similar results were obtained on six additional occasions.