Isolation and characterization of Escherichia coli phase variants and mutants deficient in type 1 pilus production

Abstract

Type 1 pili of E. coli are the prototype of the somatic class of pili found on many strains of bacteria. As a first step in the genetic analysis of type 1 piliation, an extensive series of nonpiliated derivatives of E. coli K-12, strain AW405, was characterized. None of the derivatives reacted with type 1 pilus antiserum or appeared to produce attached or free pili when examined in the electron microscope. The derivatives fell into two classes: phase variants and mutants. Phase variants that formed colonies of two distinctive types, one associated with a predominantly piliated (P+), and the other associated with a nonpiliated (P-) phase, were obtained. Each phase could give rise to the other at a relatively high rate, which was greater in the P+ to P- direction during nonselective culture on solid medium and greater in the P- to P+ direction during culture in unshaken liquid medium. In addition, 77 Pil- mutants were selected on the basis of a subtle difference in colonial morphology. The mutants reverted, if at all, at a much lower rate than that of the P- to P+ change. The stability of Pil- derivatives grown in unshaken liquid medium was used as a criterion for distinguishing between phase variants and mutants. Phase variation also affected colonial morphology and chemotactic swarming. These properties did not directly depend upon piliation since Pil- mutants were only slightly altered in colonial form and unaltered in chemotactic swarming. Piliation of Pil+ bacteria was quantitatively affected by growth conditions.