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It had been considered previously that pili might be involved in the fertility of male bacteria.3 This speculation was based on the frequent occurrence of pili on male E. coli K 12 strains and on the plausibility of chromosomal transfer being mediated by a rod-like structure which could actively traverse the cell membrane and wall. However, no correlation of piliation with maleness was found. Many female strains were richly piliated and some cultures of male strains contained only a few sparsely piliated cells. The hypothesis could not be disproved, however, since none of the many male strains subsequently examined were ever completely nonpiliated. Although the majority of cells in a culture may have had no pili, at least a few cells could always be found with a few attached pili. A new method of approach to chis problem has been provided by the recent electron microscopic observations of Crawford and Gesteland' who noted that a male-specific bacteriophage, R-17, adsorbed to pili of an Hfr and an F+ strain of E. coli but not to pili of an F- strain. We have investigated the adsorption of another male-specific bacteriophage, M 12, isolated by P. H. Hofschneider.6 Our studies revealed that M 12 adsorbs to some of the pili present on male bacteria and that phage adsorption can be used to distinguish them in electron micrographs from other types of pili occurring on the same cell. It is possible to demonstrate that these "F pili" are genetically con

A NEW TYPE OF BACTERIAL PILUS GENETICALLY CONTROLLED BY THE FERTILITY FACTOR OF E. COLI K 12 AND ITS ROLE IN CHROMOSOME TRANSFER