Covert R Factors in fi+ R+ Strains of Bacteria

Many bacterial characters, such as colicinogeny (9), antibiotic resistance (25), and production of a hemolysin (21) or an enterotoxin (22), are determined by genes carried on extrachromosomal genetic elements that are transmissible from one bacterium to another by conjugation. Two classes of sex factor responsible for conjugation have been recognized in the Enterobacteriaceae (11). The first, or F-like, class includes F itself (10) and determines the production of sex pili resembling the F pilus which acts as receptor for F-specific phages (2, 5). The second, or I-like, class determines a different kind of sex pilus typified by that detennined by the colicin factor Coll (16). This pilus does not adsorb F-specific phages but acts as receptor and confers sensitivity to I-specific phages (17). F-like sex factors can also be recognized, when they are introduced into an F+ cell, by the fi+ character (25), i.e., the ability to inhibit fertility by repressing F pilus formation. The repressor of I-like factors does not act on F, and these factors are therefore fi-. Since transmissible drug resistance due to R factors is now very common, one might well find R+ bacteria carrying more than one R factor. However, any R factor, whether fi+ or fi-, generally excludes another of the same type from a given strain (15). Doubly infected R+ strains would therefore be expected to carry one fi+ and one fifactor, not two of the same kind. Thus, strains recorded as fi+ could also carry an fifactor without its being immediately apparent. It can, however, be detected by propagation of the filamentous I phage Ifl, which specifically attacks cells forming the I sex pili determined by fi factors (17). Strains with a maskedfifactor proved to occur relatively frequently, and usually carried two