Some of the E. coli bacteria that were originally isolated from a contaminated salad were tetracycline resistant while the others were susceptible. However, when tetracycline-resistant and -susceptible strains were grown together, all of them became tetracycline resistant. This efficient transfer of resistance depends on which of the following?

Many antibiotic resistance genes in bacteria are carried on conjugative plasmids (c) also known as R factors. These plasmids are transferred from one bacterium to another, of the same or related species, by conjugation. Conjugation is a process by which the donor bacterium (male cell) is brought in direct contact with the recipient bacterium (female cell) through the conjugative pilus. In E. coli, the conjugative pilus is referred to as a sex pilus or F pilus. The conjugative plasmid carries genes necessary for its transfer including the pilus genes. The plasmid is transferred into the recipient cell as a strand of ssDNA, which re-circularizes within the recipient cell. The complementary DNA strand is then synthesized in the recipient cell. The ssDNA in the donor cell is also replicated; thus, both the donor and the recipient now carry a copy of the plasmid. Due to the direct contact between bacteria through the pilus, conjugation is an efficient process of DNA transfer. A transposon (e) is a mobile genetic element that is capable of moving from one replicon (a chromosome or a plasmid) to another but is incapable of initiating DNA transfer. Recombinase enzymes (d) are required for the recombination process that occurs between homologous regions of DNA (as in integration of a DNA in a replicon). Competent cells (b) are part of the bacterial population that is capable of taking up DNA from a solution (naked DNA). This may occur naturally or artificially (in the laboratory). E. coli competent cells are only generated in the laboratory through treatment with certain chemicals. Cell lysis and release of donor DNA (a) may be part of a mechanism through which naturally competent bacteria such as Neisseria take up naked DNA during infection.