Which method of gene transfer involves direct contact between bacterial cells?

The method of gene transfer that involves direct contact between bacterial cells is conjugation. This process allows for the transfer of genetic material, typically in the form of plasmids, from one bacterium to another through a physical connection known as a pilus. During conjugation, the donor bacterium extends a pilus to attach to a recipient cell, and then a portion of its DNA is transferred directly into the recipient. This mechanism is significant as it facilitates horizontal gene transfer, contributing to genetic diversity and the spread of traits such as antibiotic resistance among bacterial populations.

In contrast to conjugation, transformation involves the uptake of free DNA from the environment by bacterial cells, and transduction relies on bacteriophages (viruses that infect bacteria) to transfer DNA between bacterial cells. Replication, while critical for copying DNA within a cell, does not represent a method of gene transfer between cells.