A TrIP through the type IV secretion channel. The bacterial type IV secretion apparatus is localized predominantly at the pole of bacteria. In A. tumefaciens, this machinery mediates the translocation of bacterial T-DNA across both the inner membrane and outer membrane and injects the T-DNA into plant cells where it becomes integrated into the host cell genome. Virulence factors encoded by the T-DNA induce tumorlike growths (crown gall disease) in host plants. Science Volume 304, Number 5674, Issue of 21 May 2004, p. 1122.

Bacteria have evolved a series of cleverly constructed machineries that mediate the passage of proteins and DNA across their cell envelopes and into the extracellular milieu. The complexity of these transporters is especially apparent in Gram-negative bacteria, which are enclosed by two membranes (see the figure). Although the additional outer membrane offers a semipermeable barrier to protect these organisms from harmful substances, it also poses an obstacle that the bacteria must overcome to send materials to the outside environment. Consequently, transport mechanisms in Gram-negative bacteria require multicomponent structures that are organized into intricate machineries that often span both the inner and outer membranes. Possibly the most familiar example of such an apparatus is the conjugation machinery, a multiprotein complex that facilitates the transfer of genetic material from one bacterial cell to another. In the natural environment, this form of horizontal gene transfer promotes genetic diversity and survival. The discovery of conjugal transfer in the 1940s revolutionized microbial genetics, and has been widely exploited to transmit plasmids and selectable markers between bacterial strains. Although extremely useful in laboratory settings, the spread of antibiotic resistance between pathogens in clinical situations has made the treatment and control of infectious diseases extremely challenging (1). Overall, the directed delivery of materials from bacterial cells has been one of the most useful, and yet one of the most worrisome phenomena, in the field of microbiology.