A Hitchhiker's Guide to Type IV Secretion.

BACTERIA; GRAM-negative bacterial diseases; EXTRACELLULAR matrix; BACTERIAL conjugation; EXTRACELLULAR space; CYTOPLASMIC inheritance

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. 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.