Horizontal gene transfer has been a major driving force in bacterial evolution and involves the transfer of mobile genetic elements through transformation, transduction or conjugation. Two articles in this issue look at specific classes of mobile genetic elements. Chromosomal islands are discrete chromosomal segments that have been acquired by horizontal transfer and which, for the most part, are no longer mobile. One highly mobile exception is a class of elements known as phage-related chromosomal islands (PRCIs), which are found in Gram-positive bacteria. These elements hijack the reproductive cycle of helper phages for their transduction. On page 541, Novick, Christie and Penadés discuss the molecular genetics of PRCIs, focusing on the life cycle, regulation and mobility of the prototypical class, the Staphylococcus aureus pathogenicity islands (SaPIs). On page 552, Wozniak and Waldor look at integrative and conjugative elements (ICEs), which are found in a diverse array of Gram-positive and Gram-negative species. In contrast to SaPIs, ICEs are self-transmissible and encode the machinery necessary for excision from the chromosome and for conjugative transfer. Wozniak and Waldor review the key steps in the ICE life cycle, including integration, excision and conjugation.

This issue also includes the second article in our series on Applied and Industrial Microbiology. On page 578, Bernd Rehm reviews the progress that has been made in the field of bacterial biopolymers. An increased understanding of the pathways that bacteria use to synthesize polymers such as polysaccharides and polyamides should pave the way for the rational engineering of bacteria to produce tailor-made polymers.