Horizontal Gene Transfer in Planktonic and Biofilm Modes - Antibiofilm Agents: From Diagnosis to Treatment and Prevention - page 70

(Dobrindt et al. 2004 ). The coding capacity of GEIs is not limited to pathogenicity
functions, but can be very diverse, including traits such as symbiosis (Sullivan
et al. 2002 ), sucrose and aromatic compound metabolism (Gaillard et al. 2006 ),
mercury resistance, and siderophore synthesis (Larbig et al. 2002 ). Bioinformatics
studies have shown that GEIs tend to carry more novel genes (i.e., genes that do not
have orthologs in other species) than the rest of the genome (Hsiao et al. 2005 ). This
suggests that GEIs have become strongly selected for adaptive and auxiliary
functions. Juhas et al. ( 2009 ) proposed that the term GEI should be used for the
overarching family of discrete “DNA elements” which are part of a cell chromo-
some and can drive or have driven strain differentiation.
As not all GEIs contain the same components, it is difficult to define a unifying
mode of GEI functioning or lifestyle (referring to the functions required for
maintenance, excision, transfer, or integration). Interestingly, many GEIs for
which self-mobility has been shown can excise from the chromosome, encode the
full capacity for horizontal self-transfer to another cell, and reintegrate into the
target site in the new host chromosome (Juhas et al. 2009 ). GEIs that encode all
these features and self-transfer by conjugation are part of a well-defined group of
elements that have been named ICEs (Burrus and Waldor 2004 ). A wide variety of
GEIs are intimately connected to phages and conjugative plasmids through their
evolutionary origins. As a consequence, besides transformation, their transfer often
occurs via conjugation and transduction (Jain et al. 2002 ; Chen et al. 2005 ; Juhas
et al. 2009 ). GEIs do not necessarily encode the whole genetic information for self-
transfer, and several cases are known in which GEIs are packaged by another
co-residing lysogenic phage or mobilized by a plasmid or the conjugative system
of an ICE (Shoemaker et al. 2000 ).
Hall ( 2010 ) recently published an excellent review on Salmonella genomic
islands and their dissemination. Salmonella genomic island 1 (SGI1), the first island
of this type, was found in S. enterica serovar Typhimurium DT104 isolates, which
are resistant to seven different antibiotics. Early studies by Schmieger and
Schicklmaier ( 1999 ) demonstrated that SGI1 was moved into new hosts by trans-
duction via a phage produced by the DT104 isolates. SGI1 cannot transfer itself into
a new host because it does not encode a full set of conjugative transfer genes, but it
is mobilizable (Doublet et al. 2005 ). It can be transferred into Salmonella spp. or
Escherichia coli hosts if an IncA/C plasmid is present in the donor to supply the
conjugative transfer machinery (Doublet et al. 2005 ). SGI1 is found in many
different S. enterica serovars. It carries class 1 integrons containing five antibiotic
resistance genes conferring resistance to seven antibiotics, namely, ampicillin,
chloramphenicol, florfenicol, streptomycin, spectinomycin, sulfamethoxazole, and
tetracycline.