Horizontal Gene Transfer in Planktonic and Biofilm Modes - Antibiofilm Agents: From Diagnosis to Treatment and Prevention - page 65

important mechanism for this efficient horizontal gene spread. Various excellent
recent reviews exist on the mechanistic details of conjugative plasmid transfer
among bacteria mediated by T4SS, with emphasis on mechanistic details for
Gram-negative bacteria (Alvarez-Martinez and Christie 2009 ; Fronzes
et al. 2009 ; Waksman and Fronzes 2010 ; Wallden et al. 2010 ; Thanassi
et al. 2012 ; Zechner et al. 2012 ). Therefore, there is no particular need to focus
on it in this chapter. Instead, we will put emphasis on the occurrence of all three
modes of HGT, as well as on specialized modes of HGT, for which only recently
some mechanistic details have been elucidated, such as for the intra- and inter-
genomic transfer of pathogenicity islands, their regulation and consequences for
survival, and fitness of the participating microbes, in different environments.
Biofilms are the predominant mode of life for bacteria in nature. Bacteria living
in biofilms have been shown to be better protected from harmful impacts from their
environment than their planktonic counterparts. Indeed, biofilm-associated bacteria
exhibit increased resistance to antimicrobials, water stress, osmotic pressure, or
grazing by protozoans (Costerton et al. 1999 ; Hogan and Kolter 2002 ) and adapt
more readily to environmental changes via specialized communication systems,
denominated quorum-sensing (Parsek and Greenberg 2005 ; Schuster et al. 2013 ).
The evolution, adaptation, and ecology of bacteria are intertwined mechanisms.
Genes that are transferred horizontally between bacteria contribute essentially to
bacterial evolution; interspecies HGT may lead to entirely new genetic combina-
tions, which occasionally impose serious threats to human health (Madsen
et al. 2012 ). Biofilm formation is essentially a product of interbacterial relations.
Biofilms can consist of only one species, but in most cases natural biofilms contain
different species, characterized by primary colonizers, which start “conquering” the
habitat of choice, followed by secondary colonizers which establish in the matrix of
the biofilm. In any case, the formation of a stable mature biofilm is the product of
social interactions that have evolved through adaptations (Madsen et al. 2012 ). For
several decades, both HGT and biofilms have been central areas of microbiological
research in environmental as well as medical microbiology, resulting in the recog-
nition of their high significance for bacterial adaptation and evolution. A growing
number of studies showed that plasmid biology (in particular of conjugative
plasmids) and biofilm community structure and functions are intertwined through
many complex interactions, ranging from the genetic level to the community level.
This fact points towards a principal role of the concerted action of these activities in
socio-microbiology and bacterial evolution (Fig. 1 ; Madsen et al. 2012 ).
There is growing evidence that conjugative plasmids can promote the formation
of biofilms or at least increase or accelerate their formation through genetic traits
encoded on their genomes (May and Okabe 2008 ; Yang et al. 2008 ; D'Alvise
et al. 2010 ; Madsen et al. 2012 ). This chapter will review the state of the art of
interconnections between MGEs, MGE-mediated HGT, and biofilm formation.
However, not only conjugative transfer of plasmids and integrative conjugative
elements (ICE) has been shown to take place in biofilms, but experimental evidence
also exists for the occurrence of bacterial transformation via DNA uptake from the