Control of Polymicrobial Biofilms: Recent Trends - Antibiofilm Agents: From Diagnosis to Treatment and Prevention - page 324

(Gilbert et al. 2002 ; Roberts and Stewart 2005 ; Klapper et al. 2007 ; Lewis 2010 ), is
enriched in biofilms as well as aged (stationary phase) planktonic cultures
(Spoering and Lewis 2001 ; Shapiro et al. 2011 ; Wang and Wood 2011 ). Two
other biofilm-specific phenotypes that have received attention in association with
biofilm tolerance are the expression of efflux pumps and the production of cell-cell
signaling molecules. Efflux pumps can expel chemically unrelated antimicrobial
agents from the bacterial cell, and their expression may be specific to the biofilm
mode of growth or induced by antimicrobials and the biofilm lifestyle (De Kievit
et al. 2001 ; Gillis et al. 2005 ; Hoiby et al. 2010 ; Coenye et al. 2011 ). For example,
Gillis and coworkers ( 2005 ) demonstrated, using DNA microarrays, that
P. aeruginosa PAO1 cells in biofilms that were exposed to azithromycin showed
upregulation of transcripts encoding for restriction-nodulation-cell division (RND)
efflux pumps. Specifically, both the mexAB-oprM and mexCD-oprJ operons that
code for efflux pumps were required for biofilm formation in the presence of
azithromycin, but mexCD-oprJ was a biofilm-specific mechanism for azithromycin
resistance while mexAB-oprM was indicated to be important for azithromycin
resistance in both planktonic and biofilm communities. Efflux pumps can also be
involved in cell-cell signaling (Evans et al. 1998 ; Pearson et al. 1999 ; Herzberg
et al. 2006 ; Buroni et al. 2009 ; Lamarche and Deziel 2011 ).
Cell-cell signaling, the production and detection of low molecular weight signal
molecules, has received increasing attention in the last decade (Dickschat 2010 ). In
particular, this is because biofilms not only concentrate cells but also concentrate
the signal molecules that are produced by the cells (Alberghini et al. 2009 ;
Kolenbrander et al. 2010 ). Thus, the localized increase in density of cell-cell signal
molecules within a biofilm can act as a threshold-based queue for the expression of
biofilm-specific phenotypes and probably contributes to the formation of species
mosaics (Fig. 1 ) (Gu et al. 2013 ). As a consequence of changes in cellular proper-
ties, and conceivably spatial species patterning, biofilms may also have enhanced
tolerance levels and/or alter the expression of virulence factors. For example,
numerous research studies have indicated that the intraspecies and interspecies
cell-cell signaling molecule autoinducer-2 (AI-2) is responsible for a reduced
susceptibility to antimicrobials, and the threshold concentration of AI-2 is likely
only reached in biofilms (Ahmed et al. 2007 , 2009 ; Roy et al. 2011 , 2013 ). While
few examples exist to date, and considering the multispecies nature of many
biofilms, it would be interesting to examine the effects of cell-cell signaling
molecules that are produced by one species on another species that does not
produce them (e.g., AHLs are not produced by any Gram positive species), to
examine the effects of foreign signal molecules on gene expression. A fascinating
study based around such a concept was presented by Duan and colleagues ( 2003 ),
which indicated that non-AI-2 producing P. aeruginosa ( P. aeruginosa does not
possess the luxS gene responsible for the production of AI-2) can detect AI-2
produced by other bacteria in the human lung of cystic fibrosis patients. Genome-
wide transcriptional analysis demonstrated that approximately 4 % of the
P. aeruginosa genome responded to the presence of other bacterial species and
further experiments using exogenously added AI-2 indicated that some of these
genes were influenced by AI-2, including a number of virulence factors.