Control of Polymicrobial Biofilms: Recent Trends - Antibiofilm Agents: From Diagnosis to Treatment and Prevention - page 326

biofilm dispersal have been identified: erosion through de-adhesion events which
increases with greater biofilm biomass and increasing fluid shear (Characklis 1990 ),
sloughing through the loss of large multicellular aggregates of the biofilm commu-
nity due to localized physiological and physicochemical stresses (Lappin-Scott and
Bass 2001 ), and seeding which represents a triggered loss of cells from the biofilm
(Boles and Horswill 2008 ; Davies and Marques 2009 ).
While some currently used antimicrobials may have de-adhesion activities, by
altering the membrane properties of biofilm bacteria resulting in the loss of cell-cell
and/or cell-surfaces adhesion (Neu 1996 ; Rao et al. 2011 ), more efficacious
approaches are currently being investigated. In particular, unlike antimicrobials
that may have secondary dispersal effects, the use of non-antimicrobial dispersal
agents will not likely have the associated problems of developing antimicrobial
resistance and will potentially be less noxious to the environment/host in which it is
deployed. One notable example of a dispersal agent is Dispersin B. Dispersin B,
also known as DspB, is a 42 kDa glycoside hydrolase that was identified as being
produced by the human oral pathogen Aggregatibacter ( Actinobacillus ) actinomy-
cetemcomitans (Kaplan et al. 2003 ). The enzyme catalyzes the hydrolysis of poly-
N -acetylglucosamine (PNAG), one of a number of polysaccharides present in EPS
that is produced by a broad taxonomic range of Gram-positive and Gram-negative
biofilm forming species (Itoh et al. 2005 ; Chaignon et al. 2007 ). Interestingly,
Dispersin B has been shown to not only disperse biofilm species but also enhance
penetration of cetylpyridinium chloride (Ganeshnarayan et al. 2009 ). This latter
point is particularly interesting as it raises the possibility of synergy between
Dispersin B (or any other EPS degrading enzyme) and other anti-biofilm agents
and/or antimicrobials. A fascinating example of such a combinational approach was
presented by Lu and Collins ( 2007 ), who showed that by genetically engineering
bacteriophage T7 to express Dispersin B, treated E. coli biofilms were reduced by
4.5 orders of magnitude, which was about two orders of magnitude better than
treating with non-engineered phage (Lu and Collins 2007 ). While phage technology
represents an interesting approach to control biofilms, albeit potentially restricted in
taxonomic breadth, the specificity of phage for host bacteria is potentially an
advantage or disadvantage depending upon applications and species composition
of a polymicrobial biofilm. This combinational approach acts in a multifaceted
manner that is potentially self-sustaining. Specifically, engineered phage remains in
biofilms as long as suitable non-dispersed cells are present. Therefore, this approach
bypasses the problems associated with contact time—the requirement for a treat-
ment regimen to be sustained for a period of time in order to be effective.
One limitation of polysaccharide lyases such as Dispersin-B is that they exhibit a
high degree of substrate specificity. This is due to the complexity of carbohydrates,
which arises from the multiple linkages that are possible between monosaccharide
units. For example, there are nine naturally occurring disaccharides formed by the
linkage of two glucose residues (R¨diger and Gabius 2009 ). By contrast, proteins
and nucleic acids are linear chains that can be digested relatively easily by enzymes.
There is now strong evidence that extracellular DNA (eDNA) is an important
structural component in many different biofilms (Jakubovics et al. 2013 ). This