Horizontal Gene Transfer in Planktonic and Biofilm Modes - Antibiofilm Agents: From Diagnosis to Treatment and Prevention - page 73

Hennequin et al. ( 2012 ) studied the plasmid transfer capacity of a CTX-M-15
beta lactamase producing Klebsiella pneumonia e isolate in both planktonic and
biofilm conditions. Plasmid transfer frequencies in biofilms reached very high
frequencies of about 0.5 per donor cell, in comparison to the planktonic mode
where they amounted to 10 3 /donor. Ma and Bryers ( 2013 ) quantified conjugative
transfer of the Pseudomonas TOL plasmid in biofilms as a function of limiting
nutrient concentrations. Frequencies of plasmid transfer within biofilm populations
were affected by limiting substrate loading in the following way: low concentra-
tions of the limiting substrate, in this case, glucose, generated thinner biofilms
comprised of more porous biofilm clusters that allowed greater penetration of donor
cells throughout the clusters with more exposure of recipient population to donor
cells, which resulted in an increase of plasmid transfer frequencies (Ma and Bryers
2013). The opposite held true at high substrate concentrations that produced very
dense compact biofilm clusters, with corresponding low plasmid transfer
efficiencies.
ICEA fe 1 is a 291-kbp ICE that was identified in the genome of the booming
bacterium Acidithiobacillus ferrooxidans . Bustamante et al. ( 2012 ) investigated the
excision of the element and expression of relevant genes under normal and
DNA-damaging growth conditions. Both basal and mitomycin C-inducible excision
as well as expression and induction of the genes for integration/excision were
observed, suggesting that ICEA fe 1 is an actively excising SOS-regulated MGE
(Bustamante et al. 2012 ). The presence of a complete set of genes for self-transfer
functions that are induced in response to DNA damage additionally suggested that
ICEA fe 1 is capable of conjugative transfer to suitable recipients. Transfer of
ICEA fe 1 may provide selective advantages to other acidophilic bacteria in the
ecological niche through dissemination of gene clusters expressing CRISPRs and
exopolysaccharide biosynthesis enzymes, probably by resistance to phage infection
and biofilm formation, respectively (Bustamante et al. 2012 ). The presence of a
number of genes predicted to encode proteins involved in the synthesis of
exopolysaccharides that contribute to biofilm formation could enhance the persis-
tence of these bacteria in the environment and/or help to improve mineral dissolu-
tion (Rohwerder et al. 2003 ; Bustamante et al. 2012 ).
3.2.2 Conjugative Plasmid Transfer Among Gram-Positive Bacteria
in Biofilms
Roberts et al. ( 1999 ) used a constant depth film fermenter to demonstrate that
transient bacteria may be able to act as a donor to oral bacteria in an oral biofilm
community in the short time that they are present in the oral cavity. Tn 5397
(a conjugative transposon carrying tetM) in a B. subtilis donor was shown to
transfer to a streptococcal recipient growing as part of an artificial oral biofilm in
the constant depth film fermenter. The B. subtilis donor was not recovered from the
biofilm 24 h after inoculation, showing that even though the donor bacteria are no
longer present, their genetic information can persist (summarized in Roberts and