Horizontal Gene Transfer in Planktonic and Biofilm Modes - Antibiofilm Agents: From Diagnosis to Treatment and Prevention - page 76

was still visible on an agarose gel after 3.5 min incubation. Furthermore, the
presence of plasmid DNA of about 500 bp length was demonstrated by PCR after
24 h of incubation in human saliva. Duggan et al. ( 2000 ) demonstrated that pUC18
could transform E. coli to ampicillin resistance following 24 h incubation in clarified
ovine saliva. A recent study by Hannan and coworkers showed that the genomic
DNA of a Veillonella dispar strain carrying the conjugative transposon Tn 916 could
transform Streptococcus mitis to tetracycline resistance within an oral biofilm grown
in a fermenter (Hannan et al. 2010 ).
Tribble et al. ( 2012 ) studied chromosomal DNA transfer between
Porphyromonas gingivalis , a Gram-negative anaerobe residing exclusively in the
human oral cavity. Their results revealed that natural competence mechanisms are
present in multiple strains of P. gingivalis , and DNA uptake is not sensitive to DNA
source or modification status. Tribble and coworkers ( 2012 ) were the first to
observe extracellular (e) DNA in P. gingivalis biofilms and predicted it to be the
major DNA source for HGT and allelic exchange between strains. They proposed
that exchange of DNA in plaque biofilms by a transformation-like process is of
major ecological importance in the survival and persistence of P. gingivalis in the
challenging oral environment.
Certain oral streptococci produce hydrogen peroxide under aerobic growth
conditions to inhibit competing species like Streptococcus mutans . By using Strep-
tococcus gordonii as a model organism Itzek and coworkers demonstrated hydro-
gen peroxide-dependent eDNA release (Itzek et al. 2011 ). Under defined growth
conditions, the eDNA release was shown to be entirely dependent on hydrogen
peroxide. Chromosomal DNA damage seemed to act as the intrinsic signal for the
release. Interestingly, the generation of eDNA was found to be coupled with the
induction of the S. gordonii natural competence system. Consequently, the produc-
tion of hydrogen peroxide triggered the transfer of antimicrobial resistance genes
(Itzek et al. 2011 ). Thus, the eDNA found in the oral cavity can serve as a pool for
novel genetic information, since hydrogen peroxide production by one species can
induce the release of eDNA in other species. Itzek et al. ( 2011 ) argued that
hydrogen peroxide is potentially much more than a toxic metabolic by-product;
rather, it could serve as an important environmental signal that facilitates species
evolution by HGT of genetic information and an increase in the mutation rate.
The action of a competence-specific murein hydrolase, CbpD, strongly increases
the rate of HGT between pneumococci. CbpD is the key component of a bacteri-
olytic mechanism termed fratricide. It is secreted by competent pneumococci and
mediates the release of donor DNA from sensitive streptococci present in the same
environment (Wei and H ˚ varstein 2012 ). Recent data from Wei and H ˚ varstein
demonstrated that the fratricide mechanism has a strong positive effect on
intrabiofilm HGT, indicating that it is important for active acquisition of homolo-
gous donor DNA under natural conditions. Additionally, they found that competent
biofilm cells of S. pneumoniae acquire a resistance marker much more efficiently
from neighboring cells than from the growth medium. This could be explained by
the fact that externally added DNA is not able to penetrate into the biofilm and is
therefore available only to competent cells that are exposed to the growth medium.