Horizontal Gene Transfer in Planktonic and Biofilm Modes - Antibiofilm Agents: From Diagnosis to Treatment and Prevention - page 74

Mullany 2010 ). Tn 916 -like conjugative transposons have been shown to be com-
mon in tetracycline-resistant Veillonella spp. and some of them were transmissible
within a mixed-species consortium in the oral cavity consisting of 21 tetracycline-
sensitive members (Ready et al. 2006 ). Sedgley and coworkers ( 2008 ) also pro-
vided evidence that the conjugative plasmid pAM81 was able to transfer between
Streptococcus gordonii and E. faecalis in the root canals of human teeth in vivo.
Cook and colleagues demonstrated that growth in biofilms alters the induction of
conjugative transfer by a sex pheromone in E. faecalis harboring the conjugative
plasmid pCF10. Mathematical modeling suggested that a higher pCF10 copy
number in biofilm cells would enhance a switch-like behavior in the pheromone
response of donor cells with a delayed but increased response to the mating signal
(Cook et al. 2011 ). Variations in plasmid copy number and a bimodal response to
induction of conjugative transfer in populations of plasmid-harboring donor cells
were both observed in biofilms, which is consistent with the predictions of the
model. The pheromone system may have evolved such that donors in biofilms are
only induced to transfer when they are in extremely close proximity to potential
recipients in the biofilm (Cook et al. 2011 ). In contrast to the popular notion of
biofilms being the optimal niche for conjugation (Hausner and Wuertz 1999 ), Cook
and coworkers observed reduced efficiency of pCF10 transfer in biofilms. Their
mating experiments employed biofilms grown in vitro with inducing pheromone
produced by the recipient cells. The authors argued that their results reflect the
anatomy of enterococci and differences in the cell attachment mechanisms used by
the conjugative transfer machines of Gram-positive vs. Gram-negative bacteria:
E. faecalis cells are not motile. When E. faecalis cells colonize a surface and initiate
biofilm growth or attach and become part of a preexisting biofilm, they probably
remain in the same location until they die or detach to reenter the planktonic phase
(Cook et al. 2011 ). There is very low probability that donor and recipient cells get
into close/intimate contact for the exchange of pCF10. In the pCF10 T4SS, mating
pair formation is mediated by the surface adhesin Asc10 (encoded by the prgB gene
on pCF10) which can stably bind the surfaces of cells that randomly collide; there
are no sex pili that could attach cells that are not in direct wall-to-wall contact. In
planktonic cultures of sufficient population density, however, random diffusion
increases the probability of collision between donors and recipients, and induced
donors can form stable mating pairs extremely efficiently (Cook et al. 2011 ).
Ghosh et al. ( 2011 ) investigated the enterococcal populations of dogs leaving the
veterinary intensive care unit (ICU) for multidrug resistance, the capacity for
biofilm formation, and HGT. The enterococcal diversity based on 210 isolates
was low as represented by E. faecium (54.6 %) and E. faecalis (45.4 %). Most
isolates were resistant to various antibiotics. All E. faecalis strains were biofilm
formers in vitro. In vitro intra-species conjugation assays demonstrated that
E. faecium were capable of transferring tetracycline, doxycycline, streptomycin,
gentamicin, and erythromycin resistance traits to human clinical strains. High
transfer rates (10 5 -10 4 ) for both tetracycline and doxycycline resistance traits
were observed, indicating potential involvement of Tn 916 (Ghosh et al. 2011 ). The
study demonstrated that companion animals after release from the ICU and on