An indication for the presence of extracellular DNA (eDNA) was inferred
from the results described above and it was further supported
by fluorescencemicroscopywith propidiumiodide (PI) that specifically
stains DNA (Fig. 5B). When the mature biofilm produced by strain
WCFS1was treatedwith either Proteinase K orDNase I, both CV staining
and culturable cell numbers in the biofilm decreased (Fig. 5A). It can be
excluded that the addition of Proteinase K affected the initial attachment
of cells since its addition along with the inoculum did not influence
in the initial attachment up to 2.5 h (data not shown). Similar
effects after Proteinase K and DNase I additionwere observed for strains
FBR 3, FBR 5 and FBR 6 (Fig. 5A). Strains FBR 1 and FBR 2 showed no detectable
level of CV staining, however, the culturable cell numbers in the
biofilm were reduced. Remarkably, the biofilm produced by strain FBR
4, was only reduced after addition of Proteinase K, while DNase I treatment
did not have a significant effect. It cannot be excluded that other
components in the biofilm matrix produced by these strains interfere
with the accessibility of the eDNA. Proteinase K treatment resulted in
a large decrease in attached cells for all the L. plantarumstrains resulting
in CV staining below detection level.