SL-biofilms and colony biofilms were incubated and
allowed to grow for 7 days, during which a biofilm developed
(see Additional file 1: Figure S2). Static biofilms
were then harvested by mechanical disruption through
repetitive pipetting to detach the biofilm, followed by
vortexing within a conical tube. Colony biofilms were
placed into 1.5-ml tubes with 1 ml of Hv-starve medium
with sterile forceps, vortexed and then shaken at 180 rpm
for 1 h at 42°C to release cells for plating. An identical mixture
of H53 and H98 liquid culture was also left shaking at
180 rpm for 7 days as a negative control. Samples for each
condition were washed three times with Hv-starve medium,
resuspended in Hv-starve and plated on Hv-Ca with uracil
as selective screen forH53 andH98 prototrophic revertants
and Hv-YPC + thy for total viable counts. Pure culturesof H53 and H98 were included for each condition as a
negative control for mating. Plating on Hv-Ca medium
without uracil was also included in the experiment to
control for spontaneous reversion to uracil prototrophy
(although this was not likely as the pyrE2 gene is not mutated
in H53/H98, but is completely deleted) or the presence
of any ura + strain; no colonies were observed for any
of these conditions. The transfer frequency was calculated
by dividing the average number of colony-forming units
(CFU) per ml on selective medium (Hv-Ca with uracil) for
each replicate set by the corresponding averaged total CFU
count (CFU/ml on Hv-YPC + thy medium).