L. casei and L. fermentum isolates were most abundant in the yeast
propagation tank, where L. plantarum was also abundant. Results
suggest that these two tanks are the major source of these contaminants
going into the fermentation tanks. Total contaminants generally
decreased and were more variable in late fermentations, and
isolates of L. fermentum and L. plantarum decreased in late fermentations.
However, percentages of L. casei and L. mucosae increased
or remained steady throughout the fermentation, suggesting that
these species tolerate conditions in late fermentations.
3.4. Biofilm formation and inhibition of ethanol production by
contaminants
All contaminants were characterized individually for their
ability to form biofilms under laboratory conditions in a previously
described rapid assay (Rich et al., 2011). In this assay, significant
biofilm formation is considered to be represented by OD600 of
>0.3. Contaminants were also characterized for their ability to inhibit
ethanol production by S. cerevisiae in a previously described
model corn mash system (Bischoff et al., 2009). In this assay, significant
inhibition of ethanol production is considered to be represented
by a decrease in ethanol yields of greater than 10 g
ethanol/L. As shown in Fig. 3, total contaminants varied greatly
in both ethanol inhibition and biofilm formation. Twenty-two %
of total isolates exhibited ethanol inhibition of >10 g/L, while 7%
produced biofilms of >0.3 OD600. Only 2% of isolates fell into both
categories, and there was no apparent correlation between ethanol
inhibition and the ability to form biofilms. However, this result
does not mean that biofilms are unimportant in the persistence
of potentially inhibitory strains. Biofilms do not exist as pure cultures
in nature but rather as complex communities of multiple
species. It is possible that isolates that are unable to form biofilms
in pure culture can live as part of a mixed culture biofilm,
supported by isolates with stronger biofilm-forming capacity.