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Chapter 2
taken from industrial settings only displayed dense biofilm formation in
nutrient-limited environments (1/20 diluted media). This was interpreted
as suggesting that most of the clinical and retail product associated isolates
were adapted to nutrient-rich environments and body temperature [32].
The work described by Castelijn et al. suggests that the strains isolated
from the industrial environments may form a less dense biofilm due to the
environmental conditions the strains were isolated from, possibly though
adaptation to a niche environment. However, Castelijn et al. did not give
any indication of the duration the environmental isolates were present in
the environment before isolation for biofilm studies [32]. Therefore, it is
possible that the strains examined may be planktonic cells recently
introduced to the environment from food sources. As a result, these strains
may not be any different to the food related counterparts.
Castelijn et al. indicated that S. Typhimurium strains LT2 (characterised
with over expression of RpoS) and L2 (a variant of LT2 lacking expression of
RpoS) had similar biofilm density levels in the microtitre plate method. The
LT2 strain has been considered by others as a dense biofilm former due to
the expression of the stationary-phase sigma factor σS (RpoS) [167-168].
On that basis, one might anticipate that the L2 variant lacking this
expression should be a less dense biofilm former and previous studies have
in fact reported notable differences between LT2 and L2 variants [169-170].
It is possible that expression of rpoS may be less critical to biofilm
formation in the model used by Castelijn et al. for some reason however,
the authors do not address this difference between their findings and
previous reports.
Previous authors have reported that strains recovered from food
processing environments repeatedly over an extended period of time,
were more dense biofilm formers than reference strains from the S.
Montevideo and S. Agona serovars [56]. Vestby et al. also reported that
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