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Chapter 5
interpretation therefore in order to confirm the morphological traits,
previously characterised strains that display the relevant morphotypes
should be used as positive and negative controls. Positive and negative
controls generally involved the use of well characterised strains verified to
contain production of curli and cellulose such as S. Typhimurium 14028-1s
(LT2) which displays the rdar morphotype and gene knock-out strains that
display other morphotypes [77]. The S. Typhimurium LT2 strain was used as
a rdar positive control in this research however strains characterised as
pdar, bdar and sbam were not available. Although the images of colony
morphologies published elsewhere [77, 86] were used as a reference, the
lack of a negative control is a significant limitation to the morphology
characterisation work described in this chapter. However characterising
colony morphology without the use of controls has also been described
elsewhere [86, 95].
The results of the microtitre plate method experiments suggested that
particular strains of S. Agona may form a more dense biofilm under certain
conditions. As displayed in Table 5.4 the S. Agona strains had the highest
mean OD 590 measurements 22°C for 2 days based on ranking the S. Agona
strains had the 3 rd , 4 th , 5 th and the 6 th largest mean OD 590 . However, when
the same strains were incubated at 37°C this trend was less apparent.
Moreover, when the strains were categorised into their respective serovars,
there was no evidence to suggest that any differences between the
serovars was significant (Table 5.5). A similar pattern of some S. Agona
strains generally forming a more dense biofilm than strains of S.
Typhimurium and S. Enteritidis was observed when the biofilm was formed
over 168 hours, although this pattern was less consistent.
However, as displayed through the large standard deviation of mean OD 590
measurements, it was evident that there was large variation in repeated
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