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Chapter 5
5.1. Introduction
It was demonstrated in chapters 2 and 3 that Salmonella enterica can form
a biofilm on surfaces frequently associated with food processing
environments. The results show that given suitable conditions such as
nutrients and ambient temperature all 12 S. enterica strains examined
form biofilm and that the biofilm density is related to the substratum it
develops on. Overall, the results showed no major difference between the
S. Agona strain related to the outbreak (SAGOXB.0066) and other strains of
S. Agona. It was of interest to investigate biofilm density of these strains
using alternative, widely used methods such as the microtitre plate based
system.
5.1.1. Colony morphology of biofilm forming strains
As discussed previously in chapter 1, increased concentration of
components of the extracellular matrix including curli and cellulose have
been widely linked to increased density of biofilm formation [28, 77, 87, 94,
167]. The extracellular polymeric matrix components can be indentified
through the use of congo red (CR) agar. This method involves growing the
test organisms on an agar plate with a low salt concentration (Luria-Bertani
Agar without salt) and with the addition of 2 indicator dyes, (1) congo red
which binds to the amyloid protein fibres of the curli [222] and (2)
coomassie brilliant blue dye which is a non specific protein binding dye that
binds to curli and cellulose binding proteins [223]. The mixture of dyes
facilitates the detection of both cellulose and curli fimbriae. The
morphotypes displayed on CR agar are summarised in Table 5.1.
Binding of congo red and coomassie blue to the amyloid protein fibres of
the cell (curli) and coomassie blue to cellulose leads to a red, dry and rough
colony (rdar) morphology on the CR agar plate [77, 167, 224]. The brown
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