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Chapter 1
to genes involved in flagellar and LPS production (flgK, rfbA, nusA) resulted
in significantly lower biofilm formation by S. Typhimurium attachment to
stainless steel and glass in meat and poultry broths [93].
1.15.2. Identification through colony morphology
The formation of the extracellular polymeric matrix of a Salmonella biofilm
varies depending on the method of growth [28]. As discussed some of the
more consistently confirmed elements have included components of the
thin aggregative fibers referred to as curli, EPS and cellulose. Numerous
authors have identified components involved in biofilm formation through
assessing the colony morphotype on congo red (CR) agar [77, 86, 94-95].
This method involves growing the test organisms on Luria-Bertani Agar
plates with the addition of indicator dyes (congo red, coomassie brilliant
blue and calcofluor). The indicator dyes facilitates detection of cellulose
and curli fimbriae which play key roles in Salmonella biofilm production
[77].
The curli and cellulose positive phenotype can be identified as red, dry and
rough (rdar) on CR agar [82]. The curli negative phenotype is associated
with pink dry and rough (pdar) colonies. The brown dry and rough colony
morphology is associated with cells lacking synthesis of cellulose. While in
the absence of both curli and cellulose smooth and white (saw) colonies
are detected on CR agar [82]. The smooth brown and mucoid (sbam)
morphotype on CR agar is indicative of a lack of cellulose and over
production of capsular polysaccharides [86]. Efforts have been made to link
morphotype of CR agar with capacity to form biofilm. Studies have
indicated that the Salmonella rdar morphotype accounts for up to 99% of S.
enterica subgroup I [96]. However, others have questioned its reliability as
an indicator of the level of biofilm formation on food contact surfaces [67].
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