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Chapter 5
5.1.2. Previous research using the colony morphology assessment
Römiling and colleagues demonstrated that knocking out the gene
encoding for curli expression (curli subunit genes CsgA) resulted in an
altered colony morphology of S. Typhimurium on CR agar. The colony
morphology changed from rdar to pdar based on this knockout process.
This change was associated with less dense biofilm formation by strain [77,
167]. In this case, biofilm was examined through the use of a pellicle test
(biofilm formed on air-liquid interface) cells on glass [77] and by a electron
microscopic examination of stained biofilm [77]. Additionally, Römiling et al.
demonstrated that mutation of the AgfD gene (involved in encoding for
cellulose production) also altered the colony morphology resulting in a
brown, dry and rough colony morphology (bdar) [167, 225]. The bdar
strains also produced a more fragile biofilm than the wild type strain when
examined using a pellicle formation model and assessing the force needed
to disaggregate the pellicle [167, 225]. Moreover, as a result of this
research, Römiling and colleagues demonstrated that the mutants lacking
both genes in S. Typhimurium (LT2 mutants) were unable to form a biofilm
and could be identified as smooth and white (saw) on CR agar [77, 167,
225] .
Malcova et al. recently reported that S. Typhimurium strains that do not
appear to produce curli or cellulose based on morphology on CR agar, were
capable of forming a dense biofilm using the microtitre plate based
method [86]. The formation of capsular polysaccharide by the strains
resulted in a smooth brown and mucoid colony morphology on congo red
agar. Malcova et al. indicated that this phenomenon (production of a
biofilm through the formation of a capsule without cellulose or curli) was
not only associated with S. Typhimurium DT104, but was also found in 3
strains of S. Montevideo and 1 strain of S. Derby [86]. The presence of
capsular polysaccharides that may be involved in biofilm formation in the
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