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Chapter 1
of silicone disks [121]. This research demonstrated major differences in the
level of gene expression responsible for virulence and adhesion factors.
1.16.3. Effect of age of biofilm on cell recovery
It is widely accepted that if the disinfectant does not penetrate and diffuse
though the deeper layers of the biofilm, the biofilm may experience cell
recovery and persist in the environment [56]. There is both limited and
contradictory information on whether disinfectant effectiveness varies
with biofilm stage of maturation [149].
Habimana and colleagues investigated persistent Salmonella strains
(indistinguishable strains when examined using PFGE) which were
repeatedly found in the same food processing plant [70]. Habimana et al.
examined if the ability to survive stress conditions over 28 days, in
comparison to other species found in the food processing environment,
may have contributed to persistence of the Salmonella strains. The results
indicate that Salmonella survival decreased linearly over time. These
results indicated that the natural biota were able to survive in higher
quantities than Salmonella in both the dry and humid conditions [70].
However, interesting results were highlighted when looking at the S. Agona
as part of a mixed species biofilm. The cell density (measured in bio-volume
of cells visualised using microscopy) of S. Agona increased when mixed
with Pseudomonas species (3.2 fold increase) and Staphylococcus spp. (2.8
fold increase) in comparison to a mono-species culture of S. Agona. This
suggests that S. Agona biofilm persistence on food processing
environments may be supported by the presence of other organisms. It
was also noted that S. Agona had a different morphology when grown in a
mixed species biofilm and was found to grow under Pantoea spp. which
formed a super-structures on top of the S. Agona cells. This could possibly
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