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Chapter 1
shield the S. Agona cells from direct contact with the disinfectant, resulting
in lower levels of exposure to the disinfectant agents deep within the
mature biofilm [70].
Ramesh et al. also investigated the persistence of Salmonella serovars
regularly found in food processing environments (S. Typhimurium, S.
Thompson, S. Berta, S. Hadar, and S. Johannesburg) found that quaternary
ammonium compounds and an enzymatic biocide were not as effective
against a 4-day Salmonella biofilm in comparison to a 3-day biofilm on
stainless steel [150]. Similar findings were also reported by Korber et al.
who found that trisodium phosphate was not as effective at killing S.
Enteritidis when part of a 72 hour biofilm as a 48 hour biofilm [102]. Using
a glass flow cell and electron microscopy Korber found that 75% of the S.
Enteritidis biofilm cells remained attached at the base of the glass surface
after treatment of the 48 hour biofilm with trisodium phosphate, however
this increased to 91% when testing against the 72 hour biofilm [102].
The hypothesis that a mature biofilm subsequently leads to increased
resistance or co-resistance to disinfectants and antimicrobial agents has
also been substantiated elsewhere, particularly in dental research. Using
bacteria derived from dental plaque. Shen and colleagues reported the
thickness of a biofilm increased from 57µm (2 days) to 155 µm (3 weeks)
during biofilm development, reaching a steady state of 190 µm after 6
weeks [151]. When testing the multispecies biofilm against chlorohexidine
(CHX) they found that the proportion of killed bacteria in mature biofilms
(>3 weeks) was lower than a biofilm formed over a shorter period of time
(2 days, 1 and 2 weeks) after treatment with CHX. Shen et al. concluded
that bacteria in nutrient limited biofilm formed over an extended period of
time were more resistant to CHX than biofilm formed over a shorter
duration of time [151]. In the study described by Shen et al., a 3+ week
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