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Chapter 4
though a contact time of 90 minutes with a 48 hour biofilm. As previously
discussed, Wong et al. also found that benzalkonium chloride was the least
effective disinfectant against an established biofilm in comparison to 5
other disinfectants [149]. In order to achieve complete eradication of
viable cells, Wong et al. used a concentration of 1.5% which is over twice
the manufactures recommended concentration [149]. Vestby et al.
reported that the use of 0.02% benzalkonium chloride for 5 minutes
against a Salmonella biofilm formed after 48 hours resulted in a 1-2 log 10
reduction in the number of cells recovered from the surface [113].
However the log 10 reduction achieved by Vestby et al. is notably higher
than the log 10 reduction described in this chapter despite the use of a 90
minutes contact time. As mentioned previously, the biofilm substratum
influences the level of biofilm attachment therefore may also have an
effect on the level of biofilm disinfection.
4.4.4. The change in tolerance over an extended period of time
The work presented in this chapter demonstrates that a Salmonella biofilm
developed over an extended period of time is less vulnerable to the effect
of disinfectants products. The results indicate more viable cells were
recovered from a 168 hour biofilm than a 48 hour biofilm post treatment
with disinfectants. Moreover, products such as sodium hydroxide which is
effective using a 48 hour biofilm development time are no longer effective
over a biofilm formed over a longer time. Preliminary research performed
by Møretrø and colleagues suggested that the mean log 10 density of a
Salmonella biofilm on a glass surface did not increase over an extended
period of time (9 days) [134]. Moreover, the results of the work indicated
that the log 10 reduction after treatment with sodium chlorite was similar
and not dependent on the age of the biofilm, when biofilm was formed
over 2, 3, 6 and 9 days biofilm [134]. The suggestion that log 10 reduction is
not affected by the age of the biofilm is in complete contrast to the work
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