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Chapter 6
undetermined. However, the fact that the disinfectants were not effective
at eliminating the biofilm using the microtitre plate method cannot be
overlooked. Perhaps this unexpected result serves to emphasise the
necessity to consider multiple experiments and conditions when examining
the efficacy of disinfectants against a pre-established biofilm. The broad
findings from both methods indicate that the three disinfectants are of
limited use against an established biofilm. This also correlates with
indications that eradication of the S. Agona SAGOXB.0066 associated with
the outbreak from one area on the food campus using a range of
treatments including sodium hydroxide, vaporised hydrogen peroxide and
heat treatments was challenging (personal communication [58]).
6.7. Investigating repeatability of measuring biofilm density
6.7.1. Repeatability of results using the CBR
If the extent of variation that occurs with a single strain on a single
substratum is not established, there is a risk of interpreting differences
between strains or substrata as significant when it may only have occurred
through random variation. Moreover, the extent of variation may also
account for some apparent inconsistencies in the reported data on biofilm
development. As a result, it is important to investigate the limits of
repeatability in an experimental system. Repeatability refers to the ability
to produce the same results during a repeated experiment within one
laboratory [236].
The International Standards Organisation ISO (157251:1) defines
repeatability as the standard deviation of results achieved through
repeated experimentation under the same conditions [236]. The results of
chapter 2 relating to the CBR indicate that the standard deviation varied
with the strain and the surface tested. Based on the mean log density
values, the range of SD values for all 13 strain was between 0.19-0.66 on
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