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Chapter 5
5.3.2. Repeatability of measurement of Optical Density (OD)
In order to investigate difference in repeated measures (repeatability), the
optical density of a single test plate with stained biofilm (as described in
5.2) was read twice consecutively at an optical density of 590 nanometres
wavelengths (OD 590 ). As a representation of the variation in repeated
measures of OD 590 , the results of the repeated OD 590 measurements for 6 S.
Agona are displayed in Table 5.2. The results of the repeated OD 590
measurements for all 12 S. enterica strains and the P. aeruginosa strain are
displayed in Table 1 in the Appendix 2 of this thesis. As displayed in Table
5.2 in this chapter, the OD 590 was read for each individual strain in three
replicate wells. In most cases (12/18), the difference in OD 590 on repeated
measurement was less than 0.100 however it was in the range 0.1 to 0.3
for 5 wells and 1.369 for one well.
There was also a large difference in OD 590 measurement between the three
replicate wells (in same plate read) representing a single strain. For a
number of strains there was at least a two-fold difference in measured
OD 590 for one well in comparison to the other 2 measurements of OD 590 for
the same strain (the full set of data for all 13 strains is available in the
Appendices-Table 1). For the S. Agona S08-0601 strain, there was
approximately a ten-fold difference in measured OD 590 in one well in
comparison to the other OD 590 measurements of replicate wells (Table 5.2).
There was also a large difference in OD 590 measurements between the
second well of the S. Agona strain S09-0371 and the other wells for the
same strain, this was evident from both the first and the second read of
optical density (Table 5.2). The variation in OD 590 measurement from 3
replicate wells for each strain resulted in an elevated mean (higher than if
these outliers were excluded), which is denoted by a large standard
deviation of the mean for each strain which is further discussed in sections
5.3.3 and 5.3.4.
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