View/Open - ARAN - National University of Ireland, Galway

Chapter 5
5.3.3. The density of biofilm formed over 48 hours
The density of biofilm formed by 13 strains was examined using the
microtitre plate based method. The experiments were repeated on three
occasions with 3 replicate wells used for each strain for each experiment.
The results presented in this chapter (mean, standard deviation of mean
(SD) and ranking of strains based on mean) are based on these 9
measurements of optical density. However, as displayed in Appendix 2-
Table 2 and the SD’s provided in tables 5.3-5.6 there were large variations
in repeated measurements of optical density of stained biofilm measured
for each strain. This variation is most likely related to uneven biofilm
formation and incomplete resuspension of the crystal violet stain from the
biofilm matrix. This incomplete resuspension is also displayed in Figures
5.1-5.3. The large variation in repeated measurement of optical density
(denoted by SD) reduces confidence in the findings achieved when
comparing mean optical density values. This is due to the fact that some of
the mean values may appear to be elevated through the presence of a
small number of outlying OD 590 measurements. As a result, caution should
be exercised in directly comparing the mean OD 590 of the three serovars.
Table 5.3 presents the mean OD 590 and standard deviation of the repeated
OD 590 for all 13 strains when biofilm was developed for 48 hours using the
microtitre plate method. The mean and standard deviation are underlined
for 3 strains (P. aeruginosa 27853, S. Agona S08-0601 and S. Typhimurium
SL1344) due to the high OD 590 for some replicate observations for these
strains. The full set of 9 OD 590 measurements for each strain is displayed in
Appendix 2-Table 1.
There were a number of striking inconsistencies with respect to the two S.
Agona outbreak strains S08-0601 and S09-0494 strains. When the biofilm
Page 172