Quantifying the rate of biofilm growth of \textit{S. meliloti} strains in microfluidics via the diffusion coefficient of microspheres

Understanding the rate of biofilm growth is essential for studying genes and preventing unwanted biofilms. In this study, the diffusion coefficient ($D$) of polystyrene microspheres was used to quantify biofilm growth rates of \textit{Sinorhizobia meliloti}, a nitrogen fixing bacteria that forms a symbiotic relationship with alfalfa plants. Five strains were studied, two wild types (8530 $expR^{+}$ and 1021) and three mutants in the exopolysaccharide (EPS I, EPS II) synthesis (8530 $exoY$, 9034 $expG$, and 9030-2 $expA1$); 1021 and 9030-2 $expA1$ are known to be unable to form biofilms. Each strain was inserted into a microfluidic channel with the microspheres. As the cultures grew, the spheres' $D$ values were obtained every 24 hours for 4 days using fluorescence microscopy. Although the D values for 9030-2 $expA1$ were inconclusive, 8530 $expR^{+}$, 8530 $exoY$, and 9034 $expG$ showed significant decreases in $D$ between 3 days of growth ($|z| > 2.25$, $p < 0.025$). The data also indicated that 8530 $expR^{+}$ and 8530 $exoY$ grew at similar rates. There was no significant change in $D$ for 1021 ($\chi^{2}(2) = 5.76$, $p > 0.05$), which shows the lack of a structured biofilm community. Thus, $D$ can be used as an indicator of the presence of a biofilm and its development.