3.3. Correlated superoxide radical levels and silver
nanoparticle levels of individual bacteria
Having evidenced the differing SR levels of bacteria using fluorescence
microscopy and varying silver nanoparticle concentration inside a
bacterium using back reflection imaging, we finally investigate the
possible correlation between the levels of two biologically redox active
products at the single bacterium level. Experimentally, the DHE
fluorescence probes were added to a bacterial mixture after the
incubation with Ag+ to create DHE-labelled Ag
particle-containing bacteria. In turn, bright field microscopy, back
reflection imaging and fluorescence microscopy were applied to the
immoblised bacteria in a same optical field. Consequently, the optical
intensity of respective images are compared for each single entity.
Surprisingly, as the bright field image in Figure 4a presents all the
adhered bacteria in the optical field, a small fraction
(<20%) of the bacteria exhibiting an exceptional back
reflection intensity (Figure 4b) are shown to be exactly complementary
to those exhibiting a high fluorescent intensity (Figure 4c).
Ultimately, the fluorescent intensity and the back reflection intensity
of a total number of 1626 bacteria were individually measured and
plotted in the correlation chart (Figure 4d). Overall, a distinct
negative correlation between the two kinds of optical signals is
evident. This strongly suggests that a bacterium which has a potent
ability to reduce silver ions tends to possess a low superoxide radical
level. This conclusion further implies that the two different kinds of
biological activities – superoxide radical reduction and silver ion
reduction – may share to some extent the same reducing system withinD.r. bacteria: in this case, the pool of antioxidant molecules in
the bacterial cytosol as discussed in Introduction. On the ground that
most single entity research focuses on structure-activity relationship,
our work suggests an interesting perspective of looking into the
relationship between different activities of a same microbe, which may
gain fundamental implications for chemical aspects of biology.