1. Introduction
The survival of the polyextremophile Deinococcus radiodurans(D.r. ) under a variety of extreme conditions raises the question
of the underlying mechanism. Unravelling the mechanism of this
particular microbe is expected to greatly improve the understanding of
the principles of life sustainability in general. The current mainstream
view is that the fact is likely based on a single polyvalent, rather
than multiple parallel (each for a specific condition), strategy
generating various resistances.1 The key of the
strategy lies in the presence of a strong proteome protection system
that consists of an antioxidant fraction of the D.r. cytosol with
a small molecular weight of less than 3 kDa,2, 3 and
otherwise protein carbonylation can easily occur and cause cell deathvia irreversible and progressive degeneration of most vital
cellular functions.4, 5 Such a constitutive protection
is thus considered to possibly cope with many different sources causing
oxidative damages including reactive oxygen species,2heavy metal ions6-9 and ionising
radiation10-12. While the identity of the antioxidant
system is largely revealed, direct experimental evidence of the
bacterial multifunction and inter-function relationship is needed to
testify or consolidate the inferred regime for the unusual resilience of
the creature.
The functionality of a bacterium is, however, of a complex origin. Even
within a bacterial culture of a same genome, phenotypic heterogeneity
would occur and contribute the functional diversity to microbial
populations.13, 14 Statistical fluctuation, for
example, leads to a random generation of subpopulations with distinct
gene expressions.15-17 Alternatively, heterogeneity
can possibly be driven by environmental changes18 and
results in a small subpopulation with phenotypic differences from the
rest of the culture.19-21 Quorum sensing of pathogenic
bacteria can yield such type of heterogeneity to protect themselves
against host attack.22, 23 These discoveries imply
that the major body of current research on the biochemical properties of
bacteria, which is mainly based on the bulk analysis (e.g. ion
chromatography,6 UV/vis and infrared
spectroscopy7, 8, 24) of the extracts of intra- or
extracellular contents, can hardly resolve the function profile of a
particular bacterium on a colony basis. Therefore, studying more than
one biological activities and their interrelation at the single
bacterium level is crucial to provide unambiguous fundamental insights
into the complex behaviour of a bacterial strain.
Given the desired spatial resolutions and the non-invasive nature of
optical microscopic techniques for studying single living cells under
various conditions,25-29 in this work we apply optical
microscopy under suitable imaging modes for different species as a major
means to visibly characterise two different biological activities of
superoxide radical (SR) generation and silver ion metabolism of
individual bacteria. Although metal ion metabolism can increase the
production of reactive oxygen species,30 the metabolic
pathways of the silver ion reduction and superoxide radical generation
have not shown any relation in the literature. We discover that these
two seemingly independent capacities of the bacteria are clearly
correlated and this occurs only within a small subgroup
(<20%) of a clonal population. Note that we clarify here as
the D.r. bacteria predominantly appear as four linked bodies
while the latter is distinctly separate from all others, the term
‘individual bacteria’ used in this work actually means the entity of the
tetracocci form.