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.