Investigation of microbial mineral respiration remains an
experimental challenge. In this issue of Journal of Bacteriology,
Rollefson et al. (11) present a foundational study on the functionality
of the biofilm matrix in Geobacter sulfurreducens, a
model dissimilatory metal respiring bacterium (DMRB). In
this study, the investigators identify an extracellular polysaccharide
scaffold or network that entraps redox-active proteins,
thus positioning these proteins for optimal electron transfer
from the membrane-bound respiratory supercomplexes to a
mineral phase electron acceptor. The distinguishing feature of
this study is the perspective, in that the team examined specifically
exopolysaccharide formation and how it enables entrapment
and tethering of redox proteins in the vicinity of the cell.
Previous studies on Geobacter (10) and Shewanella (4) have
focused primarily on the presence and functionality of conductive
pili and nanowires, proteinaceous structures that also enable
and enhance extracellular electron transfer. Rollefson et
al. remind investigators in this field that many microbial systems
have redundancy in essential functions, and in the case of
DMRB, it is clearly critical that more than one mechanism
exists to ensure vectoral electron transport to mineral phase
electron acceptors.