This episode of This Week in Microbiology discusses two papers.
The first paper, from Nature, introduces Microbial Iron Oxide Respiration Coupled to Sulfide Oxidation (MISO). It challenges the long-held belief that the reaction between sulfide and iron oxide in anoxic environments is a slow, abiotic process. The research demonstrates that diverse microbes across 37 prokaryotic phyla actively mediate this connection, using extracellular electron transfer systems ("microbial nanowires") to "breathe" iron oxides. Experimental validation with Desulfurivibrio alkaliphilus shows this biological process is significantly faster than the chemical reaction, fundamentally altering our understanding of the global iron and sulfur cycles.
The second paper, from the Journal of Bacteriology, describes "swashing," a novel form of propulsion-independent surface migration in Salmonella and E. coli. Researchers unexpectedly found that mutants lacking flagellar filaments could still migrate on soft agar. The proposed mechanism is physicochemical: fermentation of sugars at the colony's edge creates an osmotic gradient by producing acidic byproducts. This gradient draws water from the agar, generating a fluid wave that propels the colony forward. This swashing is inhibited by surfactants and provides a new model for bacterial expansion driven by metabolic activity rather than dedicated motility structures.
Microbial Innovations: Rewriting Biogeochemical Cycles and Uncovering Novel Motility
4:50 A Foundational Discovery in Biogeochemistry: A paper in Nature reveals that the interaction between sulfide and iron oxide (rust) in anoxic environments is not a slow, purely chemical reaction as previously thought, but an active, microbe-mediated process. This discovery fundamentally rewrites models of global iron and sulfur cycling.
7:05 Microbial Iron Oxide Respiration (MISO): The process is termed MISO (Microbial Iron Oxide Respiration coupled to Sulfide Oxidation). Genomic analysis predicts this capability in 37 prokaryotic phyla. The mechanism was experimentally validated using the cultivated microbe Desulfurivibrio alkaliphilus.
10:49 Chemolithotrophy Explained: The microbes involved are chemolithotrophs, meaning they "eat rocks." They derive energy from inorganic sources (in this case, oxidizing sulfide) and use that energy to fix CO2, making them independent of organic carbon sources.
16:09 Mechanism: Microbial Nanowires: The bacteria use outer membrane multi-heme cytochromes, described as "microbial nanowires," to physically transfer electrons to solid, extracellular iron oxide, effectively "breathing" rust to oxidize toxic sulfide into harmless sulfate.
18:50 Faster Than Rocks: A key finding is that the biological MISO process significantly outpaces the abiotic chemical reaction at environmentally relevant concentrations, proving it is an ecologically important mechanism.
22:50 A New Form of Motility: Swashing: The second paper describes an unexpected observation in Salmonella and E. coli: mutants that cannot produce flagellar filaments, and are therefore non-propulsive, can still migrate across soft agar surfaces.
33:42 The Swashing Mechanism: Swashing is not driven by biological motors but by a physicochemical process. Bacteria at the colony edge ferment sugars, producing acidic byproducts. This creates a strong local osmotic gradient that pulls water out of the agar, forming a fluid wave that the colony then "surfs" to expand outwards.
34:57 Fermentation is Key: This motility is entirely dependent on fermentable sugars and is correlated with the acidification of the medium. Mutants unable to perform mixed-acid fermentation cannot swash. Unlike other forms of motility, swashing is inhibited by the addition of surfactants.
39:09 Visualizing the Wave: Optical profilometry of the colony confirms the physical model, revealing a fluid bulge approximately 60 Β΅m high at the leading edge of the migrating colony.
41:36 Physics, Not Just Biology: The discussion concludes that swashing may be an emergent property of bacterial metabolism under specific conditions, highlighting that some complex behaviors can be explained by fundamental physics and chemistry rather than a specifically evolved biological purpose.