For almost all bacteria, it is convenient to be very small, but a few highly specialist groups of bacteria have evolved to be orders of magnitude larger than ordinary bacteria. While some of these giant microbes are abnormally large in all dimensions, others are long and thin or consist of assemblages of multiple cells. These giant bacteria are spread across the domain Bacteria and have evolved multiple ways to combat diffusion constrains. Giant bacteria often thrive in nutrient and carbon‐rich environments and/or in steep redox gradients. Genomically, giant bacteria seem all to be polyploid, that is containing multiple copies of their genome. Giant bacteria have been described throughout the history of microbial research but only some of them can be traced in current taxonomy and are being actively studied. Therefore, it is likely there are other giant bacteria in nature, awaiting to be explored.
Key Concepts
Growth and activity of giant bacteria are expected to be constrained by a combination of diffusion of nutrients into the cells as well as intracellular trafficking of solutes by diffusion.
Giant bacteria can be divided into ‘truly’ giant, consisting of abnormally large cells on all dimensions, or ‘pseudo‐giant’ consisting of cells abnormally large on one axis or multicellular assemblages of bacteria.
‘Pseudo‐giant’ bacteria may not face the same diffusion limitations as ‘truly’ giant bacteria.
‘Truly’ giant bacteria likely ‘resolve’ diffusion constrains by one or more methods including, minimising cytoplasmic space by intracellular (pseudo)compartmentalisation, storage vacuoles, rapid swimming.
Giant bacteria often grow in carbon‐ and nutrient‐rich environments and/or in steep redox gradients, thus additionally minimising diffusion‐related constraints.
Current knowledge suggests that all giant bacteria contain multiple copies of their genomes being oligoploid (3–10 copies) or polyploid (10–1000s of copies).
Gigantism in bacteria occurs in multiple phylogenetic lineages covering both Gram‐negative and Gram‐positive taxa.
Early descriptions of giant bacteria suggest that the currently studied taxa do not encompass the entire diversity of abnormally large bacteria.
