Genomic Mysteries of Giant Bacteria: Insights and Implications

Abstract

Bacteria and Archaea are traditionally regarded as organisms with a simple morphology constrained to a size of 2-3 micrometers. Nevertheless, the history of microbial research is rich in description of giant bacteria exceeding tens, and even hundreds of micrometers in length or diameter already from its early days, e.g. Beggiatoa spp, to the present, e.g. Ca. Thiomargarita magnifica. While some of these giants are still being studied, some were lost to science, with merely drawing and photomicrographs as evidence for their existence. The physiology and biogeochemical role of giant bacteria has been studied, with a large focus on those involved in the sulfur cycle. With the onset of the genomic era, no special emphasis has been given to this group, in an attempt to gain a novel, evolutionary and molecular, understanding of the phenomenon of bacterial gigantism. The few existing genomic studies reveal a mysterious world of hyper-polyploid bacteria with hundreds to hundreds-of-thousands of chromosomes which are in some cases identical and in others, extremely different. These studies on giant bacteria reveal novel organelles, cellular compartmentalization, and novel mechanisms to combat the accumulation of deleterious mutation in polyploid bacteria. In this perspective paper, we provide a brief overview of what is known on the genomics of giant bacteria and build on that to highlight a few burning questions that await to be addressed.

Bacterial heterozygosity promotes survival under multidrug selection

“…While most bacterial species experience transient polyploidy during exponential cell growth 1,2 , several species, including Buchnera aphidicola 3 , Deinococcus radiodurans 4 , and Vibrio cholerae 5 , contain multiple chromosome copies (polyploidy) as a permanent feature of their life-cycle 6,7 . Polyploidy is also observed in several extremophilic archaea 8 and so-called giant bacteria 9,10 whose macroscopic cells can contain thousands of chromosomes. In some cases, polyploid chromosomes are identical 3,11 , while in other species the chromosomes are heterogeneous 12 .…”

Discovery of giant bacteria: do we need to change the definition of microorganisms?

“…For example, this value drops from 6 for a spherical cell with a diameter of 1 um to 0.6 and 0.06 for a cell with a diameter of 10 or 100 um respectively. [5][6][7] This may affect the metabolic rate of the bacteria. How do these large bacteria solve this problem?…”