Downloaded from http://cshperspectives.cshlp.org/ on October 4, 2021 - Published by Cold Spring Harbor Laboratory Press Small but Mighty: Cell Size and Bacteria Petra Anne Levin1 and Esther R. Angert2 1Department of Biology, Washington University, St. Louis, Missouri 63130 2Department of Microbiology, Cornell University, Ithaca, New York 14853 Correspondence: [email protected]; [email protected] Our view of bacteria is overwhelmingly shaped by their diminutive nature. The most ancient of organisms, their very presence was not appreciated until the 17th century with the inven- tion of the microscope. Initially, viewed as “bags of enzymes,” recent advances in imaging, molecular phylogeny,and, most recently,genomics have revealed incredible diversity within this previously invisible realm of life. Here, we review the impact of size on bacterial evo- lution, physiology, and morphogenesis. umanity has always experienced the im- bolic support. Although less than 1% of bacteria Hpact of microorganisms, most obviously canbeculturedreadily inthelaboratory(Amann through their ability to cause devastating dis- et al. 1995), the biochemical versatility among ease. For the vast majority of human history, these tiny creatures exceeds that of the plants, we were unaware of their presence, much less animals, and fungi combined (Pace 1997). the fundamental microbial processes to which Anton van Leeuwenhoek’s illustrations in a we owe our existence: from the production of letter to the Royal Society of London in the late energy by our ancient bacterial endosymbionts 17th century provide one of the earliest records (the mitochondria) to the generation of oxygen of bacterial cell form (Dobell 1960). Viewed in our atmosphere. Despite their astounding through a single lens, Leeuwenhoek pioneered global abundance (1030 cells) and their sub- studies of the human microbiome, describing stantial contribution to the total biomass of motile bacilli, cocci, and spirochetes he found planet earth (Whitman et al. 1998; Kallmeyer in scrapings taken from between his teeth (and et al. 2012), our inability to see these tiny life the teeth of others). This triumph was made forms shrouded their nearly limitless diversity possible by incomparable curiosity, lens con- in mystery. It was not until the 17th century, struction, and exceptional lighting. The simple with the careful observations and reports of An- cellular structure and glassy nature of most un- ton van Leeuwenhoek, that we became aware of stained bacteria viewed with a light microscope this previously invisible world on and around generated little interest in bacterial cell biology us. Today, we know that there are more bacteria with the exception of objects of unusual con- living in our intestinal tract than stars in the trast, such as endospores described by Robert Milky Way galaxy (and that they far outnumber Koch and the wonderfully colorful and large all the people who have ever lived). We also cyanobacteria. The bacterial nature of the latter know now that we thrive because of their meta- was itself only appreciated late in the 20th cen- Editors: Rebecca Heald, Iswar K. Hariharan, and David B. Wake Additional Perspectives on Size Control in Biology: From Organelles to Organisms available at www.cshperspectives.org Copyright # 2015 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a019216 Cite this article as Cold Spring Harb Perspect Biol 2015;7:a019216 1 Downloaded from http://cshperspectives.cshlp.org/ on October 4, 2021 - Published by Cold Spring Harbor Laboratory Press P.A. Levin and E.R. Angert tury (Oren 2004). For the most part, bacteria or 0.22 mL). Thiomargarita is slightly larger were viewed as primitive “bags of enzymes” un- than a Drosophila eye (Schulz et al. 1999) and til the 1990s, when the complexity of bacterial big enough to be seen by the human eye (Fig. 1). subcellular structure and regulators of cell re- A closer look at Thiomargarita reveals a central- production finally began to emerge. Tools and ly located fluid-filled vacuole, which takes up reagents developed for eukaryotic cell biol- 98% of the cell volume and is a nitrate reser- ogy (e.g., stains for DNA, membranes, and fluo- voir used to fuel sulfide oxidation. Even when rescent protein tags), once applied to bacterial accounting for the intracellular vacuole, a large cells, revealed astonishing insights including the Thiomargarita cell has a tremendous biovolume specific and even dynamic localization patterns to support (4.4  106 fL). Epulopiscium spp., of proteins, and the accuracy of chromosome intestinal symbionts of certain marine surgeon- organization. fish, are the largest known heterotrophic bacte- ria. These cigar-shaped cells are up to 600 mm  80 mm with an active cytoplasmic volume of BACTERIAL SIZE RANGE 2  106 mm3 or 0.02 mL (Angert et al. 1993). Bacillus subtilis, Staphylococcus aureus, Escheri- Unlike Thiomargarita, Epulopiscium cells con- chia coli, and Caulobacter crescentus, the prima- tain no storage vacuoles or other inert inclu- ry models for bacterial cell biology, are more sions (Fig. 1). The difference in size between or less typical in size, with individual cell vol- Candidatus Actinomarina minuta and these gi- umes between 0.4–3 mm3 (or 0.4–3.0 femto- ants is equivalent to the difference between a liters; femtoliter or fL is equal to 10215 L). Free- mouse and the Empire State Building (E. coli living marine ultramicrobacteria, appropriately could be represented by a small skunk or a rabbit named Candidatus Actinomarina minuta, have on this scale). We refer the reader to Niklas an average cell volume 1% that of E. coli (2015), wherein he discusses how cell features (0.013 mm3, range 0.6  1022 to 2.4  and geometry factor into considerations of cell 1022 fL). At the other end of the spectrum, size. the marine sediment–dwelling Thiomargarita What limits bacterial cell size? The smallest namibiensis, the “Sulfur pearl of Namibia,” is cells need enough volume to accommodate ad- a spherical organism with a volume eight or- equate genetic resources to support the cell’s ders of magnitude more than that of E. coli lifestyle (Koch 1996). The cell must also contain (750 mm in diameter, volume 2.2  108 fL the basic machinery required to express those Figure 1. Giant bacteria. On the left is a chain of Thiomargarita namibiensis cells. In this bright-field image, sulfur granules can be seen in the cytoplasm. The panel on the right shows an exceptionally large Epulopiscium cell with two large internal offspring. Scale bars, 100 mm. 2 Cite this article as Cold Spring Harb Perspect Biol 2015;7:a019216 Downloaded from http://cshperspectives.cshlp.org/ on October 4, 2021 - Published by Cold Spring Harbor Laboratory Press Cell Size and Bacteria genes as well as housekeeping proteins and port becomes numbingly unreliable. A small biochemicals to maintain its metabolism and molecule like oxygen at room temperature cellular reproduction. Genomic and metabol- would typically take about an hour to diffuse ic streamlining is seen in obligate intracellular 1 mm (Schulz and Jorgensen 2001). Similarly, symbionts, pathogens, and organelles that have the acquisition of nutrients relies on diffusion given up metabolic capabilities because those and capture of molecules at the surface of a cell. needs are supplied by the host (McCutcheon Consequently, free-living cells tend to be small, and Moran 2012; Wernegreen 2012). The loss with a large surface area relative to their cyto- of genes for sensing environmental change and plasmic volume, so that capture of nutrients, at responding to those contingencies can allow for low concentrations, can support the cell’s met- substantial genome reduction but not always a abolic needs. Cell form and function have been corresponding reduction in cell size. subjected to these constraints. For comparison, The structure and function of all large cells E. coli has a surface-area-to-volume ratio of appear bounded by the limits of diffusion 3.7 mm2 to 1 mm3, whereas the largest Thio- (Schulz and Jorgensen 2001). Encounters with margarita cell has a surface-to-volume ratio of nutrients, elimination of waste, and the timely 8.2  1023 mm2 to 1 mm3. Clearly, these large movement of biomolecules within the cell to bacteria are bending the rules. support metabolic needs all impact the ability of a large bacterium to survive in its environ- How Epulopisicum Has Overcome ment. The compartmentalization of cellular the Issue of Diffusion functions, the motor protein-facilitated traf- ficking over a complex cytoskeletal network, Most large bacteria either maintain a high sur- the expansion of genomic resources, and the face-to-volume ratio by being long and thin acquisition of endosymbionts that became en- like Spirochaeta plicatilis (a 250-mm-long, ergy-generating organelles have all been credit- 0.75-mm-diameter corkscrew-shaped bacte- ed for the advancement of the size and com- rium) or adopt a morphology in which no plexity of eukaryotic cells (Angert 2012). part of the cytoplasm is much more than a mi- cron away from the external environment. Thi- omargarita is an example of the latter, maintain- THE PROBLEM OF DIFFUSION ing a thin layer of cytoplasm surrounding a large The identification of giant bacteria required a fluid-filled vacuole. Epulopiscium spp. are the reexamination of the long-held beliefs about exception to this rule. maximum bacterial cell size. For all cells, growth Although their elongated form undoubted- and reproduction is limited by the speed of ly helps increase their surface-area-to-volume chemical communication and the availability ratio (Koch 1996), the largest Epulopiscium has of nutrients to fuel metabolism. Diffusion is a ratio of 0.6 mm2 to 1 mm3, 1/6 that of E. coli. the random, three-dimensional movement of Despite this difference, Epulopiscium maintains a molecule. For movement of molecules inside a high metabolic rate.