Abstract
Bacteria have been traditionally classified in terms of size and shape and are best known for their very small size. Escherichia coli cells in particular are small rods, each 1–2 μ. However, the size varies with the medium, and faster growing cells are larger because they must have more ribosomes to make more protoplasm per unit time, and ribosomes take up space. Indeed, Maaløe’s experiments on how E. coli establishes its size began with shifts between rich and poor media. Recently much larger bacteria have been described, including Epulopiscium fishelsoni at 700 μm and Thiomargarita namibiensis at 750 μm. These are not only much longer than E. coli cells but also much wider, necessitating considerable intracellular organization. Epulopiscium cells for instance, at 80 μm wide, enclose a large enough volume of cytoplasm to present it with major transport problems. This review surveys E. coli cells much longer than those which grow in nature and in usual lab cultures. These include cells mutated in a single gene (metK) which are 2–4 × longer than their non-mutated parent. This metK mutant stops dividing when slowly starved of S-adenosylmethionine but continues to elongate to 50 μm and more. FtsZ mutants have been routinely isolated as long cells which form during growth at 42°C. The SOS response is a well-characterized regulatory network that is activated in response to DNA damage and also results in cell elongation. Our champion elongated E. coli is a metK strain with a further, as yet unidentified mutation, which reaches 750 μm with no internal divisions and no increase in width.
Highlights
Escherichia coli has astonished investigators with its remarkable metabolic efficiency packed into such a small size
Involvement of Methionine We have found several cases in which production of long cells in E. coli is associated with alterations in methionine and S-adenosylmethionine (SAM) metabolism
Escherichia coli normally exists as a small rod, but we have discussed many conditions that cause it to elongate when cell division is inhibited
Summary
Escherichia coli has astonished investigators with its remarkable metabolic efficiency packed into such a small size. In its 0.5–2 μ length, it packs its genetic material, its metabolic machinery, and an impressive variety of adaptive strategies It can make a new cell as fast as every 30 min with scarcely an error. In order to produce a new cell, E. coli must approximately double its cell contents and distribute them between 2 daughter cells It must exactly duplicate and segregate its DNA, and it must double its length and divide itself at midcell. It becomes longer using a cell wall synthesizing system based on penicillin binding protein 2 (PBP2) to elongate. The direction of cell wall synthesis changes when the length has doubled, uses a different enzyme system based on PBP3, and coincides with synthesis of a septum at midcell. This system, known as binary fission, is an alternation between elongation via a PBP2 complex and division via a PBP3 complex (Lutkenhaus et al, 2012)
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