staphylococci) or (c) one parallel plane (e.g. Neisseria or pediococci), (b) three perpendicular planes (e.g. These can be observed as isolated cells, diplococci or small chains, depending on the degree of cell separation.īacterial division over successive division cycles in (a) two perpendicular planes (e.g. 1a and b) (2) organisms whose cells are elongated ellipsoids, such as enterococci, streptococci or lactococci (and some other genera such as Lactovum, Leuconostoc, Weissella and Oenococcus, Melissococcus, Vagococcus), which divide in successive parallel planes, perpendicular to their long axis ( Fig. what determines the shape of a coccus cell?īefore answering this question, it is useful to distinguish between two classes of cocci: (1) organisms with truly round cells such as pediococci, micrococci, deinococci, staphylococci or Neisseria (except Neisseria elongata), which usually divide in either two or three alternating perpendicular planes during consecutive division cycles, leading to arrangements in tetrads or in three-dimensional cuboidal packets of eight cells, respectively ( Fig. This review will address the question of the origin of cocci, not from an evolutionary point of view, but from a morphogenetic point of view, i.e. Genetically, it is also easy to convert a rod into a coccus, for example by loss ( rodA, pbpA) or overexpression ( bolA) of a gene ( Aldea et al., 1988 Murray et al., 1997 Henriques et al., 1998), but there is no report of genetic alterations that convert coccal cells into stable rod-shaped cells. The stability of coccal shape in evolution can be due to the inherent difficulty to regain genes for rod morphology, and/or to the absence of selective pressure for rod shape. This indicates that coccal morphology, which appears to have evolved multiple times during bacterial history, is evolutionarily irreversible ( Siefert & Fox, 1998). Interestingly, once a particular lineage exhibits coccus morphology, clusters that result from that lineage become homogeneous for coccus morphology. Yet, in the current era of molecular phylogenetics, comparative analysis of small subunit RNA sequences indicates that bacteria with different morphologies exist within single branches of phylogenetic trees and species with coccus morphology are present in clusters with a predominantly rod morphology ( Siefert & Fox, 1998). Historically, determining the morphology of bacterial cells has been an important phylogenetic tool. However, much can be learned from comparative studies of morphologically diverse bacteria. Caulobacter crescentus, a bacterium that undergoes a developmental cycle, has also emerged as a powerful model system to investigate morphogenesis ( England & Gober, 2001 Briegel et al., 2006). Most studies of cell division and morphogenesis have been centered on the two rod-shaped laboratory workhorses: Escherichia coli and Bacillus subtilis, due mainly to the wide array of genetic tools available to probe the life and death of these organisms ( Errington et al., 2003 Goehring & Beckwith, 2005). The present review aims to integrate older ultra-structural data with recent localization studies, in order to clarify the relation between the mechanisms of cell wall synthesis and the determination of cell shape in various cocci.īacterial division, penicillin-binding proteins, peptidoglycan, murein Introduction While only one peptidoglycan biosynthesis machinery seems to exist in staphylococci, two of these machineries are proposed to function in ovoid-shaped bacteria, reinforcing the intrinsic differences regarding the morphogenesis of different classes of cocci. Interestingly, there seems to be a correlation between the shape of an organism and its set of penicillin-binding proteins – the enzymes that assemble the peptidoglycan, the main constituent of the cell wall. While staphylococci or Neisseria cells, for example, are truly round-shaped, streptococci, lactococci or enterococci have an ovoid shape. Even among genera with the suffix ‘cocci’, which are the focus of this review, different shapes exist. The shape of bacteria is determined by their cell wall and can be very diverse.
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