The bacterium's way for safe enlargement and division.

The origin of the bacterial sacculus probably caused the origin of the domain Bacteria. Manufacture of this exoskeleton is complex because it involves the formation of polymerized peptidoglycan external to the cell membrane as a strong, stress-resistant sacculus. This process involves the formation of the disaccharide penta-muropeptide, its transport through the membrane, and its polymerization in the glycan and peptide directions to form an intact fabric covering the cell. Beyond that, the precursor must be inserted into the wall without risk of lysis and the wall must in some cases be safely turned over. These complex processes imply that in the time before the First Bacterium and in the time of the Last Universal Ancestor a set of diverse mechanisms arose and gave rise to variant mechanisms that functioned together to allow the formation of a strong covering for the bacterial cell. Our understanding of the main aspects of the development of life on this planet are becoming established by the findings of biochemistry, biophysics, and cell biology, including those of the study of DNA sequences of different organisms (72). One important conclusion is that life leading to the organisms that are alive today started only once (2, 3, 55, 56). The other is that only after extensive evolution to gain the major attributes of cellular processes did organisms differentiate and develop stable diversity (31). We can be quite certain that the domain of bacteria arose as one line branching off from the remainder of organisms, which in turn later branched into the archaea and the eucarya. While many mutations occurred, only the most successful would have persisted, and thus stable diversity was probably delayed until after the general aspects of cell biology had been developed; this was simply because a successful mutant continued to eliminate the weaker competitors (4), according to the competitive-exclusion principle of Gause. The hypothesis that diversity finally arose because cell metabolism became more effective and caused the internal concentrations of solutes to increase has been suggested. These increased concentrations caused osmotic stress to become so great that the rupture of the cell membrane occurred (for a more detailed exposition of this argument, see references 31, 33, and 36). As a critical advance to compensate for this situation, bacteria that had developed an external wall, called the sacculus or the exoskeleton, arose. To deal with this osmotic stress, shortly thereafter the successful members of the remainder of organisms developed other ways to cope with osmotic problems, primarily by developing an endoskeleton. Thus, according to this hypothesis stable diversity probably arose because multiple noncompeting solutions to the same problem were independently developed (36). The purpose of this review is to outline the technology and architecture of the cell wall that even the first successful bacterium must have developed. It is based on what we know about bacterial wall biology, i.e., the biochemistry, biophysics, and microbial physiology of the exoskeleton. So far, information for just a few bacteria is available. These are Enterococcus hirae, a gram-positive coccus, Bacillus subtilis, a gram-positive rod, and Escherichia coli, a gram-negative rod. Over the last 20 years, my research, speculation, and thinking have been aimed at trying to resolve how bacteria originated and to determine the physical and chemical bases of the bacterial wall, which is outside the cytoplasm and gives bacteria strength and shape. In this short review it is not possible to give a full explanation of the experiments and logic that gave rise to the concepts presented here. Moreover, much of the early work and ideas of workers in the cell wall field have not been given adequate coverage but are covered in references 5, 7, 10, 11, 15, 16, 17, 20, 22, 25, 30, 32, 36, 39, 48, 52, 54, 63, 68, and 70.