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A theoretical methodology has been developed for studying the growth kinetics of bacterial cells. It utilizes the steady-state cell length distribution in a bacterial population to predict the dependency of growth and division rates on cell length and age. The mathematical model has been applied to the analysis of two bacterial populations, a wild-type strain of Bacillus subtilis, and a minicell-producing strain that carries the divIVB1 mutation. The results show that our model describes the wild-type population very well and that the assumptions typically used in traditional methods are unrealistic. In the case of the minicell-producing mutant we find evidence that the rate of cell division must be a function not only of cell size but also of cell age., (Copyright 2000 Academic Press.)

A new mechanism to segregate daughter genomes in bacterial cells is suggested that is based upon the rules of geometry governing the helix clock (Mendelson, 1982a). The reorientation of cell surface string arrays used as a timing reference in the helix clock is capable of drawing apart the initial products of DNA replication. Physically linking the sister DNA replication origins to the ends of the initial cell surface string inserted into the cell surface at the start of a helix clock cycle, and linking the DNA terminus to a point along the length of the same string provides a means to mark the locations to which the genomes will segregate as well as the place where cell division will occur. The parallel packing of additional cell surface strings into an array which includes the string to which DNA is attached provides the necessary spatial rearrangements. The helical segregation model can account for the precise registration of cell divisions with the completion of replication forks in a multifork replication system, provides a basis for determining the relationship of sister cell sizes at division, and can also accommodate the asymmetrical divisions associated with minicell production and sporulation. Examination of the helical segregation theory under multifork DNA replication conditions moreover reveals that adjacent helical clocks are physically linked to one another although totally independent in terms of their progression through the clock cycle. A relationship between the initiation of DNA replication forks and the insertion of the first cell surface string associated with the start of a helix clock cycle is predicted by the model.

Bacillus subtilis, normally a rod-shaped organism, can grow in the form of a helix with pitch ranging over a spectrum from tight right-handed to tight left-handed depending upon the growth environment and genetic composition of the strain. Five factors have been identified which contribute either to the helical shape deformation or its maintenance: 1) a biomechanical component involving blocked rotation during growth; 2) cell wall polymer conformation; 3) a protein(s) concerned with the left-hand form produced at high temperature; 4) electrostatic aspects of the cell wall; and 5) water, as it affects the mechanical properties of cell walls and the structure of cell wall polymers. The findings are compatible with a model in which the cell wall polymers are inserted in helical orientation along the cylindrical portion of the cell during growth.

Experiments are described in which the tensile strength, the extensibility and the initial Young's modulus of bacterial cell wall have been determined as functions of relative humidity in the range 11-98%. Data on stress relaxation and recovery are also given. Standard fibre-measuring technique has been used on 'bacterial thread', made from a cell-separation-suppressed mutant of Bacillus subtilis. The data show that peptidoglycan, the load bearing polymer in the cell wall, behaves very much like other viscoelastic polymers. Its mechanical behaviour when dry is that of a glassy polymer with tensile strength about 300 MPa and modulus about 20 GPa. When wet, it is weaker and much less stiff with tensile strength about 3 M Pa and modulus 10 M Pa. The relaxation data indicate a wide spectrum of relaxation times. The results are discussed in terms of the structure of peptidoglycan and its orientation in the bacterial cell wall. The way in which mechanical behaviour depends strongly on humidity is compared with that of other biopolymers in terms of possible hydrogen-bond density and the ordering of water molecules. The possibility of a well-defined glass transition is briefly examined.