GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

6. MICROBIAL GROWTH

6.4. PHYSIOLOGY OF GROWTH

6.4.2. Bacterial Reproduction

Most commonly, Bacteria reproduce by binary fission, where a single Cell divides into two, each of which subsequently divides again. The division process is always preceded by the duplication (Replication) of DNA. There are two types of division: by constriction and by means of a transverse septum.

Division by constriction involves the narrowing of The Cell at the site of division, with all layers of the cell envelope participating in the process. The invagination of the cell envelope from both sides inward progressively narrows the cell and ultimately divides it into two (Fig. 6.3). Constriction is characteristic of Gram-negative bacteria.

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Fig. 6.3. Cell Division of Listeria monocytogenes by constriction

Division with The formation of a transverse septum is typical of Gram-positive bacteria (Fig. 6.4). However, some groups of bacteria exhibit alternative modes of division (thiobacteria, mycobacteria, and arthrobacteria).

Fig. 6.4. Cell division of Staphylococcus aureus via a transverse septum;

cm — cytoplasmic membrane; cw — Cell wall; ts — transverse septum

The period from one division of a bacterial cell to the next is termed the Cell Cycle (bacterial ontogeny). Several types of vegetative cell cycles are distinguished in bacteria: monomorphic, where only a single morphological cell type is produced (bacilli, Escherichia coli); dimorphic, giving rise to two cell types (bacteria of the genus Caulobacter); and polymorphic, resulting in several morphologically distinct cell types (actinomycetes, arthrobacteria).

Spherical bacteria may form not one, but multiple transverse septa (Fig. 6.5). If a single septum forms, the cell divides in one plane, producing micrococci, diplococci, and streptococci. Two septa located in two mutually perpendicular planes result in the formation of tetrads upon division. When septa are formed in three mutually perpendicular planes, cell clusters resembling sarcinae are produced. During division by a transverse septum, the cytoplasmic membrane together with The cell wall grows inward, the invaginations approach each other, fuse, and the septum splits.

Bacteria can also reproduce by budding, which is a variant of binary fission. This mode of division is characteristic of bacteria belonging to the genera Hyphomicrobium, Rhodomicrobium, and Nitrobacter, which feature dimorphic and polymorphic cell cycles.

Bacteria are characterized by a high rate of reproduction. For instance, Escherichia coli divides every 20-30 min, meaning that within 24 hours, a single cell can produce 4.72 × 10¹⁰ Cells (2⁷², 72 generations). Assuming that one billion dry cells weigh 1 mg, 4.72 × 10¹⁹ cells would have a mass of 4720 tons under conditions where cell death is excluded.

Fig. 6.5. Diagram of transverse septa arrangement in a Myxococcus species cell

s — septum; cm — cytoplasmic membrane; cw — cell wall