MICROBIOLOGY Study Guide - 2012

CHAPTER 3. BACTERIAL MORPHOLOGY

3.5. BACTERIAL REPRODUCTION

Most Bacteria reproduce by equal binary fission, resulting in The formation of two identical daughter Cells. In Gram-positive bacteria, Cell Division occurs via the synthesis of a transverse septum growing inward from the periphery to the center, while The Cell diameter remains unchanged. Gram-negative bacteria divide by constriction, leading to a sharp decrease in cell diameter at the constriction site. The Separation of the newly formed cells is driven by the action of a lytic enzyme on the Middle layer of the cell septum.

Cell division is preceded by the Replication of the bacterial chromosome. Chromosomal METABOLISM/36.html">DNA replication initiates at the ori site (from English origin). The bacterial chromosome attaches to the cytoplasmic membrane at the ori region. It is often observed that this DNA-cytoplasmic membrane contact is mediated by mesosomes. DNA replication is catalyzed by DNA polymerases. First, The Double Helix of the DNA molecule unwinds, forming a Replication fork (a branched chain). Then, a complementary strand is synthesized on each of the polynucleotide templates. The subsequent separation of the two daughter Chromosomes is facilitated by the localized synthesis of a cytoplasmic membrane region between them, leading to the formation of distinct nucleoids.

The replication of the bacterial chromosome triggers The Mechanism of cell division. A constriction or division septum forms within the cell (Fig. 11). Following the autolysis of the division septum, the cells separate. This autolysis may proceed unevenly, causing dividing cells to remain joined at an angle to each other in certain areas, which is characteristic of corynebacteria, for instance.

Class="center">

Fig. 11. Diagram of Chromosome replication and Bacterial cell division (Gusev, Mineeva, 2005):

a — cell with a partially replicated chromosome: 1 — DNA; 2 — attachment of the chromosome to the cytoplasmic membrane; 3 — cytoplasmic membrane; 4 — Cell wall; b — chromosome replication completed: 5 — localized Synthesis of the cytoplasmic membrane region; 6 — new cell wall material; c — onset of cell division via transverse septum formation

A variant of binary fission is budding, which is typical of bacteria of the genus Rhodomicrobium. The budding process in Rhodomicrobium strains resembles Yeast budding. In bacteria of the genus Hyphomicrobium, budding involves the formation of a very thin, filamentous outgrowth—a hypha—at the pole of an oval, non-motile mother cell. This hypha is about 0.3 µm thick and sometimes several times longer than the cell itself. A bud forms at the tip of the hypha, into which one of the replicated nucleoids migrates. The bud enlarges to become a daughter cell. The latter develops flagella and either detaches from the mother cell to swim away or remains attached, forming multicellular complexes. Once the budded cells begin to grow, their flagella are shed.