Bacteria and Viruses by Britannica Educational Publishing

By Britannica Educational Publishing

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Once connected, the two cells are brought into direct contact, and a conjugal bridge forms through which the DNA is transferred from the donor to the recipient. Many conjugative plasmids can be transferred between, and reproduce in, a large number of different Gram-negative bacterial species. Some chemolithotrophic bacteria (bacteria that derive energy from inorganic 29 7 Bacteria and Viruses 7 compounds) carry plasmids that are very large, ranging in size from 400,000 to 700,000 base pairs. Although plasmids are not essential for the bacterium, they may confer a selective advantage.

Consequently, endospores do not possess metabolic activity until nutrients become available, at which time they are able to differentiate from spores into vegetative cells. Only one spore is formed inside each bacterial cell during sporulation. The formation of a spore begins with invagination of the cytoplasmic membrane around a copy of the bacterial chromosome, thus separating the contents of the smaller cell from the mother cell. The membrane of the mother cell engulfs the smaller cell within its cytoplasm, effectively providing two concentric unit membranes to protect the developing spore.

A particularly important nutrient of bacteria is iron, an abundant element in the Earth’s crust. Iron is a component of heme proteins, such as hemoglobin in red blood cells and cytochromes in electron transfer chains, as well as many other iron-containing proteins involved in electrontransfer reactions. Iron is needed for the growth of almost all organisms. In aerobic environments at neutral pH values, ferrous iron (iron in the +2 state) is oxidized to ferric iron (iron in the +3 state), which is virtually insoluble in water and unable to enter cells.

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