at least a billion years, Bacteria and Archaea ruled the Earth as the only existing organisms.
Even though life is much more diverse on Earth today, bacteria (singular: bacterium) are
still the most abundant organisms on Earth. You probably know bacteria as “germs” that
cause disease, but as you will see, they can also do many helpful things for the environment
and humankind.
Characteristics of Bacteria
Bacteria are so small that they can only be visualized with a microscope. When viewed
under the microscope, they have three distinct shapes. These three shapes allow bacteria to
be classified by their shape. Thebacilliare rod-shaped, thecocciare sphere-shaped, and
thespirilliare spiral-shaped (Figures8.1,8.2and8.3).
Figure 8.1: Bacteria come in many different shapes. Some of the most common shapes are
bacilli (rods), cocci (spheres), and spirilli (spirals). Bacteria can be identified and classified
by their shape. ( 7 )
Bacteria are surrounded by a cell wall consisting of peptidoglycan, a complex molecule
consisting of sugars and amino acids. The cell wall is important for protecting the bacteria.
In fact the cell wall is so important that some antibiotics, such as penicillin, work to kill
bacteria by preventing the proper synthesis of the cell wall. In parasitic bacteria, which
depend on a host organism for energy and nutrients, capsules or slime layers surround the
cell wall help defend against the host’s defenses.
Recall that all prokaryotes, including the bacteria, lack the membrane-bound organelles and
nucleus of eukaryotic cells (Figure8.4). Like eukaryotic cells, however, prokaryotic cells do
have cytoplasm, the fluid inside the cell; a plasma membrane, which acts as another barrier;
and ribosomes, where proteins are assembled. The DNA of bacteria is mostly contained in a
large circular strand, forming a single chromosome, that is compacted into a structure called
thenucleoid.Many bacteria also have additional small rings of DNA known asplasmids.
Some bacteria also have tail-like structures calledflagella(Figure8.5). The flagella assist
the bacteria with movement. As the flagella rotate, they spin the bacteria and propel them