Bacteria are single-celled microorganisms that can come in a vast number of species and with a variety of lifestyles. Some of them are free living, not requiring other organisms for their survival, and existing in everything from the soil and fresh water to extreme environments such as deep ocean volcanic vents and radioactive waste, for example. Some bacteria can be beneficial to other organisms or have at least benign interactions with them. Some of them, however, are pathogenic. When they come into contact with another organism they harm it.
The key to understanding the characteristics of pathogenic bacteria is to consider the morphologies of the bacteria in question. What this refers is the shape properties of the bacterial cells that are provided to it by its genes through the structures that they generate such as the cell wall and the cytoskeleton. These structures can generate a variety of shapes that the bacteria can exist in such as the cocci, which are spherical, for example. The bacilli, meanwhile, are rod-shaped bacteria, which can also exist in shapes known as spirilla (which are spiral shaped) and spirochaetes (which are tight coils).
The importance of these different morphologies is that they determine the potential functions of the bacterium. They determine what behaviour the bacteria can exhibit such as what they can attach to, what nutrients they can take in, how they can move, and whether they can avoid predators and predate successfully themselves. In the case of the pathogenic bacteria the morphology determines the effects that the particular bacterial species can have on another organism such as destroying cells, taking nutrients, or impeding performance with toxins.
One example of a pathogenic bacterium is that of Bacillus anthracis. This is a bacterium of the rod-shaped bacillus type and is responsible for the various forms of anthrax infection such as cutaneous, pulmonary, and gastrointestinal, which can be spread by contact with infected animals. Transmission can be through inhalation or through abrasions on the skin of the organism being infected. There is an anthrax vaccine for prevention. Treatment could involve penicillin or doxycycline, for example.
A phylum known as the proteobacteria provide plenty of examples of pathogenic bacteria. Escherichia coli, for example, causes E. coli infection. Salmonella food poisoning, which is caused by the Salmonella typhi bacterium, is another example. But there are plenty of other examples of pathogenic bacteria, such as Listeria monocytogenes bacterium, which causes listeria, Clostridium botulinim, which causes botulism, and Legionella pneumophila, which causes Legionnaire's disease.