Bacteria are a type of single-celled microorganism. They can come in many different species and in several very different forms that have implications for how they live and how they interact with other organisms. One of these types is the pathogenic bacteria, which are harmful to other organisms. Another form is the free living bacteria, which survive independently of other organisms. Of course, some bacteria are not free living but not pathogenic either, depending upon other organisms but also being beneficial to those organisms, or at least neutral in their activities.
Pathogenic bacteria are responsible for a wide variety of illnesses, both in human beings and other animals. The proteobacteria, for example, are a phylum of bacteria filled with a variety of potentially deadly species of microorganisms. The species of this phylum are responsible for a variety of diseases that can affect human beings and other animals. One such disease is E. coli (caused by the bacterium Escherichia coli), for example, which can lead to urinary tract infections, diarrhoea, and potentially even meningitis in the young.
Another example of a disease caused by pathogenic proteobacteria is that of salmonella food poisoning. This is caused by the Salmonella typhi bacterium. But there are many more examples of these, in some cases deadly, pathogenic bacteria. The Listeria monocytogenes bacterium is responsible for listeria outbreaks, for example. Meanwhile, the Clostridium botulinim bacterium causes botulism, and the Legionella pneumophila bacterium causes Legionnaire’s disease.
Another form of bacteria is the free living bacteria. These are the bacteria that live an existence independent of other organisms. The free living bacteria, which make up the large majority of bacterial species, can exist in basically any habitat on the planet. This could be anything from the fairly modest surroundings of the soil, where they exist in abundance, to the extreme conditions found in hot springs and even in radioactive waste, for example.
It is worth mentioning a third possibility to the pathogenic and free living bacteria. That is the so-called good bacteria. These are bacteria that are not free living and that do interact with or depend upon higher organisms such as human beings but which do not cause disease in those higher organisms. Indeed, in some cases, bacteria can be beneficial to the organisms that they interact with. The thousands of bacterial species found in the human gut provide a variety of benefits such as improved digestion and immunity, for example.