Although bacteria and viruses both are very small to be seen without a microscope, there are many differences between Bacteria and Viruses.
Bacteria are typically much larger than viruses and can be viewed under a light microscope. Viruses are about 1,000 times smaller than bacteria and are visible under an electron microscope.
Bacteria are single-celled organisms that reproduce asexually independently of other organisms.Viruses require the aid of a living cell in order to reproduce.
The most prominent point of difference between these two, is the fact that aerobic bacteria require oxygen to survive, while anaerobic bacteria don't.
This can be attributed to the fact that aerobic species have the ability to detoxify oxygen.
In contrast, anaerobic species lack the ability to sufficiently break down food molecules like their aerobic counterparts.
Anaerobic bacteria can even grow in places wherein oxygen is not available, it is not possible for aerobic species to do that. This implies that these species can also grow in parts of the human body wherein the amount of oxygen supplied is very low. For instance, there also exist some examples of bacteria which can grow in the gut, i.e., the alimentary canal between the stomach and anus.
In terms of respiration, aerobic bacteria use oxygen in the process of energy metabolism, while anaerobic bacteria don't, and thus, the former has an edge in terms of the amount of energy produced. When both these bacteria types are collected in a liquid medium, aerobic species come to the surface of the medium in order to take in as much oxygen as possible, while anaerobic species settle at the bottom in order to avoid it.
Other than these two, there exists yet another type of bacteria―facultative bacteria. These species, which carry out aerobic respiration in the presence of oxygen, also have the tendency to switch over to the process of fermentation in the absence of oxygen. In other words, facultative bacteria are capable of adapting to a range of conditions.