There are several key differences between Bacteria and Viruses, the most obvious being physological. Bacteria as prokaryotes lack the membrane bound organelles and nucleus found in Eukaryotic cells. However like Eukaryotes Bacteria have a lipid based cell membrane that encloses their cytoplasm. Outside of this membrane is the bacterial cell wall, unlike plants and fungi walls, the bacterial cell wall is composed of peptidoglycan. There are two classes of Bacteria, they are defined by the composition of their respective cell walls. Gram Positive Bacteria possess a thick wall consisting of a large number of peptidoglycan layers and teichoic. Gram Negative Bacteria have a thin cell wall consisting of 1-2 layers of petidoglycan enclosed by another lipid membrane containing lipopolysaccharides and lipoproteins. The outermost layer of Bacteria is known as the capsule which houses protruding pili and flagellum which is responsible for the mobility of Bacteria. Bacterial genetic material is typically a single circular chromosome located in the cytoplasm in an irregularly shaped body called the nucleoid.
Viruses have a genome that usually consist of either single stranded DNA or RNA, however there are known viruses that have a double stranded genome. The nucleic acid of a virus is surronded by a protein coat known as a capsid, a virus in this form is known as a virion or complete virus particle. A virus can also have an outer lipid based shell called an envelope which is manafactured by the machinery of the host cell. Viruses are about 100 times smaller than bacteria. Most viruses which have been studied have a diameter between 10 and 300 nanometres. Some filoviruses have a total length of up to 1400 nm, however their diameters are only about 80 nm.
The most evident difference between the two is the life cycle which includes reproduction and metabolism. Viruses are not considered living organisms because they do not undergo cell division or replication unless in a host cell. There they hijack the transcription and metabolism machinery of the host cell in order to copy its genome. The life cycle of a virus can be understood in six stages; the first is attachment, where the free virion attaches to the surface of the host cell. Next is penetration where the virus enters the host cell via endocytosis, followed by uncoating in which the capsid is removed exposing the viral genome. Replication of the genome commences as the virus uses the transcription machinery of the host. The virus then enters the assembly phase where complete virion are produced, followed by the final stage of release where they virions break free of the host cell and are capable of affecting several more cells.
Bacterial cell growth and and reproduction by cell divison are intimately linked, Bacteria grows to a fixed size and then undergoes asexual reporduction via binary fission. The product of binary fission are two identical daughter cells; growth and reproduction are only possible if Bacteria is in its optimal growth conditions (i.e. aerobic bateria can only grown in oxygen rich environments). Bacteria when consistently growing and reproducing form colonies, these colonies are formed in 3 phases. The first is the lag phase, the period in which the bacteria is becoming accustomed to its environment and absorbing the necessary nutrients. The second is the exponential growth phase in which reproduction increases and the number of bacteria increase rapidly and dramtically. The last is the stationary phase in which growth levels off and numbers stabalize or begin to fall, this is due to the near complete consumption of the nutrients.