Final answer:
The presence of a capsid is a characteristic of a virus but not of bacteria. Capsids are protein coatings that surround a virus's genetic material, which is a distinctive feature that is not found in bacteria.
Step-by-step explanation:
The characteristic that distinguishes viruses from bacteria is the presence of a capsid. A capsid is the protein coat that surrounds the viral genome, and it is a distinct feature of viruses. Viruses, including bacteriophages, which are viruses that infect bacteria, all have capsids. In contrast, bacteria do not have capsids; they have cell walls and membranes that protect their cellular contents.
Bacteriophages are particularly interesting because they betray typical characteristics of viruses by infecting bacterial cells. Moreover, viruses have a disease-causing phenotype due to their intrusion and use of the host's cellular machinery to replicate. Despite their ability to cause disease, however, this is not a feature exclusive to viruses, as bacteria can also be pathogenic.
Some animal viruses utilize the budding method of exiting a host cell, which allows them to take a portion of the host's plasma membrane during release, further differentiating them from bacteria.