Virus or bacteria?

Anna Rull Aixa
Researcher from the Infection and Immunity research group (INIM)
anna.rull(ELIMINAR)@urv.cat

Viruses and bacteria that cause infectious diseases are called pathogens. These pathogens usually enter the body through the mucous membranes and then spread throughout the body. To combat viruses, we have developed antiviral drugs and vaccines, whereas to treat bacterial infections, we use antibiotics. The improper and excessive use of antibiotics can lead to bacterial resistance, a phenomenon that jeopardises our ability to combat common bacterial infections.

A virus is a very simple and tiny organism composed mainly of a nucleic acid that contains genetic information and is protected by a protein coat and, in some cases, a fatty envelope that surrounds it when it is not inside a cell. They cannot reproduce independently so in order to survive and replicate they need to infect other organisms, be they animals, plants, fungi, bacteria or other even viruses. Once there, they force the cells that they infect to copy their genetic material and thus reproduce by coopting the host organism into producing thousands of additional viruses.

Viruses are classified according to their morphology, chemical composition and mode of replication; they are present in every ecosystem and spread in many different ways. It is very important to know how each type of virus spreads in order to prevent infections and epidemics. There are many infections caused by viruses, including the common cold, flu, laryngitis, herpes, chickenpox, Ebola, AIDS and COVID-19.

Bacteria are single-celled prokaryotic organisms that exhibit great diversity of shapes and sizes. Most are spherical (cocci) or rod-shaped (bacilli), but they can also be cylindrical (vibrio), corkscrew-shaped (spirilla) or spiral (spirochaetes). Bacteria often attach to surfaces and form dense aggregations called biofilms that can contain multiple bacterial species.

Biofilms are of medical importance because they are often present in chronic bacterial infections or in infections from implanted medical devices. The bacteria living in biofilms form complex structures (microcolonies), which make them more difficult to eliminate than any individual isolated bacterium.

These organisms are vital for ecosystems and are found in most habitats; in fact, some species can live in extreme conditions of temperature and pressure. Although most of the bacteria in the body cause no harm, they are responsible for diseases such as whooping cough, pneumonia, tonsillitis, otitis or food poisoning.