Visible only under an Electron Microscope.

Viruses typically have spherical (polyhedral), rod-shaped, or helically shaped capsids while some viruses, such as bacteriophages, have complex shapes.

Intracellular organisms (they infiltrate the host cell and live inside the cell).

Mostly intercellular organisms (i.e. they live in-between cells); some intracellular.

Genetic material within a capsid, some have an envelope membrane.

Some are enveloped, but no membrane function.

Cell membranes present. No sterol except in Mycoplasma cells which have cholesterol.

1 nucleocapsid except in segmented or diploid viruses

DNA or RNA is enclosed inside a coat of protein.

DNA and RNA floating freely in the cytoplasm.

Some have poly-cistronic mRNA and post-translational cleavage.

It invades a host cell and takes over the cell causing it to make copies of the viral DNA/RNA. Destroys the host cell releasing new viruses.

Able to reproduce by itself.

The need for host cell

Besides viruses, two other acellular forms exist Viroids and Prions.

In some spore-forming bacteria, sporulating forms can be seen.

A viral infection may or may not cause a fever.

Most viral illnesses last 2 to 10 days.

A bacterial illness commonly will last longer than 10 days.

The virus does not respond to antibiotics.

Most bacteria are susceptible to antibiotics.

27

Beneficial use

Some bacteria are beneficial (as normal flora, probiotics, fermenters, etc.)