Bacteria are microscopic prokaryotes that are generally spherical, rod-shaped, or spiral. They have cytoplasm and DNA surrounded by a cell membrane and cell wall. While most bacteria do not cause disease, some are pathogens that can damage tissue or release toxins. Viruses are strands of DNA or RNA surrounded by protein that can only replicate inside a host cell. Unlike bacteria, viruses are not considered living and require a host cell to reproduce. Both bacteria and viruses can cause disease in humans, but bacteria can reproduce on their own while viruses rely on host cells.

1. BACTERIA AND VIRUSES

2. WHAT ARE BACTERIA?
 Bacteria are microscopic prokaryotes. A prokaryote is a
single cellular that does not have a nucleus. Bacteria live in
almost every organism.
 Bacteria are often shaped like a sphere, a rod, or a spiral.
 Bacteria are much smaller than a plant or animal cell. They
are generally only 1-5 micrometers wide.

3. STRUCTURE OF BACTERIA
 Bacteria typically has cytoplasm and DNA
surrounded by a cell membrane and a cell
wall. Most bacteria have the DNA that is in
one coiled, circular chromosome. The
cytoplasm contains ribosomes.
 Some bacteria have special structures. For
instance, a bacterium could have a capsule
around its cell wall that protect it from frying
out. Many bacteria have capsules with hair-
like structures called pili, which help the
bacteria stick to different surfaces.

4. PATHOGENS
 Only a few species of bacteria are considered pathogens,
or agents that cause disease. Pathogens normally live in
your body, but only cause illness when your immune system
is weakened.
 Bacteria can harm you and cause disease in one of two
ways. One way is to damage tissue. Other bacteria can
cause diseases by releasing toxins.

5. PASTEURIZATION
 Unless food has been treated or processed, it contains
bacteria. The bacteria begin to reproduce and start to
break down the food, causing it to spoil.
 Pasteurization is a process in which food is heated to a
temperature that kills most harmful bacteria.

6. WHAT IS A VIRUS?
 A virus is a strand of DNA or RNA surrounded by a layer of
protein that can infect and replicate in a host cell.
 Scientists do not consider viruses to be alive because they
do not have the characteristics of a living organism.
 Viruses can make copies of itself in a process called
replication, but it must rely on a living organism to do so.

7. VIRUSES AND ORGANISMS
 Viruses have no organelles so they are not able to do things
that are usually associated with living organisms. Therefore
they cannot replicate without using the cellular parts of
another organism. The living cell the virus infects is called
the host cell.

8. VIRAL REPLICATION
 A virus cannot infect every cell. A virus can only attach a
host cell with specific molecules.

9. DIFFERENCES BETWEEN VIRUSES AND
BACTERIA
Virus
• Requires a host cell to
reproduce
• Not a living organism
• Most viruses do cause disease
• Viruses are much smaller than
bacteria
Bacteria
• They can reproduce on their
own
• Bacteria are living single-
celled organisms
• Most bacteria do not cause
diseases
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