Microbes and Pathogens
Microbes are things like bacteria and viruses that are too small to see with the naked eye. They exist on every surface and in every environment on Earth. They’re in the air, in the water, in the soil — even in your body.
Most microbes can’t hurt you, and many of them are beneficial to the environment or your body. In fact, life on Earth couldn't even exist without microbes! But a few microbes, called pathogens, grab all the headlines because they’re the ones that cause diseases in humans.
Most microbes on Earth are beneficial to life and to humans. Microbes in the environment are nature’s recyclers. They break down the molecules in dead organisms and make them available again to living things. Humans harness the power of microbes for the production of fermented food like wine and cheese and for industrial applications like paper production.
The bacteria that normally live in and on your body are your normal microbiota, and they protect your health by making your body less vulnerable to pathogens and by producing vitamins that aid with digestion and blood clotting.
A small percentage of microbes cause infectious diseases in humans. Infectious diseases are simply those diseases caused by things that can spread, like bacteria and viruses. To cause disease, microbes must be able to enter and colonize your body, overcome your immune system’s defenses, and cause damage to your body.
Bacteria and viruses are examples microbes that can cause infectious disease. Bacteria are tiny, single-celled organisms that live in the environment all around you. The cells of bacteria have many things in common with your own cells, but they also have differences because bacterial cells are prokaryotic, while human cells are eukaryotic.
Viruses, on the other hand, aren't cells at all! Think of viruses as the pirates of the microbial world — they hijack host cells and take them over to get what they need to reproduce.
Viruses aren’t made of cells. They’re just tiny particles of genetic information protected by a protein coat.
Viruses are much smaller than bacteria — so small you can’t even see them with a light microscope. They don’t have cell walls, ribosomes for protein synthesis, or the ability to transfer energy to ATP.
Because they have so little of their own cellular components (such as ribosomes), viruses can’t reproduce unless they enter a host cell, which is why scientists call viruses obligate intracellular parasites. When viruses do find a host, they reprogram the cell with their own genetic information and convert the cell into a tiny factory that makes lots of copies of the virus.
The following figure shows an HIV virus attacking a human cell as an example of how viruses reproduce. HIV is a type of virus called a retrovirus, so its multiplication cycle is a little more complicated than that of many viruses. The figure shows all the steps of the HIV cycle, but we'll focus on just the steps shared by all viruses. Here are the essential steps the virus takes:
Attachment: The virus attaches its proteins to a cell’s receptor.
Think of this like inserting a key into a door. If your key doesn’t fit, you can’t get in. In #2 in the figure, you can see the HIV virus attaching to a protein called CD4 that’s found on the surface of certain human white blood cells.
Penetration: The virus inserts its nucleic acid into the cell, taking over the cell.
In #3 in the figure, you can see the viral genetic material of HIV entering the human cell. The viral nucleic acid reprograms the cell, turning it into a viral production factory.
Biosynthesis: Instead of doing its job for the body, the cell starts making viral nucleic acid and proteins.
The cell even uses its own molecules and energy reserves (ATP) to produce the viral parts. In #6 in the figure, you can see more viral genetic material being made by the cell.
Some viruses, like HIV, can also insert a copy of their genetic material into the host cell chromosomes (you can see this happening is #4 and #5 in the figure). Viruses that can do this become almost invisible to the immune system and are almost impossible to get rid of. And when the host cells reproduce, they make a copy of the viral genetic material, along with their own DNA, increasing the number of infected cells in the host.
Maturation: The viral components pull themselves together to form mature viruses.
In #7 in figure, you can see a viral particle in the process of maturing as it begins to exit the host cell.
Release: The host cells release viral particles to go wreak havoc in other cells in the host’s body.
The number of viruses that go on attack at this point can range from ten to tens of thousands, depending on the type of virus. In #10 in the figure, you can see completed viral particles exiting the human cell.
For questions 1–5, use the following terms to identify which type of pathogen best fits the characteristic in each question. Some questions may have more than one correct answer.
a. Bacterial cell
Has its own ribosomes.
Always requires a host cell in order to reproduce.
May insert its genetic material into the host chromosome.
Can cause infectious disease.
Could be killed by an antibiotic that targets cell walls.
For questions 6–10, use the following terms to label the basic steps in the viral life cycle in the blanks provided in the preceding figure.
The following are answers to the practice questions presented.
The answer is a. Bacterial cell.
The answer is b. Virus.
The answer is b. Virus.
The answers are a. Bacterial cell and b. Virus.
The answer is a. Bacterial cell.
Viruses aren’t cells, so they don’t have cellular structures like cell walls. Because antibiotics target cellular structures, they don’t work on viruses.
The answer is b. Attachment.
The answer is d. Penetration.
The answer is c. Biosynthesis.
The answer is a. Maturation.
The answer is e. Release.