Pathogens: How Do They Affect the Human Body?

FROM THE LECTURE SERIES: An Introduction to Infectious Diseases

By Barry C. Fox, M.D., University of Wisconsin

The human body contains billions of bacteria which can be both beneficial and harmful. Some of the harmful bacteria, known as pathogens, are germs that can cause damage and disease via interaction with a host. However, there are some “good bacteria” in the human body that play an important role by aiding in the digestion process and boosting the immunity system.

3D illustration of bacteria.
The study of bacteria has given scientists insight into how microbes affect physiology, human development, immunity, and nutrition. (Image: nobeastsofierce/Shutterstock)

How Do Bodies First Attract Bacteria?

The cover of the National Geographic magazine’s 125th anniversary featured “Discovering Microbes Within Us” that highlighted scientists’ renewed interest in the bacteria known as the “microbiome”. The Human Microbiome Project, launched in 2007, gave scientists an insight into how microbes affect physiology, human development, immunity and nutrition.

Today, five microbiome sites are being studied—the nose, mouth, intestines, vagina, and skin. The beginning of its development is at birth, as infants go through the birth channel. They catch bacteria, lactobacilli, in the process of childbirth, through the vaginal canal. Over the next two or three months, the infants’ microbiomes diversify, while at the same time, their immune systems are maturing and learning not to attack the good bacteria.

Babies born by caesarian section start out with less diverse microbiomes, which may have an impact on the development of their immune system. It was found that infants born by C-section acquire organisms from the environment, predominantly skin bacteria. Specifically, C-section babies are missing the lactobacillus species of bacteria from the mother’s vaginal birth canal.

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Paradoxical Nature of Pathogens

Humans are often home to bacteria with pathogenic potential on our skin or mouth. But, unless the right circumstances occur that lead to an invasion, there is generally no consequence for the simple colonization of these bacterial pathogens. Instead, a person may serve as a carrier to transmit diseases to others.

For example, a deadly germ, Neisseria Meningitidis, is present in the back of 20 per cent of teenagers’ throats that causes a deadly infection of the brain. However, most teenagers never become ill with this bacterium. It just lives there, causing no harm.

The Case of Mary Mallon, or Typhoid Mary

Another example of the paradoxical nature of pathogens was Typhoid Mary. In the early 1900s, Mary Mallon was a cook for a banker’s family on Long Island. She had no symptoms of typhoid. First, one of the banker’s daughters became ill with fever, caused by a strain of the salmonella bacteria. Next, the wife and two maids became ill, then the gardener and another daughter.

An investigation traced the source of the outbreak back to Mary’s cooking responsibilities. Analysis of her stools confirmed that she carried the bacteria, which she transmitted during food preparation. Since there were no cures, she was placed in isolation on an island for years until she agreed never to work again as a cook.

Unfortunately, she broke her promise, and she did work as a cook again, under an assumed name in a maternity hospital in Manhattan. This caused an outbreak of 25 cases of typhoid fever and two deaths. She was subsequently imprisoned on the island for the rest of her life.

This is a transcript from the video series An Introduction to Infectious Diseases. Watch it now on The Great Courses Plus.

What Makes Bacteria Pathogenic?

Bacteria have the ability to create toxins, which are chemical poisons that interfere with cell function such as digesting normal human enzymes, evade infection-fighting white blood cells, and immune clearance. They can produce protective capsules or slime material, so they cannot be swallowed by white blood cells, or adapt DNA that can encode for antibiotic resistance.

Even good bacteria in the wrong place can cause infections. The microbes that inhabit the skin and mucous membranes such as the mouth are known as “normal flora” or “normal microbiota”. They play a part in the normal, healthy human physiology unless they become too populous, or they inhabit places in the body where they don’t belong, such as the bloodstream.

There are some anatomical sites within the body that are normally sterile. These include areas around the brain and spinal cord, or the lower portions of the lung. For instance, bacteria in the spinal fluid will cause bacterial meningitis. There are numerous entry points through which bacteria can infect these sterile parts of the body such as cuts in the skin, mucous membranes of the mouth, or bleeding intestines.

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How to Distinguish Among Bacteria?

One of the simplest ways to classify bacteria visually is on the basis of a specific stain known as the Gram stain. This method is named after the Danish bacteriologist, Hans Gram, who devised the technique in 1882. This is one of the most important staining techniques in microbiology and is usually the first test performed by the microbiology lab and infectious disease specialists in the identification of bacteria.

The Gram Stain Method

Gram staining is a method of differentiating bacterial species into two large groups: Gram-positive and Gram-negative. (Image: Schira/Shutterstock)

The Gram stain procedure distinguishes between Gram-positive or violet-colored and Gram-negative or red-colored bacteria.

It has three steps—staining with a violet dye, decolorizing with acetone or alcohol, and counter-staining with safranin, a red dye. Gram-positive bacteria stain purple due to the presence of a specific sugar layer in their cell membrane, which retains crystal violet stain and is not washed out by alcohol. Gram-negative bacteria, on the other hand, stain pink, because they have a thinner sugar wall, which allows the crystal violet to get washed out during the decolorization process with alcohol. Restraining with a safranin dye results in red color. Examples of Gram stain positive include diphtheria and tetanus, while pertussis and plague are Gram stain negative.

Common Questions about Pathogens

Q: What are pathogens?

Pathogens are germs that can cause damage and disease via their interaction with a host.

Q: Identify a characteristic that make bacteria pathogenic.

Pathogenic bacteria have the ability to create toxins, which are chemical poisons that interfere with cell function such as digest normal human enzymes, evade infection-fighting white blood cells, and immune clearance.

Q: What is the Gram stain?

The Gram stain is one of the simplest ways to classify bacteria visually on the basis of a specific stain.

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