How Does Bacteria Cause Diseases

One of the common causes of illnesses in humans is germs. And when one says the term 'germs', they either mean bacteria, viruses, fungi, protozoa or helminths. Of these infectious agents, one of the most common cause of serious illnesses in humans is bacteria.

They are really, tiny microorganisms that are visible only with the aid of a microscope. These tiny microorganisms have a specific pathway that leads to development of an infection in an otherwise healthy person.

What Makes Bacteria Pathogenic?

Bacteria are known as pathogens as they are able to cause a disease in plants, insects, animals as well as humans. The ability of bacteria to cause disease is called pathogenicity. This pathogenicity is expressed in terms of virulence. This means the degree of pathogenicity a particular organism exhibits.

There are a few determinants of virulence that give bacteria its ability to cause a disease. These determinants of virulence can either be a genetic code, biochemical pathway or structural feature. Let us have a look at the factors that makes bacteria pathogenic:

☛ Fimbriae
The fimbriae, (also called pili) are hair-like structures present on the surface of the bacteria body. These fimbriae help the bacteria to attach themselves to certain sites within the body. This helps prevent the bacteria from being washed away by body fluids. Fimbriae are commonly seen in most of the enterobacteria, like E.coli.

These bacteria generally cause urinary tract infections. Thus, the pili prevents the bacteria from being washed away from the bladder through urine.

☛ Flagella
Flagella are long tail-like structures that help the bacteria swim or move. These flagella help the bacteria migrate to its site of infection and survive. Thus, flagella in this way helps increase bacterial pathogenicity.

☛ Toxins
Bacteria produces toxic compounds that cause harmful effects on the body. These compounds are nothing but toxins that cause vomiting and diarrhea. These toxins are very harmful and can even induce severe pain, high fever as well as lead to paralysis.

Most bacteria are completely harmless, if they fail to produce toxins. One of the classical examples where toxins are commonly seen is in the case of bacteria that cause food poisoning.

☛ Invasion
Some bacteria have the ability to invade the body cells and cause pathogenicity. Some bacteria destroy the cells as they feed on the cellular content. For example, Salmonella typhimurium has the ability to destroy intestinal cells and cause severe diarrhea.

One of the chronic diseases caused due to invasion is tuberculosis. The Mycobacterium tuberculosis invades the cells of the lungs and destroys the cells.

However, not all bacteria are able to cause disease in humans. Although they are pathogenic in nature, the body's natural defense prevents them from doing any harm. Thus, only if the bacteria is smart enough to hide, evade or fight back the immune reactions of the body, it will be able to cause disease.

Let us now have a look at the strategies undertaken by bacteria to cause diseases in humans.

How Does Bacterial Invasion Lead to an Infection?

Before we go into the details of how does bacteria cause diseases, it is important to understand the difference between 'infection' and 'infectious disease'. When an organism is able to lodge itself in or the tissue of the host and multiply, it is called an infection. However, not every infection leads to a disease.

The immune system acts quickly and curbs the development of disease. When a disease can be transferred from an infected person to a healthy individual, resulting in a disease, it is termed as an infectious disease. So, how does one get infected with an infectious agent?

In order to cause disease in humans, the bacteria first has to break the barriers of the skin and tissues. Only after it gets entry into the body, bacteria can cause disease. There are different modes of transmission of bacteria as follows:

☛ Contact
When an infection spreads through direct or indirect contact, it may lead to an infection. In order to differentiate between direct and indirect transmissions, the term contagious disease is used to specify a disease that is caused by direct contact. Infectious disease is a term that is generally used to specify diseases spread by other modes.

Thus, sexually transmitted diseases are examples of infections spread by direct contact. Infections spread by contact with infected objects like pencils, glasses, towels, toys, etc. are termed as indirect infections like in the case of diphtheria.

☛ Inhalation
Most of the respiratory infections are spread through inhalation of the infectious bacteria. These bacteria tend to be present in air in form of aerosols. They are released in the environment by sneezing, coughing, talking, spitting, etc. Most of the time these respiratory droplets dry off.

However, some bacteria are resistant to drying and can remain suspended in air for long period. Thus, when a healthy person inhales these droplets, it may lead to respiratory infection.

☛ Ingestion
Gastrointestinal infections are usually caused by ingestion of the pathogens or their toxins. Thus, giving rise to different illnesses like waterborne, food-borne and hand-borne illnesses. These pathogens enter the gastrointestinal tract through the mouth and in some cases through the nose or eyes.

Examples of diseases caused through ingestion include cholera, dysentery and food poisoning.

☛ Inoculation
When bacteria is inoculated into the subcutaneous body tissue, it may lead to infection. For example, a deep wound may give a chance to Clostridium tetani a chance to cause a tetanus infection. Similarly, gangrene causing bacteria may also cause cellular death and tissue decay.

☛ Congenital
Pathogens that are able to cross the placental barrier and infect the fetus in the uterus are called congenital infections. These infections can lead to congenital disorders in the baby.

We have already understood the terms pathogenicity and virulence. We have also understood the factors that give bacteria its virulence and the modes of transmission of a pathogenic bacteria.

☛ Adhesion
As we have seen earlier, fimbriae or pili helps the bacteria to attach itself to the site of infection. This is called adhesion. This does not occur by chance, but a specific reaction helps in adhesion.

The surface receptors on the epithelial cells and the adhesive structures (adhesins) on the bacterial surface are involved in this specific adhesion reaction. These adhesins are present on the fimbriae or fibrillae or pili.

They contain the virulence factors that makes the bacterial strain virulent. If these adhesins are lost, the bacteria becomes avirulent. Thus, people are immunized with specific adhesins. This gives the body a chance to build immunity against the infectious bacteria.

☛ Invasiveness
When bacteria is able to invade the host tissues, it can cause a generalized or localized infection. For example, a wound infections can lead to streptococcal septicemia that is a generalized infection. A Staphylococcus abscess is more of a localized infection.

☛ Toxigenicity
We have already seen, bacteria are able to produce toxins that lead to an infection. There are two types of toxins produced by bacteria, these include exotoxins and endotoxins. The exotoxins can diffuse in the surrounding medium and tend to be highly potent when present even in minute amounts.

These endotoxins are easily destroyed by heat. There are certain exotoxins known as the most poisonous substances in the world. For example, 1 million guinea pigs can be killed by just 1 mg of botullinum toxin.

They are generally produced by gram-positive bacteria and a few gram-negative bacteria like E.coli , Cholera vibrio, etc. Exotoxins show specific affinity towards certain tissues and every exotoxin has a different effect on the host.

The integral part of the gram-negative bacteria cell wall are endotoxins. They are polysaccharide-protein-lipid complexes, that are highly heat stable. The lipid A component imparts the toxicity to these endotoxins. They are released into the surrounding medium, only when the cell wall disintegrates.

These endotoxins are harmful only when present in large doses. They do not have specific pharmacological activities and have similar effects on host.

☛ Other Factors
Some bacteria contain bacteriophages that give the organism its virulence. For example, the diphtheria bacteria contains bacteriophage that has a gene for toxin production. There are bacteria that contain plasmids and they are responsible for the presence of surface antigens.

These plasmids give the bacteria multiple drug resistance and hence the infection becomes difficult to treat. Klebsiella pneumoniae and haemophilus influenzae are capsulated bacteria.

These bacterial cells are covered by a capsule that helps them avoid phagocytosis. There are bacteria that carry their antigens on the capsules to carry out lytic activity within the body cells.

Do All Bacteria Cause Diseases?

No, not all bacteria cause diseases. You will be surprised to know, less than 1% bacteria have the ability to cause diseases. There are a few beneficial bacteria that help plants, animals as well as humans in various ways. Many commensal bacteria are harmless organisms that live on or within our body, without ever causing any kind of disease or infection.

This is all about how bacteria cause diseases. As you can see, there are various modes of infection and different strategies of bacterial cells that lead to an infection. Remember, not all infections lead to disease. It is a very rare natural event, that may occur when the body's immune system fails to get rid of the infectious agents.