Pathogenic bacteria and infectious diseases

What Are Infectious Diseases?

Infectious diseases (also known as transmissible diseases or communicable diseases) are clinically evident illnesses caused by microorganisms, such as bacteria, viruses, fungi, or parasites, that can be spread - directly or indirectly - from one vector to another. Some infectious diseases are caused by person-to- person contact, others are passed via animal or insect bite, and still others are caused by ingestion of infected, contaminated food, water, or other environmental exposures. Many of the more well known infectious diseases are caused by bacteria.

What Are Bacteria?

Bacteria (singular: bacterium) are a large domain of prokaryotic microorganisms, generally a few micrometers in length. Bacteria have many shapes, which range from rods to spirals to spheres, and were among the first lifeforms to appear on earth. Bacteria live in almost every habitat on the planet, in soil, hot springs, water, deep within the Earth, in radioactive wastes, in addition to the living bodies of humans, plants, and animals. Bacteria, which often are given a bad reputation thanks to the pathogenic bacteria that cause infections, play a vial role in recycling nutrients in the environment. In humans, it has been found that there are about ten times as many bacterial cells inside the human body as there are human cells, with large numbers of bacteria lining the gastrointestinal tract and living on the skin. Most of these bacteria are rendered harmless by the immune system of the body, and others are beneficial to the body. However, it's important to note that there are pathogenic species of bacteria that can lead to infectious diseases.

What Are Bacterial Infectious Diseases?

While most species of bacteria are harmless, and others beneficial for us, there are a number of disease-causing bacteria, which are called "pathogenic bacteria." Pathogenic bacteria can contribute to many worldwide diseases, including tuberculosis, cholera, anthrax, leprosy, the bubonic plague, pneumonia, and food-borne illnesses. The most common fatal bacterial infectious diseases are respiratory infections, with tuberculosis (caused by the bacteria Mycobacterium tuberculosis) killing approximately two million people each year, generally in sub-Saharan Africa. Bacterial infections can be caused by a wide range of bacteria which can lead to mild illness to life-threatening illnesses (like bacterial meningitis), which require immediate hospital interventions. Infections caused by bacteria are one of the leading causes of death in both the very young and the very old. Under normal circumstances, people are protected against bacterial infections by a healthy immune system.

Mycobacterium tuberculosis the causative agent of tuberculosis

Neisseria meningitidis the causative agent of bacterial meningitis

Vibrio cholerae the causative agent of cholera

Yersinia pestis the causative agent of the bubonic plague

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?

Mode of Infection

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.

Staphylococcus aureus the causative agent of nosocomial infections

Klebsiella pneumoniae the causative agent of nosocomial infections

Neisseria gonorrhoeae the causative agent of gonorrhea

Corynebacterium diphtheriae the causative agent 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.

Streptococcus pneumoniae the causative agent of pneumonia and meningitis

Haemophilus pneumoniae the causative agent of pneumonia and meningitis

Mycobacterium tuberculosis the causative agent of tuberculosis

Moraxella catarrhalis the causative agent of pneumonia

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

Vibrio cholerae the causative agent of cholera

Salmonella enterica the causative agent of salmonellosis; typhoid fever

Shigella the causative agent of shigellosis

Campylobacter jejuni the causative agent of gastroenteritis

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.

Clostridium tetani the causative agent of tetanus

Clostridium perfringens the causative agent of gas gangrene

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.

Treponema pallidum the causative agent of syphilis

Factors that Lead to Bacterial Infection in Humans

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 also 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.

Pili on E.coli cells

Bacterial biofilm formed by S.aureus

SEM of S.aureus biofilm

SEM of P.aeruginosa biofilm

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 usually 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 botulinum toxin. They are generally produced by Gram-positive bacteria and a few Gram-negative bacteria like E.coli , Vibrio cholerae, Shigella dysenteriae 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.