Bacteria are a type of microscopic ‘bug’ of germ that can cause an infection. There are about 3 trillion bacteria that exist on or in the average human body without causing any disease. The gastrointestinal tract has the majority, followed by the skin surface and then the mouth (oral cavity). These bacteria do not cause an infection unless suitable conditions arise for it to invade tissue. Some bacteria are pathogenic and once it makes contact with the body, it rapidly causes an infection even in a person with an otherwise healthy immune system.

Types of Bacteria

Bacteria are known as prokaryotes because it has a cell membrane but its nucleus lacks a membrane. There are a number of terms used to categorize bacteria and this can sometimes be confusing. Firstly bacteria are single-celled organisms that may either need oxygen to survive (aerobic) or can sustain itself in the absence of oxygen (anerobic). Secondly,  most bacteria have a cell wall which may vary in thickness. Those bacteria with a thick cell wall are known as gram-positive bacteria because its wall can stain with certain dyes used to highlight the microbe for microscopic examination in a laboratory. Bacteria with thin walls are gram-negative. Lastly, bacteria come in various different shapes and this too can be used for classification. With regards to shape, rod-shaped bacteria are known as bacilli while spherical bacteria are known as cocci.

Below is a picture of E.coli bacilli which are found in large numbers in the gut. Picture from Wikimedia Commons.

Bacterial Infections

Some bacteria are pathogenic meaning that it can cause disease. This is because of certain genes that allow the bacteria to have this effect. These bacteria will then cause tissue damage by releasing toxins, adhering to cells or invading cells which are then destroyed. Some bacterial species may be able to do more than one of these actions thereby making it more virulent (harmful).

Pathogenic vs Non-Pathogenic Bacteria

The reason why some bacteria are pathogenic and others not is due to the genetic structure. Pathogenic bacteria have virulence genes which allow it to cause an infection. However, non-pathogenic bacteria which may lack these genes can be transformed when infected by viruses. These plasmids and bacteriophages are viruses that genetically ‘reprogram’ the bacteria and make it virulent. Apart from this virulence, bacteria can consume any living or dead matter and the body’s immune system keeps it at bay. If the immune system is severely compromised, even non-pathogenic bacteria that live on or in the human body will being to consume tissue that is the most prone.

Bacterial infections tend to progress rapidly once it arises and if the immune system cannot contain it, or medical treatment by the means of antibiotics are not commenced soon enough, the infection can spread. It directly infiltrate surrounding tissue, cause the formation of abnormal passages between two hollow organs known as a fistula and infect the new site, or spread to distant sites through the blood (bacteremia), lymph or other fluid and semi-solid media in the body.

Signs and Symptoms of a Bacterial Infections

The clinical features are largely associated with localized inflammation and systemic response to the invading pathogen. These signs and symptoms may include :

• Pain at the site of the infection
• Redness of the tissue
• Swelling of the affected area
• Warmth at the site
• Pus, oozing or accumulated in an abscess
• Fever