Overview 

Bacteria are microorganisms which are single celled (ubiquitous), microscopic living in the environment  and prokaryotic.

A few examples of bacteria are – salmonella , E. coli, cyanobacteria, Streptococcus and Staphylococcus.

There are different types of bacteria present in the environment. Some are airborne while some live in water, soil, plants and on animals too. They can live in extreme temperatures and pressure.

Bacteria vary in shape and size where there  are spherical, oval, and rod shaped bacteria. The human body consists of more bacterial cells than human cells.¹ 

Many species of bacteria are harmless and beneficial, such as bacteria present in the gastrointestinal tract of the human body which are involved in breaking down the food for digestion and release vitamins.

Only a few (less than 1%) of bacterial species are harmful and cause infections in humans. This kind of bad bacteria which cause disease or infection are known as pathogenic bacteria.²

Importance of understanding causes and symptoms

Most bacterial infections if left untreated for a long period of time can lead to septicemia.

Septicemia is a condition in which infectious bacteria enters the bloodstream and leads to blood poisoning, a life threatening situation which calls for a medical emergency.

Septicemia is an extreme response to the infection by our body. The blood poisoning results in septic shock and tissue damage, organ failure and sometimes it is fatal.³

What causes bacterial infection?

Two main categories of bacteria

Gram positive bacteria and gram negative bacteria

Gram staining method is a method first introduced by Danish bacteriologist Hans Christian Gram in 1882 to identify organisms causing pneumonia. This staining method involves the use of crystal violet or methylene blue color.⁴

The cell wall of gram positive bacteria has a thick layer of peptidoglycan which retains the violet stain and appears purple under optical microscope.

Examples include - Actinomyces, Clostridium, Mycobacterium, Streptococci, Staphylococci, and Nocardia 

The cell wall of gram negative bacteria, however, has a thin layer of peptidoglycan which does not retain the violet stain and appears pinkish or red in color under optical microscope.

Examples include E. coli, Salmonella, Shigella, and other Enterobacteriaceae, Pseudomonas, Moraxella, Helicobacter, Stenotrophomonas, acetic acid bacteria, Legionella.

Pathogenic bacterias 

Staphylococcus aureus

• Scientific name: Staphylococcus aureus
• Domain: Bacteria
• Class: Bacilli
• Family: Staphylococcaceae
• Genus: Staphylococcus
• Order: Bacilates 
• Phylum: Bacillota

It is a gram positive, spherical shaped bacteria and is usually found on the skin or nose of an individual. It causes upper respiratory infection and skin infection.

Streptococcus pneumoniae

• Class: Bacilli 
• Genus: Streptococcus
• Domain: Bacteria
• Family: Streptococcaceae
• Order: Lactobacillales
• Phylum: Bacillota
• Species: S. pneumoniae 

It is a gram positive, spherical shaped bacteria and is found commonly in the upper respiratory tract, causing conditions such as meningitis and pneumonia.

Escherichia coli 

• Class: Gammaproteobacteria
• Genus: Escherichia
• Family: Enterobacteriaceae
• Order: Enterobacterals
• Phylum: Pseudomonadota
• Species: E.coli  

It is a gram negative, anaerobic and rod shaped bacteria. It is usually found in the lower intestine of human beings and causes gut infection, food poisoning and diarrhea through contaminated food.

Salmonella spp 

• Scientific name: Salmonella
• Class: Gammaproteobacteria
• Domain: Bacteria
• Family: Enterobacteriaceae
• Genus: Salmonella; Lignières, 1900
• Order: Enterobacterals  

It is a gram negative, rod shaped bacteria. There are two common types of salmonella species : salmonella enterica and salmonella bongori. They are found in human and animal intestines and cause acute diarrheal disease through contaminated food and water.

Clostridium difficile

• Lower classifications: Clostridium perfringens, clostridium difficile, clostridium tetani
• Class: Clostridia
• Scientific name: Clostridium
• Family: Lachnospiraceae
• Genus: Clostridium; Prazmowski 1880
• Order: Eubacteriales
• Phylum: Bacillota  

It is an anaerobic gram positive bacteria. It is found in the soils and intestines of humans and animals and it causes colon infection and tetanus.

How bacterial infection is transmitted?

Most of the bacteria are present on the body and are not harmful. The bacteria can enter the human body through contaminated air, water, soil or vectors. Once the infected bacteria enters the body they multiply rapidly causing illness. 

Direct contact 

Bacteria can enter the human body through cuts, direct contact via skin to skin (streptococcal impetigo) or mucous membrane to mucous membrane (gonorrhea) or faecal to oral transmission (salmonella) or transfusion of contaminated blood (syphilis).

Other ways that bacteria can be transmitted include: Unprotected sex from person to person , being bitten or scratched by animals and from mother to unborn child through placenta or vaginal birth.⁵ 

Through air (airborne)

Some bacteria are carried through air or transmitted through air for example tuberculosis.

Health care associated infections or hospital infections

The bacteria can enter the human body through surgery or intubation. 

Vehicular (vector borne)

Vectors such as mosquitoes, flies (shigella) , lice (epidemic typhus), and ticks (Lyme disease) carry the infected bacteria and transfer them to healthy individuals. 

Lifestyle 

Bacteria (such as Escherichia coli) can enter the human body by consuming contaminated food (campylobacter) and water or unpasteurized dairy products.⁶

Risk factors

• Compromised immune system - A healthy individual’s immune system fights against the infection whereas if the immune system is compromised then it cannot fight against the infections and becomes more susceptible to bacterial infections. 
• Age - Very young or elderly individuals may be more susceptible to bacterial infections due to developing or declining immune systems. 
• Chronic illness - Chronic health conditions such as diabetes, heart disease, lung disease and cancer patients are more susceptible to bacterial infections due to their immunocompromised health.
• Environmental factors and lifestyle choices - Traveling to places where bacterial infection cases are more prevalent can increase the chances of catching an infection. Consumption of contaminated food and water is also a key risk factor as well as not washing hands after gardening or after being in touch with soil. This is because soil is an inhabitant of certain bacteria. Not maintaining proper sanitation measures is another way of getting a bacterial infection.⁷ 

Symptoms 

The bacterial infection symptoms depend on the type of infection and the part of the body affected.⁸

Common symptoms of bacterial infection include:  

• Fever
• Chills 
• Malaise
• Pain or discomfort in the affected area 

Organ specific symptoms

Respiratory Tract 

If the respiratory tract is infected by pathogenic bacteria then the individual may experience symptoms like : 

• Persistent cough
• Chest pain with sputum
• Shortness of breath
• Discomfort in chest area

Gastrointestinal Tract

If gastrointestinal infection occurs, the symptoms are: 

• Diarrhea
• Vomiting
• Nausea
• Abdomen pain

Urinary Tract

Urinary tract infection presents symptoms such as:  

• Burning sensation while urinating
• Frequent urination
• Painful intercourse
• Thick or cloudy urine, sometimes with blood

Skin and soft tissue

Skin and soft tissue infection shows symptoms like:

• Ulcers
• Blister formation
• Painful and swollen skin
• Redness of the affected skin
• Itching of the affected area

Central Nervous System 

Symptoms of infection in the central nervous system are:  

• Headache
• Sensitivity to light
• Confusion
• Stiffness of the neck

Summary

Bacterial infections are infections caused by pathogenic bacteria. Many bacteria live in the environment and on/in the human body which are not harmful. Such beneficial bacteria aid in digestion of the food. On the other hand pathogenic bacteria can enter the human body through various ways like skin cut, nose, etc and multiply quickly causing illness. Some of the common bacterial infections are tuberculosis, urinary tract infection, pertussis, ear infection, tetanus, pneumonia and sinus infection.

If the bacterial infection is left untreated for a long period of time then it leads to septicemia, blood poisoning resulting in a life threatening condition. The symptoms should be treated and monitored later.