Streptococcus Bacteria: Everything You Need to Know

Streptococcus is one of the most common bacterias in the world. Unfortunately, it can also spread easily and cause many conditions, some of which can be life-threatening.

Streptococcus Bacteria: Everything You Need to Know

Although strep throat is the most recognizable condition, streptococcus can also cause many more diseases and infections, especially in elderly patients, newborns, and those who are immunocompromised. 

There are several groups of streptococcus bacteria which we’ll explore more below. Each group is responsible for the formation of different conditions.

Sometimes though, streptococcus can live in the body and not cause any symptoms to its host.  So what exactly is streptococcus? 

In this article, we’ll walk you through everything you need to know about streptococcus. This includes its classifications, its infections, gram stain, observations, and more. 

What Is Streptococcus?

Streptococcus is a type of bacteria that’s gram-positive and belongs to the Streptococcaceae family.

Streptococcus bacteria are usually found in the respiratory tract or mouth membrane, where they can cause several diseases and infections, most notably pneumonia, pharyngitis, and sepsis. 

Certain species of Streptococcus can cause illnesses such as endocarditis, meningitis, pink eye, and necrotizing fasciitis, although most species are not pathogenic.

There are approximately 50 species of Streptococcus, but only five can cause diseases in humans. 

What Are Gram-Positive Bacteria?

Gram-positive bacteria are bacteria that show up as a positive result in a gram stain test. Examples of gram-positive bacteria include staphylococcus and streptococcus. Staphylococcus tends to grow in clusters, whereas streptococcus grows in chains.

Staphylococcus Vs. Streptococcus: What’s The Difference?

Staphylococcus and Streptococcus are both gram-positive cocci. They can be distinguished from each other in a catalase test because the staphylococcus bacteria have the capacity to produce catalase, an enzyme that can catalyze the reduction of hydrogen peroxide. 

One of the main ways that these bacteria differ is in terms of their pathogenesis. Most staphylococcal species are non-pathogens, whereas streptococcus can cause many diseases.

However, staphylococcus can still be responsible for conditions including skin diseases, impetigo, cellulitis, bacterial conjunctivitis, food poisoning, and more. 

Streptococcus tends to cause more severe diseases, which can include scarlet fever, toxic shock syndrome, necrotizing fasciitis, blood infections, meningitis, and pneumonia. 

To date, around 40 species of staphylococcus have been identified. In addition, 50 streptococcal species have also been identified. Staphylococci tend to be found on the skin, whereas streptococci are predominantly found in the respiratory tract. 

Where Did Streptococcus Come From? 

The first time a streptococcus infection was documented was in 1875 by Austrian surgeon Theodor Billroth, who discovered this organism in cases of wound infections and erysipelas. 

However, it wasn’t until 1879 that the first formal entry of streptococci made it into the history books. In 1879, Louis Pasteur decided to isolate the microorganism from the blood and uteruses of women who had been diagnosed with puerperal fever.

He discovered that the streptococcus bacteria was responsible for this disease which, at the time, caused the highest mortality rates among women and newborns. 

Streptococcus Infection 

Most Streptococcus bacteria live in the bodies of humans and animals. Group A streptococci are usually found in the throat and on the skin. Group A strep can also live in the nose and be spread easily to other people through sneezing, talking, and coughing. 

Like all bacteria, Streptococci need hosts to survive. Evidence has shown that Streptococcus thrives due to its relationship with humans and animals, making streptococci a widely distributed bacteria worldwide.

The streptococcus pathogen tends to colonize the nasopharynx (the top part of your throat) in an asymptomatic manner. However, it can move to organs and sterile tissues over time. 

Streptococci can either exist as commensals or as parasites. The term commensal refers to a relationship between different organisms that can act on the host’s immune system by giving the host essential nutrients.

In other words, if streptococcus is commensal, it can live within the host without causing any harm, but it will still derive essential nutrition from the host. If streptococcus is a parasite, it will harm the host by causing diseases. 

Streptococci can be divided into several groups, depending on the organisms they infect and the specificity of the host. These groups are: 

The Pyogenic Group

The pyogenic group of streptococci is a species that reside in the intestines. They’re usually found in homeothermic hosts. However, some species may only be found in specific hosts called host-specific bacteria. 

The Subthermophilic Group

Some species of Streptococcus can infect both animals and human beings such as Streptococcus bovis and Streptococcus equinus. These species belong to the subthermophilic group, and they also live in the intestines of their hosts. 

The Anaerobic Group

Unlike the intestinal dwellers above, Streptococcus from the anaerobic group can be found existing as part of the healthy flora in the skin, genital tract, and mouth.

Several species of streptococcus are responsible for the multiple variations in the anaerobic group, which can cause several diseases that range in severity. 

Alpha Hemolytic Streptococcus

This group of streptococcus is made up of gram-positive bacteria. These can generate in both short or long chains, and the gram-positive bacteria are non-motile. This group of streptococcus is anaerobic and is responsible for the oxidation of iron in our hemoglobin. 

Streptococcus in the group can live naturally in humans without causing any harm. However, some species may cause infections that can range in severity. Some species of streptococcus in this group that cause human infections are: 

Streptococcus pneumoniae 

Streptococcus pneumoniae is caused by the streptococcus bacteria and can spread through coughing or sneezing. This bacteria can cause infections in several parts of the body, including the lungs. 

Streptococcus pneumoniae is an infectious pathogen that’s carried by as many as 27 to 65 percent of children. However, less than 10 percent of adults are carriers of Streptococcus Pneumoniae.

It’s thought that infection rates are higher in children because close contact with infected respiratory droplets is needed to pass on the pathogen, and children tend to play close together. 

This pathogen usually exists commensally in humans. However, it’s somewhat of an opportunist. In some cases, this pathogen can enter the respiratory system and access the bloodstream – it’s from here that Streptococcus pneumoniae can cause infections in other parts of the body. 

When this bacteria is transmitted, several factors are responsible for the colonization of the bacteria. There are two important enzymes at play here, both of which can modify the peptidoglycan, stick to local cells and tissue, and allow the bacteria to colonize and persist, resulting in infection.

If the streptococcus pneumoniae bacteria are able to invade the tissue and enter the bloodstream, more severe infections, or infections in different parts of the body, may occur. 

Streptococcus Pneumoniae can cause meningitis if it reaches the membrane that protects the brain and the spinal cord. 

Streptococcus viridans

Streptococcus Viridans are a form of alpha-hemolytic streptococcus. Like streptococcus pneumoniae, Viridans can also exist as a regular part of our microbial flora, and it’s usually found in the oral cavity. 

Sometimes, streptococcus viridans can be found in other parts of the body, including the upper respiratory tract, the gastrointestinal tract, and the female genital tract. 

Although Viridans can exist commensally, they can occasionally invade the human tissue and cause a range of diseases. Immunocompromised humans are most at risk of these infectious and opportunistic pathogens. 

One of the most common ways for Viridans to enter the bloodstream is through trauma in the oral mucosa. If streptococcus viridans enter the bloodstream this way, it can cause bacteremia (the presence of bacteria in the bloodstream).

This provides the perfect opportunity for Viridans to make their way to the heart, which can cause serious conditions such as endocarditis. Streptococcus viridans may be responsible for between 45 to 80 percent of cases of native valve endocarditis. 

If Viridans makes its way to the heart, they can stick to the surface of the human heart valves. If it multiplies here, it can be responsible for the formation of abscesses, pericarditis, peripheral emboli, and macroscopic verrucous lesions. 

Some of the most common symptoms of a viridans streptococci infection are: 

  • Rashes
  • Chills
  • Fever
  • Toxic shock-like syndrome
  • General weakness 
  • Flushing pharyngitis 

Beta Hemolytic Streptococci 

Another group of the streptococci bacteria is beta-hemolytic streptococci. Members of this group are also gram-positive and tend to form chains and grow in pairs. Beta hemolytic streptococci can cause lysis of human red cells through the use of hemolysins O and S. 

There are two main groups of beta-hemolytic streptococci in this group, including: 

Group A Streptococci 

Group A Streptococci is a gram-positive bacteria that are prevalent in the mucous and skin of humans. In some cases, it can also be found in the rectum and vagina. 

Group A streptococci is one of the most recognizable forms of streptococcus, and it’s responsible for a number of human infections, ranging from moderate to life-threatening. 

Like other forms of Streptococcus, group A streptococci are spread through respiratory droplets and direct contact with infected skin.

After transmission occurs, group a streptococci bacteria begin to stick to the cell’s proteins, called extracellular matric proteins, using proteins including lipoteichoic acid. 

Infections caused by Group A Streptococci include: 


Bacteremia is the term used to describe the presence of bacteria in the bloodstream. Sometimes, bacteremia can have no symptoms.

However, in some cases, bacteremia can cause infections in the blood that develop into serious complications. Therefore, bacteremia should not be confused with sepsis, which is a clinical diagnosis. 


Cellulitis is a common but potentially serious condition that’s characterized as a bacterial skin infection. Patients with cellulitis experience swollen and inflamed skin in the affected area.

Their skin may also be warm and sore to the touch. In most cases, cellulitis affects the lower legs, but it can also develop on the arms, face, and other parts of the body. 

Cellulitis infections develop when a break in the skin allows bacteria to enter the body. If cellulitis isn’t treated, it can spread to the bloodstream and lymph nodes, where it can quickly become a medical emergency. Cellulitis usually isn’t infectious. 


Myositis is the term used to describe a group of rare conditions. Myositis usually gets worse with time, and the onset is slow. Myositis is usually caused by a weak immune system, where it starts to attack healthy tissue in the body.

Symptoms of myositis include tiredness after walking or standing and frequent trips or falls. You may also experience depression, issues with swallowing or holding your head up, muscle weakness, and trouble sitting upright or standing after a fall. 

Necrotizing fasciitis 

Necrotizing fasciitis is a rare and serious infection that targets the tissue beneath the skin and the muscles and organs surrounding the affected tissue. This bacterial infection has a mortality rate of between 24 to 34%, and it’s often called the ‘flesh-eating disease.’

Early symptoms can include a small, painful cut or scratch, intense pain that doesn’t correlate with the damage on your skin, and a high temperature. 

After a few hours or days of experiencing the early symptoms, you may experience pain, swelling and redness in the area (this will usually be warm to the touch), vomiting and diarrhea, and dark spots on the skin, which turn into blisters.

If necrotizing fasciitis isn’t treated, it can spread fast through the body and cause weakness, confusion, and dizziness. 


Lymphangitis is the inflammation of lymphatic channels in the body. This usually occurs due to an infection. This infection of the lymph vessels or channels is a rare complication of bacterial infections, and if left untreated, it can spread to the blood.

One of the most common symptoms of lymphangitis is a red streak or strike extending from the injury site. It will spread towards areas with lots of lymph glands, such as the armpits and groin. 


Pneumonia is an infection that can affect one or both of the lungs. It’s usually caused by viruses, bacteria, or fungi.

Pneumonia is a serious infection that causes the air sacs in the lungs to become filled with pus or other liquid. You may also experience other symptoms such as a cough, difficulty breathing, a fever, and chest pain.

If you’re elderly or immunocompromised, you’ll be more likely to experience complications with pneumonia. 

Streptococcal Toxic Shock Syndrome (STSS)

Streptococcal Toxic Shock Syndrome (STSS) is a rare and serious bacterial infection that can develop quickly, and spread through the body. It can cause low blood pressure, multiple organ failure, and in some cases, death.

This condition is often life-threatening. Symptoms include nausea and vomiting, fever and chills, muscle aches, hypotension, and tachycardia. 


Vulvovaginitis is the inflammation of both the vagina and the vulva. Symptoms of vulvovaginitis can include irritation, burning, itching, and thick, mucus-like vaginal discharge.

Vulvovaginitis is a common condition that affects people with vaginas of all ages. It’s rarely serious, but it can be irritating and unpleasant to experience. 

Group B Streptococcus 

Group B Streptococcus usually lives commensally in the female genitals and the digestive tract. Because it lives commensally, group b streptococcus doesn’t tend to be harmful to human health in normal conditions.

Group B streptococcus is also beta-hemolytic, meaning it causes complete lysis of the human red cells. 

This form of streptococcus can cause meningitis and sepsis in newborn children, and infections in elderly and immunocompromised patients. In newborns, the transmission of group b streptococcus can occur through the amniotic fluid.

The bacteria can also be transmitted during the fecal-oral route during sexual contact between adults. 

Once you’ve been infected with group b streptococcus, you may be at risk of developing any of the following infections: 

  • Urinary tract infections (UTIs)
  • Endocarditis
  • Cellulitis
  • Meningitis
  • Osteomyelitis
  • Pneumonia 

Streptococcus Classification

Streptococcus Classification

Now, let’s take a closer look at the classification of Streptococcus. 

Kingdom: Bacteria

Streptococci are members of the kingdom bacteria. By nature, they are prokaryotes. Prokaryotes are microscopic-sized single-celled organisms that lack a distinct nucleus and membrane and have no specialized organelles.

Streptococci also have a layer of peptidoglycan in their cell walls. 

Subkingdom: Posibacteria

Streptococci can also be defined as subkingdom posibacteria, which contain largely gram-positive bacteria. 

Class: Bacilli

Bacilli are a type of bacteria defined by their low G + C content. Bacilli groups have gram-positive bacteria that are divided into both Bacillales and Lactobacillales. 

Phylum: Firmicutes

Firmicutes are a phylum consisting largely of gram-positive bacteria. Members of this phylum, including streptococcus, usually form a healthy part of the guts microbiota. 

Family: Streptococcaceae

Members of the streptococcaceae family are usually spherical, grow in pairs and chains, and are gram-positive. They can also ferment carbohydrates. 

Order: Lactobacillales

These orders are defined under the Bacilli class. Bacteria in this class can be both spherical and rod-shaped, and they’re all gram-positive. Each bacteria in the lactobacillales class has a low G + C content, and they’re sometimes called lactic acid bacteria.  

Genus: Streptococcus

Listed below are several examples of the streptococcus bacteria: 

  • Streptococcus canis
  • Streptococcus bovis
  • Streptococcus mutans
  • Streptococcus pneumoniae 
  • Streptococcus oralis 
  • Streptococcus salivarius 

Streptococcus Gram Stain

In microbiology, a gram stain is a test that seeks to assess the presence of bacteria at the site of a suspected infection or in certain bodily fluids. Gram staining is one of the most important staining techniques in microbiology. 

Gram staining is one of the tests used to find the presence of streptococcus bacteria in an infected sample. Other tests include the bile solubility test, the catalase test, and the optochin test. 

To successfully gram stain for Streptococcus, the following materials are needed: 

  • Water
  • Oil immersion
  • A Sample
  • A compound microscope
  • Gram stain reagents 
  • A burner
  • A glass slide

Streptococcus Gram Stain Procedure

Below are the steps required to gram stain for streptococcus. 

  • With a sterilized applicator stick or a sterilized wire loop, make a small smear of the sample in the middle of the glass slide. 
  • Now, attach the specimen to the slide by placing it over a burner. When you’re doing this, take care not to overheat the sample. 
  • Once you’ve done this, allow the slide to cool down, and then cover it with a primary stain for around 60 seconds. 
  • Now, gently rinse the slide with water. Then, add the iodine solution (mordant) to the slide for 60 seconds. This will create a complex barrier between the iodine and your primary stain. 
  • Wash the slide with water.
  • Now, cover the slide with ethyl alcohol or acetone for several seconds. This could also be done by using a decolorizer dropwise. 
  • Add the counterstain to the slide and leave it to stand for another 60 seconds. 
  • Finally, place the slide on a microscope and observe streptococcus under 100x oil immersion. 

Streptococcus Observations

If you were to view streptococcus under a microscope of 100x magnification, you will see streptococcus pneumoniae emerge in pairs or single cells. They are often purple. In some cases, though, chains consisting of as many as four or six cells have been observed. 

Other variations of streptococci may have longer chains consisting of as many as ten cells. One example is streptococcus agalectiae, whose chains average at ten or more cells. 

Streptococcus Shape

Streptococcus is an incredibly small bacteria with a diameter ranging from 0.5 to 2.0 micrometers. Most streptococci bacteria will appear to be an ovoid or spherical shape, and when viewed through a microscope, these bacteria can show up in either chains or pairs. 

The way streptococcus propagates will determine how its shape is maintained. Streptococcus pneumonia, for example, experiences the synthesis of peptidoglycan through septal and peripheral systems.

Other spherical species of streptococcus experience this process through the septal system only. 

In Streptococcus pneumoniae, the division occurs at the inward growth of the cross wall, which can be found at the cells equator.

When this part of the division process occurs, the equatorial ring (or wall band) is followed by the generation of two more wall bands; peptidoglycan gradually begins to establish itself between these things.

When this happens, peripheral growth is encouraged to continue until the internal hemisphere in the cell reaches the size of its predecessors. 

When peripheral growth reaches its final stages, the septal synthesis of peptidoglycan occurs. When this happens, peptidoglycan hydrolases begin to split the septum, separating the daughter cells. 

Streptococcus can sometimes appear to have an elongated shape, unlike other bacteria such as micrococci and deinococci, which are completely round.

This may be due to the fact that their division occurs across an entirely and successively parallel plane that sits perpendicular to their axis. 

When compared to round cocci such as micrococci and staphylococci, this is entirely different. Division can occur in two or even three perpendicular planes in this cocci.

This means that both the plane division and the cell wall may play a significant role in the general shape of most bacterial cells, but specifically streptococci. 

The length of a streptococcus bacterial chain may play a pivotal role in virulence. However, this may vary according to species. 

There are also several factors that can determine the nature and length of a bacterial chain. For example, cultures can sometimes produce clustered chains, whereas regular short and long chains tend to be formed in vivo. 

Final Thoughts

Although it can be potentially life-threatening, streptococcus is a fascinating bacteria that affects the body in profound ways.

Thankfully, most conditions caused by streptococcus can be treated, and if caught early enough, even those conditions with high mortality rates don’t have to be a death sentence. 

Since streptococcus was first documented in the 1800s, our understanding of this bacteria has come leaps and bounds.

While it’s unlikely that this bacteria will ever completely disappear, modern science has given us the knowledge and treatment we need to help combat even the most serious infections. 

Whether you experience group a or group b strep, treatments are available. In most cases, streptococcus is harmless, and you may not even need treatment at all. However, this bacteria is an opportunist in the right conditions, which could put you at risk of infection. 

Jennifer Dawkins

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