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What is Streptococcus?

Last updated date: 29th Jan 2023
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If you feel pain in your throat which leads to swallowing issues then you might think of it as a common cold and sore throat. Having a sore throat could be harmless or it could indicate a more severe throat infection called streptococcus.


In general, sore throats are caused by viruses whereas strep throat is due to the streptococcus bacteria. This bacterial streptococcus infection of the throat and tonsils is caused by group A streptococcus. 


The diseases caused by streptococcus bacteria could range from minor illnesses to extremely serious and fatal consequences. In this article, we will find out more about what is streptococcus, the streptococcus classification, what implications streptococcal skin infections have, and learn streptococcus pathogenesis.


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Streptococci Definition

The other names for streptococcus bacteria are GAS (Group A streptococcus) or GABHS (Group A (beta-hemolytic) streptococcus). Streptococcus are gram-positive bacteria that have an ovoid or spherical shape. They are chiefly parasitic and nonmotile bacteria that divide only in one plane and occur in chains or pairs.

  • Streptococcus is an organism with no nuclear membrane and has no organelles in the cytoplasm (except for ribosomes) hence its pyogenes are prokaryotes.

  • The genetic material of streptococcus is in the form of a single continuous chain which forms loops or coils.

  • These cocci are anaerobic i.e. they can live without needing oxygen and they have complex nutritional requirements.

  • Many of the known species of group A streptococcus are parasitic in humans as well as animals. Some of them are important pathogens.

  • Strep throat is not very prevalent and accounts for only a small percentage of sore throats.

  • If strep throat is not treated, it can cause many complications for instance rheumatic fever, kidney inflammation, etc. Rheumatic fever can give painful and inflamed joints, damage to heart valves, and rashes.

  • Streptococcus infection mostly occurs in children but it can be seen in adults too.


Streptococci Has Two Key Groups

  1. \[\alpha\] (alpha)-hemolytic Streptococci 

This is the most common group and many strains live inside humans without causing any symptoms.

This group is further divided into Streptococcus pneumonia and Viridans Streptococci.

  1. \[\beta\](beta)-haemolytic Streptococci 

This is further divided into Group A Streptococci (Streptococcus pyogenes) and Group B Streptococci (Streptococcus agalactiae)

Group A Streptococci is transmitted through coughs, sneezes, or direct contact. It can be non-invasive (i.e. does not spread inside the bloodstream) or invasive (which means it spreads to the bloodstream and other body sites).

Group B Streptococci in general lives harmlessly inside female genitals and digestive system. It is transmitted either through sexual contact or from the mother to a baby during birth. That is why Group B Streptococci mostly affects newborns as the bacteria get passed from mother to infant through the amniotic fluid.


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Streptococcus Pyogenes Morphology

  • They are gram-positive bacteria.

  • They have a diameter of five to one μm.

  • They are non-motile (can not move) and non-sporing (do not form spores).

  • They contain a protein called Protein F.

  • Some of the streptococcus strains are capsulated and can be seen best in young cultures.

  • They are fimbriated. It means they have a border or fringe of fingerlike or hairlike projections.

  • The walls of its cell contain group-specific carbohydrates.


Streptococcus Habitats

  • They are found in the skin and throat of humans.

  • They obtain their nutrients from the upper respiratory tract.

  • They are opportunistic pathogens which means they do not infect healthy hosts but cause infections in hospitals to immunosuppressed people.

  • They can survive in dust for a little while.

  • The carrier rate of this pathogen in adults is very few and around 10% in kids going to kindergarten.


Streptococcus Classifications

There are many ways that streptococcus bacteria can be classified. They can be segregated into different categories based on:

  • Oxygen requirement.

  • The hemolytic pattern on sheep blood agar (Brown classification).

  • Physiological characteristics (Shermann’s classification).

  • Serological classification (Lancefield’s classification).

  • Biochemical classification.

  • 16 rRNA sequence.


We will look into the details of some of the classifications mentioned above.

  1. Streptococcus Classification based on oxygen requirement:

    1. Aerobic anaerobes such as Streptococcus spp

    2. Obligate anaerobes such as Peptostreptococcus


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  1. Brown Classification

    1. Αlpha-Haemolysis Group - These show greenish discoloration and form incomplete hemolysis on blood agar.

    2. Beta-Haemolysis Group - These form complete hemolysis on blood agar.

    3. Non-Haemolysis Group - These do not cause hemolysis at all for example Streptococcus faecalis.


  1. Sherman's Classification

    1. Lactococci - They are a non-hemolytic group and are found in dairy products for example Lactococcus.

    2. Enterococci - They are present as normal human intestinal flora for example Enterococcus.

    3. Viridans Streptococci - They are present as normal human upper respiratory tract flora.


Streptococcal Skin Infection

You can get many skin diseases if you come in direct contact with streptococcus bacteria. Some of the common ones are:

  • Impetigo - A superficial bacterial infection that forms honey-colored crusted erosions (also called “school sores”).

  • Ecthyma - It forms crusted sores and ulcers get produced beneath these crusts. It is a more severe form of impetigo. 

  • Cellulitis - This infection occurs in subcutaneous tissues and the lower dermis. It gives localized red, swollen, and painful skin. If not treated, cellulitis can turn life-threatening.

  • Erysipelas - It affects the upper dermis and is a superficial form of cellulitis that extends into superficial cutaneous lymphatics. Owing to the intense rash caused by this infection, it is also referred to as St.Anthony’s fire.

  • Necrotizing Fasciitis - It is an extremely serious bacterial infection that occurs in fascia and soft tissues. It causes thrombosis (blood clots) in blood vessels as this bacteria multiplies and releases toxic enzymes.

  • Tropical ulcers


Streptococcus is a bacterial infection caused by Streptococcus pyogenes and it affects the throat and skin region. This infection can range from mild to extremely severe and cause fatalities. Streptococcus is a highly contagious infection and usually spreads by the cough and sneeze of an infected person. Streptococcus is seen more in kindergarten-going kids than in adults.

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FAQs on Streptococcus

1. How do you catch strep throat?

Group A strep is present in the nose and throat and can quickly spread to other people. Many times people infected with strep do not show symptoms or appear sick. Strep mostly spreads if you are near an infected person and s/he coughs or sneezes.

  • Sneezing releases small respiratory globules and if they are breathed in by others they can get strep.

  • If someone touches a surface with those strep globules on it and then touches their nose or mouth, the disease can spread.

  • If you eat or drink from the same plate as the infected person.

  • If you touch sores on the body which are caused by group A streptococcus.

2. What are some of the other diseases caused by streptococcus bacteria?

Group A strep cause the following diseases:

  • Scarlet fever - Also called scarlatina, Scarlet fever is a reaction to the toxin circulation which is caused by some specific strains of streptococcus. It is a mild infection and if someone is exposed to streptococcus bacteria, it will take two to five days to show symptoms of this disease.

  • STSS - Streptococcal Toxic Shock Syndrome is a serious bacterial infection that is not so common. It can develop rapidly to low blood pressure, multiple organ failure, and even cause death.STSS is rarely contagious.

  • PSGN - PSGN stands for Post-streptococcal glomerulonephritis and is a very rare kidney disease that is caused by group A strep infection. PSGN results from the body's immune system fighting off the streptococcus infection. People cannot catch PSGN from others infected by streptococcus bacteria.

3. What are gram-positive bacteria?

The staining method was developed by the biologist Hans Christian Gram in 1884, and gram-positive or gram-negative bacteria are given after his name. Gram-positive bacteria are classified based on the colour they turn into in the staining method. Gram-positive bacteria have a thick mesh-like cell membrane called peptidoglycan. Gram-positive bacteria are rod-shaped and do not have an outer cell wall apart from the peptidoglycan hence they are more absorbent.


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