Streptococcus mitis characteristics, taxonomy, pathologies

Alexander Pearson

The Streptococcus mitis It is a species of bacteria that makes up the bacterial flora of the oral cavity. It is part of a subgroup of bacteria of the genus Streptococcus known as Viridans, which also include: S. mutans, S. salivarius and S. sanguis.

It is found mainly on the back and body of the tongue, and on the mucous membranes of the oral cavity. Likewise, it has been frequent to find it adhered to the teeth. It generally does not behave as a pathogenic agent, that is, under normal conditions it is not the cause of any pathology.

Streptococcus mitis growth. Source:

Medical problems can manifest when Strepctococcus mitis leaves its natural habitat and travels through the bloodstream to other parts of the body, where it can cause serious infections.

Article index

  • 1 Taxonomy
  • 2 Morphology
  • 3 Biological characteristics
  • 4 Pathologies caused by bacteria of the species Streptococcus mitis
    • 4.1 Cause of acute endocarditis
    • 4.2 Symptoms
    • 4.3 Diagnosis of acute endocarditis
    • 4.4 Echocardiography
    • 4.5 Blood culture
    • 4.6 Treatment of acute endocarditis
  • 5 References


To get an idea of ​​the characteristics that an organism can have, as well as its physiology, it is pertinent to know its taxonomic classification. In the case of Streptococcus mitis, it is the following:

Domain: Bacteria.

Edge: Firmicutes.

Class: Bacilli.

Order: Lactobacillales.

Family: Streptococcaceae.

Genus: Streptococcus.

Species: S. mitis.


The characteristics of the bacteria of the species Streptococcus mitis are:

-As its name implies (Strepto, means chain and coccus means cocci, rounded cell), these bacteria are composed of chains of medium-sized bacterial cells..

-On its cell surface you can see extensions of various sizes, called appendages.

-They have a cell wall made up of a carbohydrate called Polysaccharide C, as well as another compound that is a type of polymer belonging to the group of teichoic acids..

Biological characteristics

With regard to biological characteristics, it is important to clarify that reference is made to the biological behavior that this organism has when it is in its natural habitat, as well as that which has been observed through experimental activities in the laboratory..

Gram positive

In bacteriology studies, two large groups of bacteria can be seen, Gram Positive and Gram Negative..

This name is due to the staining that bacteria acquire when they undergo a staining process known as Gram Stain, in honor of its creator, Christian Gram, a Danish microbiologist..

In the case of Gram Positive bacteria, when they are subjected to this staining process, they acquire a violet color. This occurs because they have a thick cell wall that has peptidoglycan.

This characteristic violet color is due to the fact that part of the dye is trapped between the peptidoglycan filaments, without dissolving, giving it the color already described..

It belongs to the group of alpha hemolytic

This has to do with whether the bacteria are capable of hemolysis of erythrocytes, also known as red blood cells..

By hemolysis is understood the process by which a total destruction of erythrocytes occurs, with the consequent release of their content, especially hemoglobin.

Bacteria, according to this criterion, can cause a total destruction of red blood cells, being called beta hemolytic. There are others that only generate partial destruction and are known as alpha hemolytic. And finally, there are those that do not have this capacity and are called gamma hemolytic.

To determine this, a bacterial cell culture must be made in a special culture medium called blood agar, which contains 5% blood..

In the case of alpha hemolytic bacteria (such as Streptococcus mitis), a greenish halo is observed in the culture, produced by the oxidation of hemoglobin molecules, an unequivocal sign that there has been incomplete hemolysis.

It is facultative anaerobic

The word anaerobic means in the absence of oxygen. There are organisms that are obligate anaerobes, that is, they must necessarily develop in an oxygen-free environment.

There are other organisms that can survive, both in the presence and in the absence of oxygen. These are known as facultative anaerobes..

Being facultative anaerobic makes the metabolism of bacteria of the species Streptococcus mitis quite versatile, since it can take advantage of various substances in the environment in which it is found to metabolize them and transform them into others..

It is catalase negative

Catalase is an enzyme that some bacteria possess. When this enzyme comes into contact with hydrogen peroxide (H2O2) it is able to decompose it into water (H2O) and oxygen (O).

When a study is done to determine whether a bacterium possesses the enzyme catalase, it comes into contact with hydrogen peroxide. The appearance of bubbles is an unequivocal sign of the release of oxygen present in the molecule.

This is a characteristic that contributes to the identification of bacteria at an experimental level..

It is mesophilic

In order to develop, living beings need to be in specific environmental conditions that favor the proper functioning of their body. An element that has fundamental importance in this is the temperature.

Bacteria are no exception. This is why, according to their morphological and genetic characteristics, they will need specific temperature levels..

The word mesophyll refers to those organisms that can develop at intermediate levels of temperature.

In the case of Streptococcus mitis, the temperature it requires to survive is approximately 36 - 37 ° C. Above or below these values, the proteins they contain are denatured and lose their function, causing cell death..

Pathologies caused by bacteria of the species Streptococcus mitis

Of all the bacteria that make up the bacterial flora of the oral cavity, Streptococcus mitis is one of the most innocuous and harmless. This is so, as long as the balance that exists in this habitat is maintained..

When an unexpected event occurs that affects that balance, those bacteria that are normally in the oral cavity can transform into pathogens. That is, they can generate pathologies, among which the most common are infections.

This can occur when there is a considerable injury or wound in the oral cavity, which may be the result of some dental treatment. Likewise, it can happen when people have a medical condition that generates a weakening in their immune system..

Among the pathologies that are caused by Streptococcus mitis, the most frequent is endocarditis.

Cause of acute endocarditis

The first thing to clarify is that acute endocarditis is not spread from one person to another, but is caused by the disruption of the balance that exists in the oral bacterial flora, together with the existence of a wound or injury.

The human heart is made up of three layers: an external (pericardium), an intermediate (myocardium) and an internal (endocardium).

When bacteremia occurs, that is, Streptococcus mitis leaves its natural habitat in the oral cavity and enters the bloodstream, it is transported to the heart.

Among the structures of the heart for which Streptococcus mitis has preference are the atrioventricular valves. When it gets there, it fixes and begins to reproduce, causing damage to their structure and of course, affecting their proper functioning..

It is important to clarify that, although Streptococcus mitis is one of the organisms most frequently associated with this pathology, it is not the only one. The other bacteria that colonize the oral cavity can also become causative agents..


Unfortunately, the symptoms of acute endocarditis are nonspecific and can correspond to many other pathologies.

When it comes to acute infections or illnesses, its symptoms appear suddenly and unexpectedly. This is the case with acute endocarditis caused by Streptococcus mitis. Symptoms include:

  • Fever: is the most common symptom of a bacterial infection. In the case of acute endocarditis, it is usually elevated, between 39 and 40 ° C.
  • Changes in heart rate: because the bacteria are attached to any of the heart valves or the inner layer of the heart, its proper functioning is affected. This is reflected in a disturbance of the normal heart rhythm.
  • Usually the heart rate accelerates, which is known as tachycardia.
  • Fatigue: many people with acute endocarditis report feeling tired and weak to carry out daily activities. This is due to the malfunctioning of the heart..
  • Dyspnea - Another common symptom is shortness of breath. It is present in approximately 40% of patients with acute endocarditis. It is also caused by a deterioration of heart function.
  • Other symptoms: anorexia, weight loss, cough, skin lesions, chest pain, abdominal pain.

Diagnosis of acute endocarditis

When a patient presents at the health center showing symptoms that can be related to this pathology, it is important that the doctor perform a series of tests that will allow him to make a proper diagnosis..


It is a medical test that consists of obtaining images of the internal organs of the body through the use of sound waves. It is a non-invasive procedure that does not carry risks because it does not use ionizing radiation.

Through echocardiography the doctor can visualize certain structures of the heart and detect abnormalities such as abscesses, vegetations or regurgitations, characteristic signs of this disease.

Blood culture

This is perhaps the most reliable test to irrevocably diagnose infective endocarditis due to Streptococcus mitis..

It consists of taking a blood sample and performing cultures in Petri dishes, enriched with a suitable culture medium for the bacteria that are believed to be present.

Later, once the culture has developed, staining techniques can be applied for identification, or other criteria can be taken into account, such as the ability to perform hemolysis..

Treatment of acute endocarditis

The effective treatment for bacterial infections is the use of antibiotic therapy. However, bacteria are organisms that can sometimes develop resistance to certain antibiotics..

The first choice for doctors is penicillin, a broad-spectrum antibiotic, which has been shown to be very effective against streptococci. If the microorganism happens to be resistant to penicillin, another antibiotic can be used.

When a bacterial culture is performed, a susceptibility test is done to determine which antibiotics the bacteria found in it are susceptible or resistant to..

The results of this test will determine which will be the most appropriate antibiotic to treat the pathology.

Likewise, if the infection has advanced a lot and has generated significant damage to cardiac tissues, cardiovascular surgery is also indicated..


  1. Brooks G., Carroll K., Butel J., Morse S., Mietzner T. Medical Microbiology. 25th edition. Mc Graw Hill Interamericana. 2010.
  2. Kilian M., Mikkelsen L., Henrichsen J. Taxonomic Study of Viridans Streptococci: Description of Streptococcus gordonii sp. Nov. and Emended Descriptions of Streptococcus sanguis (White and Niven 1946), Streptococcus oralis (Bridge and Sneath 1982), and Streptococcus mitis (Andrewes and Horder 1906). International Journal of Systematic and Evolutionary Microbiology. 1989. Recovered from
  3. Cruz S., Díaz P., Arias D., Mazón G. Microbiota of the ecosystems of the oral cavity. Cuban Journal of Stomatology. 2017. Recovered from
  4. Poveda J., Soriano T., Cañas A., Rodríguez L. Diagnosis and management of infective endocarditis. Costa Rican Journal of Cardiology. 2003. Recovered from
  5. Senior, JM, Gándara-Ricardo, JA. Infectious endocarditis. Iatreia [Internet]. 2015; 28 (4): 456-471. Recovered from:
  6. Verger E. (2017). Gram stain: how it is done and what it is used for. Recovered from:
  7. Byrd V., Nemeth A. A Case of Infective Endocarditis and Spinal Epidural Abscess Caused by Streptococcus mitis Bacteremia. Case Reports in Infectious Diseases. 2017. Recovered from
  8. Lonks J., Dickinson B., Runarsdottir V. Endocarditis Due to Streptococcus mitis with High-Level Resistance to Penicillin and Cefotaxime. The New England Journal of Medicine. 1999. Recovered from
  9. Streptococcus mitis. Recovered from
  10. Prats G., Clinical Microbiology. 1st edition. Mc Graw Hill Interamericana. 2005.

Yet No Comments