Taxonomic Classification And Characteristics Of Streptococcus Pyogenes

Taxonomic Classification of Streptococcus Pyogenes

This paper will discuss a medically important microorganism. Streptococcus pyogenes has been selected for the same. The taxonomic classification, appearance, lifecycle, and natural hist/environment of this microorganism will be discussed in this paper. Different methods and techniques to identify the microorganism will also be explained with proper literature.

In the field of biology, the term taxonomy can be defined as the scientific study through which the biological organisms are named, defined, and classified according to the characteristics they share. Taxa (singular: taxon) are groupings of organisms that are assigned a taxonomic rank; collections of a particular rank can be combined to produce a much more comprehensive group of higher rank, resulting in a taxonomic hierarchy. In present usage, the main rankings are domain, kingdom, division, class, order, family, genus, and species (Kunz, 2013).

Based on this the taxonomic classification of Streptococcus Pyogenes is (ITIS, 2022):

Kingdom: Bacteria

Phylum: Firmicutes

Class: Bacilli

Order: Lactobacillales

Family: Streptococcaceae

Genus: Streptococcus

Species: Pyogenes

When viewed under the microscope, Streptococcus appear as cocci bacterial cells. When the technique of Gram staining is done on them, they appear purple, which indicates that they are Gram positive as indicated in Figure 1. These bacteria are non motile and do not form spores. They measure between 0.5 and 2 μm in diameter. The species of Streptococcus shows growth in pairs or chains because cellular division happens along a single axis or plane. After incubating these bacteria for 18-24 hours at the temperature of 35-37°C on blood agar, these bacteria have greyish white colonies which appear smooth, shining, and translucent as shown in figure 2 with zones of haemolysis (Spellerberg & Brandt, 2016). These zones are Alpha, beta, or no hemolysis. Streptococci were historically classed depending on the type of hemolytic action observed on blood agar. Beta -Hemolysis is characterized by the full disintegration of red blood cells around the colony, while alpha-hemolysis is characterized by incomplete or “greening” hemolysis and a decrease in the hemoglobin present in the red blood cell. Nonhemolytic colonies are referred to as γ-hemolytic colonies (Spellerberg & Brandt, 2016).  Streptococcus pyogenes show beta hemolysis and are characterized under Group A streptococcus (Kanwal & Vaitla, 2020). 

Figure 1

(Source: Batra, 2018) 

Figure 2

(Source: Spellerberg & Brandt, 2016)

Several species of streptococcus can have structures which appear like pilus or fimbria extending from their surfaces, such as S. pyogenes’ M protein-containing fimbriae as indicated in figure 3. 

Figure 3

(Source: Hossain, 2014)

Streptococcus pyogenes are bacteria that grow in long chains. It is present in 5 to 15 percent of all healthy people and is shown no danger or threat to them. But when the circumstances become optimum for their growth it can cause severe infection which can be fatal. It causes infections such as strep throat, toxic shock syndrome, scarlet fever, flesh-eating bacteria syndrome, impetigo, and erysipelas (Ferretti, Stevens & Fischetti, 2016).

As Streptococcus pyogenes is bacteria, the mode of reproduction is binary fission. This mode of reproduction is asexual through which single celled organisms reproduce. Single cell divides into two cells of the equal size during this phase. Most prokaryotes undergo binary fission.

Appearance and Lifecycle of Streptococcus Pyogenes

These bacteria can be transmitted from the respiratory tract of an infected person to a healthy person through the droplets of moisture in the air. For instance, when a person infected with the bacteria coughing or sneezing, another person may inhale in sneezing particles. The bacteria can be transmitted through different modes such as sharing bodily fluids, for example using the toothbrush of the person who is infected or coming in contact with an open wound. Once the bacteria have entered the body, the pathogen can spread to other parts of the body via the circulation of blood (Avire, Whiley & Ross, 2021).

When a population of bacteria resides on or inside a human body without causing any kind of disease, they are considered the normal flora. In the respiratory tract of the human being, Streptococcus pyogenes is one such normal flora. But once the immune system of the body compromises, this normal flora can start causing infection, such as in situations of other diseases or if the normal flora moves to the other area of the body (Martin, 2016).

There are many factors in Group A Streptococcus that can help the bacteria in adhesion and forming colonies in the nasopharynx region including tonsil epithelium and skin. These factors are hyaluronic acid capsule (HA), fimbrious structures or pili which are long rod shaped structures that protrude from the cell wall of the bacteria, M proteins, and the S. pyogenes fibronectin-binding adhesin (SfbI) (Castro, S. A., & Dorfmueller, 2021).

The process of cell attachment is very much complicated and has not been fully characterized fully. This process is characterized by two steps. The first stage or the initial attachment is supported by various elements such as GAS surface carbohydrate, lipoteichoic acid by having a poor but adequate affinity for pharyngeal or cutaneous epithelial cells of the host via hydrophobic linkages (Castro, S. A., & Dorfmueller, 2021).  The second or the later stage is facilitated through events that have a high affinity toward binding, it is initiated by pili and subsequent affinity via lectin–carbohydrate and protein to protein interactions (Castro, S. A., & Dorfmueller, 2021).  These types of interactions are facilitated through proteins that help in the adhesion of Group A Streptococcus and initiate a very tight and firm kind of adhesion to different tissues present in the body of the human being. Bacterial adhesion is seen as a continuous phenomenon owing to the pathogen’s capacity to separate from the hard surface of adhesion and migrate to an environment which is more suitable where it can live and thrive. For the adhesion of Group A Streptococcus there are different types of human extracellular proteins which act as components that are necessary for binding. These proteins are collagen, fibronectin, fibrinogen, laminin, and vitronectin. Significantly, human fibronectin is a common site of binding for streptococcal adhesins, which contributes to GAS-specific binding affinities with host 51 integrin receptors on epithelial cells. GAS strains generate a minimum of 11 fibronectin-binding adhesins, including multiple M proteins, and they attach to host fibronectin in the extracellular matrix (ECM) whether soluble or immobilized. When GAS adheres to the skin and pharyngeal surface of the host, microcolonies emerge, which seem like macroscopic entities that spread and cause streptococcal infections (Castro, S. A., & Dorfmueller, 2021) as shown in figure 4.  

Identification of Streptococcus Pyogenes

Figure 4

(Source: Castro, S. A., & Dorfmueller, 2021).

It is not possible to differentiate the diagnosis of pharyngitis or tonsillitis caused by S pyogenes just on the basis of symptoms. Even skilled clinicians struggle to distinguish bacterial pharyngitis from viral pharyngitis, hence bacteriologic procedures must be used.

Throat cultures are generally recommended for the isolation of S pyogenes. Aspirates taken from the developing periphery of the infection may be indicated in situations of cellulitis or erysipelas presumed to be generated by S pyogenes. The first step in the identification of S pyogenes is Gram staining and biochemical tests. When the technique of Gram staining is done on them, they appear purple, which indicates that they are Gram positive. The agar suitable for culturing throat swabs will be 5 % blood agar with a trypticase soy base. It is then incubated in the air. It remains the gold standard and guideline technique for the diagnosis of S. pyogenes acute pharyngitis when conducted and read correctly. As evidenced by research using double throat cultures, these circumstances constitute dependable that is well accepted and procedures with a specificity of 90% or higher. After 24 hours of incubation at the temperature of 35-37°C, most instances of acute streptococcal pharyngitis show the abundant formation of characteristic colonies. Table 1 depicts biochemical assays that give sensitive traits that are specific to the group that allows for the preliminary diagnosis of Gram-positive, catalase-negative cocci.

Table 1

Once the beta-hemolytic colonies depict the typical morphology of S. pyogenes, the catalase testing should be done. It helps in confirming that the isolated bacteria are streptococci. After this, there are several laboratory tests that are performed rapidly and can be used to identify the particular species. Because many of the tests described below has significant limits, the most accurate identification findings can be obtained by merging two of the procedure listed below.

This technique was first developed by Rebecca Lancefield so that the streptococci which are β-hemolytic can be differentiated into different types of species. Using antibodies to detect the availability of Lancefield antigens on the surface of streptococcus (Lancefield, 1993). Commercially accessible Lancefield antigen grouping sera from a variety of sources are nevertheless routinely used in clinical microbiology laboratories for the distinction of β–hemolytic streptococci. These kits that are available in the market provide substrates that can help in extracting the antigens very rapidly and helps in the subsequent process of agglutination by some particular antibodies. These are then directed towards the Lancefield antigens which are A, B, C, F, and G. Although there is a strong association between the existence of some Lancefield antigens and the existence of particular streptococcal species, it is not perfect in the situation of Lancefield antigens group A, C, or G (Gera & McIver, 2013). With the exception of unusual mutations, the cell surface of all strains of S. pyogenes has the Lancefield group A antigen; nevertheless, the existence of the group A antigen is not exclusive to S. pyogenes. It has also reportedly been detected in Streptococcus anginosus species and rare isolates of Streptococcus dysgalactiae subspecies equisimilis. As a result of the identification of Lancefield group A, additional analysis for a definitive species identification of S. pyogenes is required, which can be accomplished using bacitracin sensitivity discs or a PYR determination test (Nishiki, Yoshida & Fujiwara, 2019).

The PYR test is a quick chromogenic approach that is frequently employed to differentiate S. pyogenes from similar beta-hemolytic species of streptococci with similar appearance (like S. dysgalactiae subspecies equismilis) and looks for the existence of the enzyme pyrrolidonyl aminopeptidase. Whenever a cinnamaldehyde reagent is introduced, this enzyme hydrolyzes L-pyrrolidonyl—naphthylamide (PYR) to -naphthylamide, which creates a red hue. In just a few minutes, the procedure can be done on strips of paper containing dried chromogenic precursors for pyrrolidonyl aminopeptidase. This technique must only be conducted on cultures that are pure to avoid results that are false positives generated by other species of bacteria that are positive for PYR that may be found in mixed cultures (Agarwal, 2018).

Streptococcus pyogenes is distinguished from similar non-group A -hemolytic streptococci by its higher sensitivity to bacitracin. Because other -hemolytic streptococci that may carry the group A antigen are immune to bacitracin, the bacitracin test, in conjunction with the Lancefield antigen A test, is employed for increased accuracy in the diagnosis of S. pyogenes (Spellerberg & Brandt, 2016).

References

Agarwal, J. (2018). Streptococcus pyogenes. Microbiology Practical Manual, -E-book, 75.

Avire, N. J., Whiley, H., & Ross, K. (2021). A Review of Streptococcus pyogenes: Public health risk factors, prevention and control. Pathogens, 10(2), 248.

Batra, S. (2018). Morphology And Culture Characteristics Of Streptococcus Pyogenes.

https://paramedicsworld.com/streptococcus-pyogenes/morphology-and-culture-characteristics-of-streptococcus-pyogenes/medical-paramedical-studynotes

Castro, S. A., & Dorfmueller, H. C. (2021). A brief review on Group A Streptococcus pathogenesis and vaccine development. Royal Society open science, 8(3), 201991.

Ferretti, J. J., Stevens, D. L., & Fischetti, V. A. (2016). Streptococcus pyogenes: basic biology to clinical manifestations [internet].

Gera, K., & McIver, K. S. (2013). Laboratory growth and maintenance of Streptococcus pyogenes (the Group A Streptococcus, GAS). Current protocols in microbiology, 30(1), 9D-2.

Hossain, Z. (2014). Bacteria: Streptococcus. https://www.sciencedirect.com/science/article/pii/B9780123786128001165

ITIS. (2022). Streptococcus pyogenes.

https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=966482#null

Kanwal, S., & Vaitla, P. (2020). Streptococcus Pyogenes.

https://pubmed.ncbi.nlm.nih.gov/32119415/#:~:text=Streptococci%20are%20gram%2Dpositive%2C%20catalase,%2Dhemolytic%20(no%20hemolysis).

Kunz, W. (2013). Do species exist?: Principles of taxonomic classification. John Wiley & Sons.

Lancefield, R. C. (1933). A serological differentiation of human and other groups of hemolytic streptococci. The Journal of experimental medicine, 57(4), 571-595.

Martin, J. (2016). The Streptococcus pyogenes carrier state. Streptococcus pyogenes: Basic Biology to Clinical Manifestations [Internet].

Nishiki, I., Yoshida, T., & Fujiwara, A. (2019). Complete genome sequence and characterization of virulence genes in Lancefield group C Streptococcus dysgalactiae isolated from farmed amberjack (Seriola dumerili). Microbiology and immunology, 63(7), 243-250.

Spellerberg, B., & Brandt, C. (2016). Laboratory diagnosis of Streptococcus pyogenes (group A streptococci). Streptococcus pyogenes: Basic Biology to Clinical Manifestations [Internet].

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