What is the Difference Between Streptococcus Pneumoniae and Streptococcus Viridans

The main difference between Streptococcus pneumoniae and Streptococcus viridans is that S. pneumoniae is pathogenic to mice, whereas S. viridans is non-pathogenic to mice.  

S. pneumoniae and S. viridans are two Streptococcus species. They are Gram-positive, spherical-shaped bacteria

Key Areas Covered

1. What is Streptococcus Pneumoniae
– Definition, Features, Importance
2. What is Streptococcus Viridans
– Definition, Features, Importance
3. Similarities Between Streptococcus Pneumoniae and Streptococcus Viridans
– Outline of Common Features
4. Difference Between Streptococcus Pneumoniae and Streptococcus Viridans
– Comparison of Key Differences

Key Terms 

Streptococcus Pneumoniae, Streptococcus Viridans

What is Streptococcus Pneumoniae

S. pneumoniae is a spherical, Gram-positive bacterium. Also, it undergoes alpha-hemolysis, in which the hydrogen peroxide produced by the bacterium oxidizes hemoglobin in the culture producing green color, oxidized methemoglobin. Additionally, the bacterium does not form spores. It occurs in pairs (diplococci). It is a nonmotile bacteria as well. Significantly, S. pneumoniae is a pathogenic bacterium being the primary cause of pneumonia. It occurs symptomatically in healthy individuals’ sinuses, respiratory tract, and nasal cavities. But when the immune system is weakened in adults and young individuals, it becomes pathogenic, causing pneumonia.

Figure 1: S. pneumoniae

Furthermore, person-to-person contact is the leading cause of the spread of the disease, and respiratory droplets can also carry the disease. However, S. pneumoniae is the leading cause of community-acquired pneumonia. Also, it causes meningitis in children and adults. Also, it causes sepsis in HIV-infected patients.   

What is Streptococcus viridans

S. viridans is a commensal Streptococcus that is a Gram-positive bacterium. In contrast, S. viridans undergo gamma hemolysis, and therefore, it is a non-hemolytic bacterium, producing no change in the blood agar. Meanwhile, the optochin test is the primary method to distinguish between S. viridans and S. pneumoniae. S. viridans is optochin-resistant, while S. pneumoniae is optochin-sensitive. However, S. viridans and other Streptococcus bacteria are most abundant in the mouth. One member of this group is S. mutans which causes dental caries. S. sanguinis is another cause of dental caries.

Figure 2: S. viridans

Moreover, when S. viridans is introduced into the bloodstream, it causes endocarditis, damaging heart valves. Subacute bacterial endocarditis is also commonly caused by S. viridans. Additionally, S. viridans can produce dextrans from glucose, allowing them to adhere to fibrin-platelet aggregates in the damaged heart valves. 

Similarities Between Streptococcus Pneumoniae and Streptococcus Viridans

• S. pneumoniae and S. viridans are two Streptococcus species that are Gram-positive bacteria.
• They belong to the family Streptococcaceae.
• They are a type of lactic acid bacteria.
• Both types of bacteria appear in pairs or chains due to their single-axis cell division. 

Difference Between Streptococcus Pneumoniae and Streptococcus Viridans

Definition

Streptococcus pneumoniae refers to lancet-shaped, gram-positive, facultatively anaerobic bacteria with more than 100 known serotypes, while Streptococcus viridans refer to usually a harmless inhabitant of the mouth.

Occurrence

S. pneumoniae resides asymptomatically in the respiratory tract, sinuses, and nasal cavity, while S. viridans is abundant in the mouth.

Bile Solubility Test

S. pneumoniae is soluble in bile, while S. viridans is insoluble in bile.   

Fermentation of Inulin

S. pneumoniae ferments inulin with acid production, while S. viridans does not ferment inulin.

Sensitivity to Optochin

S. pneumoniae is sensitive to optochin, while S. viridans is not.

Pathogenicity to Mice

S. pneumoniae is pathogenic in mice, while S. viridans is not pathogenic in mice.

Quellung Test

S. pneumoniae is positive for the Quellung test, while S. viridians are negative for the Quellung test. 

Hemolysis

S. pneumoniae undergoes alpha-hemolysis, while S. viridans undergoes gamma-hemolysis. 

Conclusion

In brief, S. pneumoniae and S. viridans are two species of the genus Streptococcus. They belong to the family Streptococcaceae. They are Gram-positive bacteria. S. pneumoniae is a pathogenic bacterium in the respiratory tract, sinuses, and nasal cavity. It is soluble in bile ad ferments inulin. Also, it is sensitive to optochin. Additionally, S. viridans is pathogenic in mice. It undergoes alpha-hemolysis. In comparison, S. viridans mainly occurs in the mouth. It is insoluble in bile and does not ferment inulin. However, it is sensitive to optochin. It is pathogenic in mice as well. It undergoes gamma-hemolysis. Therefore, the main difference between S. pneumoniae and S. viridans is their pathogenicity. 

References:

1. Centers for Disease Control and Prevention. (2022, January 27). Streptococcus pneumoniae: Information for clinicians. Centers for Disease Control and Prevention. https://www.cdc.gov/pneumococcal/clinicians/streptococcus-pneumoniae.html
2. Viridans streptococci. Viridans streptococci – an overview | ScienceDirect Topics. (n.d.). https://www.sciencedirect.com/topics/medicine-and-dentistry/viridans-streptococci

Image Courtesy:

1. “Pneumococcus CDC PHIL ID1003” By CDC/Dr. M.S. Mitchell – Own work (Public Domain) via Commons Wikimedia
2. “Streptococcus viridans 01” By w:en:User:Dmcdevit – Own Work (Public Domain) via Commons Wikimedia