Streptococcus pneumoniae vs. Streptococcus viridans

What's the Difference?

Streptococcus pneumoniae and Streptococcus viridans are both species of bacteria belonging to the Streptococcus genus. However, they differ in several aspects. Streptococcus pneumoniae is a major human pathogen responsible for causing pneumonia, meningitis, and other respiratory tract infections. It has a capsule that helps it evade the immune system and is known for its ability to cause invasive diseases. On the other hand, Streptococcus viridans is a group of bacteria that are part of the normal flora in the human mouth, throat, and gastrointestinal tract. While some strains of Streptococcus viridans can cause infections, they are generally considered opportunistic pathogens and are associated with dental caries, endocarditis, and other localized infections.


AttributeStreptococcus pneumoniaeStreptococcus viridans
Gram StainPositivePositive
PathogenicityPathogenicOpportunistic pathogen
Virulence FactorsCapsule, pneumolysin, autolysinNone
Associated DiseasesPneumonia, meningitis, otitis mediaEndocarditis, dental caries
Optimal Growth ConditionsAerobic or facultative anaerobicAerobic or facultative anaerobic
Antibiotic SusceptibilitySome strains are resistantGenerally susceptible

Further Detail


Streptococcus pneumoniae and Streptococcus viridans are two species of bacteria that belong to the Streptococcus genus. While they share some similarities, they also have distinct attributes that set them apart. Understanding the characteristics of these bacteria is crucial for medical professionals in diagnosing and treating infections caused by them. In this article, we will explore the attributes of Streptococcus pneumoniae and Streptococcus viridans, highlighting their differences and similarities.

1. Morphology and Arrangement

Both Streptococcus pneumoniae and Streptococcus viridans are Gram-positive cocci, meaning they have a spherical shape and retain the crystal violet stain in the Gram staining process. However, they differ in their arrangement. Streptococcus pneumoniae typically appears in pairs or short chains, while Streptococcus viridans forms long chains or clusters. This distinction in arrangement can be helpful in differentiating between the two species when observed under a microscope.

2. Capsule Formation

One of the key differences between Streptococcus pneumoniae and Streptococcus viridans lies in their ability to form capsules. Streptococcus pneumoniae is known for its polysaccharide capsule, which plays a crucial role in its pathogenicity. The capsule helps the bacteria evade the host's immune system, making it more virulent. In contrast, Streptococcus viridans does not possess a capsule or has a very thin capsule, which makes it less pathogenic compared to Streptococcus pneumoniae.

3. Pathogenicity

Streptococcus pneumoniae is a well-known pathogen responsible for various infections, including pneumonia, meningitis, and otitis media. Its polysaccharide capsule, along with other virulence factors, allows it to colonize and invade host tissues, leading to disease. On the other hand, Streptococcus viridans is considered a part of the normal flora of the human oral cavity, gastrointestinal tract, and genitourinary system. While it is generally harmless, it can cause opportunistic infections, particularly in individuals with compromised immune systems or in the presence of certain risk factors.

4. Antibiotic Susceptibility

Both Streptococcus pneumoniae and Streptococcus viridans have developed resistance to various antibiotics over time. Streptococcus pneumoniae, in particular, has become a major concern due to the emergence of multidrug-resistant strains. This resistance is primarily attributed to the acquisition of specific genes that encode for antibiotic resistance mechanisms. On the other hand, Streptococcus viridans generally exhibits lower levels of antibiotic resistance compared to Streptococcus pneumoniae. However, it is important to note that antibiotic susceptibility patterns can vary among different strains and geographical regions.

5. Clinical Manifestations

Streptococcus pneumoniae is associated with a wide range of clinical manifestations, including pneumonia, sinusitis, otitis media, meningitis, and septicemia. These infections can occur in individuals of all age groups, but certain populations, such as young children, the elderly, and immunocompromised individuals, are more susceptible. In contrast, Streptococcus viridans is commonly associated with dental caries, endocarditis, and abscesses. It can also cause infections in other sites, such as the respiratory tract and urinary tract, particularly in individuals with underlying conditions or invasive procedures.

6. Laboratory Identification

When it comes to laboratory identification, both Streptococcus pneumoniae and Streptococcus viridans can be identified using standard microbiological techniques. However, additional tests may be required to differentiate between the two species. Streptococcus pneumoniae can be identified by its sensitivity to optochin and its characteristic alpha-hemolytic colonies on blood agar. On the other hand, Streptococcus viridans is typically optochin-resistant and exhibits a wider range of hemolysis patterns on blood agar, including alpha, beta, and gamma hemolysis.


In conclusion, Streptococcus pneumoniae and Streptococcus viridans are two distinct species of bacteria within the Streptococcus genus. While they share some similarities in terms of morphology and Gram staining characteristics, they differ in their arrangement, capsule formation, pathogenicity, antibiotic susceptibility, clinical manifestations, and laboratory identification. Streptococcus pneumoniae is a significant pathogen associated with various infections, while Streptococcus viridans is part of the normal flora but can cause opportunistic infections. Understanding these differences is crucial for accurate diagnosis and appropriate management of infections caused by these bacteria.

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