Streptococcus viridans is a group of bacteria commonly found in the human oral cavity, gastrointestinal tract, and genital tract. They are considered part of the normal flora and, in general, are not harmful to healthy individuals. However, these bacteria can cause infections under certain conditions, especially in individuals with weakened immune systems. 

The epidemiology of S. viridans infections needs to be better defined, as these bacteria are not typically reported in surveillance systems. However, they cause various infections, including dental caries, endocarditis, and infections of the respiratory tract, skin, and soft tissue. 

The risk factors for S. viridans infections include underlying medical conditions that weaken the immune system, such as HIV/AIDS, cancer, and diabetes. Patients who have received chemotherapy or radiation therapy and those who have undergone surgical procedures that disrupt the normal flora are also at increased risk. 

Scientific Classification: 

Domain: Bacteria 

Phylum: Firmicutes 

Class: Bacilli 

Order: Lactobacillales 

Family: Streptococcaceae 

Genus: Streptococcus 

Species: Streptococcus viridans 

Structure: 

• Streptococcus viridans is a spherical, Gram-positive bacterium typically occurring in chains or pairs. It is a facultative anaerobe, which can grow in the presence or absence of oxygen. The cell wall of S. viridans is composed of peptidoglycan, which provides structural support and protection to the bacterium. The cell wall also contains teichoic acids involved in cell wall maintenance and cell division. 

• S. viridans does not have a capsule, a common feature of many pathogenic bacteria that allows them to evade the host immune system. Instead, S. viridans produces a slime layer that helps it to adhere to surfaces and other bacteria. 

• Like other bacteria, S. viridans contains a single, circular chromosome that carries its genetic information. It also has plasmids, small, circular pieces of DNA that can replicate independently of the chromosome. Plasmids can contain genes that confer antibiotic resistance or other advantageous traits. 

• S. viridans produces several necessary extracellular enzymes and metabolites in its pathogenesis. For example, it produces hydrogen peroxide, which can damage host tissues and contribute to the development of infections. It also produces enzymes that degrade host tissues and allow the bacterium to spread within the host. 

In S. viridans, the antigenic types are typically determined by the composition of the bacterial cell wall, which can include different types of polysaccharides and glycoproteins. 

• The most well-studied species within the S. viridans group are S. mitis, S. sanguinis, and S. mutants, each with multiple antigenic types. 

• S. mutants, a significant contributor to dental caries, have multiple antigenic types determined by the composition of their cell wall polysaccharides. These antigenic types can differ in their ability to adhere to tooth surfaces, colonize the oral cavity, and evade the host immune system. 

Streptococcus viridans is a commensal bacterium that usually inhabits the human oral cavity, upper respiratory tract, and gastrointestinal tract. However, it can cause infections when it gains access to areas of the body where it is not usually found, such as the bloodstream or deep tissues. 

The pathogenesis of S. viridans infections is multifactorial and involves a combination of bacterial virulence factors and host factors.  

Virulence factors of S. viridans include: 

• Adhesins: These surface proteins allow the bacteria to adhere to host cells and tissues, allowing colonization and invasion. S. viridans produces a variety of adhesins, including FimA, which is involved in dental plaque formation, and Hsa, which mediates adherence to heart valve endothelium in endocarditis. 

• S. viridans can produce thick capsules that protect the bacteria from host immune defenses, making them more challenging to eradicate. Capsule types differ among S. viridans species and are essential in their pathogenesis. 

• S. viridans can produce enzymes such as hyaluronidase, streptokinase, and DNase, which can degrade host tissues and extracellular matrix and help the bacteria to spread and evade host defenses. 

• Some S. viridans can produce toxins such as hydrogen peroxide, which can cause oxidative damage to host cells and tissues. 

• Physical barriers: The human body has physical barriers, such as the skin, mucous membranes, and epithelial cells, that can prevent the entry of S. viridans into the body. 

• Normal microbiota: The normal microbiota in the human oral cavity and other body sites can compete with S. viridans for resources and prevent overgrowth and colonization. 

• Immune system: The immune system is the body’s primary defense against S. viridans infections. It involves innate and adaptive immune responses, which can recognize and eliminate the bacteria. The innate immune response includes neutrophils, macrophages, and natural killer cells, while the adaptive immune response includes B cells and T cells, which produce antibodies and target infected cells. 

• Complement system: This group of proteins can recognize and eliminate foreign microorganisms, including S. viridans. The complement system can also activate the immune system and enhance the phagocytosis and killing of bacteria. 

• Dental and oral infections: S. viridans is a common cause of dental caries, periodontitis, and other oral infections. 
• Endocarditis: S. viridans is a leading cause of infective endocarditis, an infection of the heart valves or lining of the heart. Symptoms may include fever, fatigue, heart murmur, and other signs of heart failure. 
• Septicemia and bacteremia are serious bloodstream infections that can lead to septic shock, multiple organ failure, and death.
• Deep tissue abscesses: S. viridans can cause deep tissue abscesses, which are localized collections of pus that can occur in various parts of the body, such as the brain, liver, and lungs.
• Osteomyelitis: S. viridans can cause osteomyelitis, an infection of the bones and bone marrow. It can lead to severe pain, bone destruction, and disability. 
• Necrotizing fasciitis: S. viridans can cause necrotizing fasciitis, a rapidly progressing infection of the skin, subcutaneous tissue, and fascia. If not treated promptly, it can lead to tissue destruction, sepsis, and death. 

The choice of diagnostic method depends on the clinical presentation, site of infection, and availability of resources. 

• The most common method for diagnosing S. viridans infection is by culturing the bacteria from clinical specimens such as blood, urine, or pus. The bacteria can be identified by their characteristic morphology on culture plates and by their biochemical properties. 

• S. viridans is a Gram-positive bacterium that appears purple under the microscope after Gram staining. 

• Serological tests such as the latex agglutination test or enzyme-linked immunosorbent assay (ELISA) can detect antibodies against S. viridans in the patient’s serum. 

• Polymerase chain reaction (PCR) and DNA sequencing can be used to detect and identify S. viridans by amplifying and analyzing specific regions of the bacterial genome. 

• Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS): This method is increasingly used in clinical microbiology laboratories to identify bacterial species rapidly and accurately, including S. viridans. 

Good oral hygiene can lower the risk of infection. Brushing teeth twice daily, flossing regularly, and using an antiseptic mouthwash can help prevent S. viridans from colonizing the mouth and causing infections such as dental caries or endocarditis. 

• Good hand hygiene is essential to prevent the spread of S. viridans and other infectious agents. Hands should be washed frequently with soap and water or alcohol-based hand sanitizer, especially after contact with contaminated surfaces or materials. 
• In healthcare settings, strict infection control measures such as isolation precautions, disinfection of surfaces and equipment, and proper handling and disposal of contaminated materials should be implemented to prevent the spread of S. viridans infections. 
• Individuals with underlying medical conditions such as diabetes, heart disease, or immunodeficiency should be adequately managed to reduce the risk of S. viridans infections. 
•  Antibiotic prophylaxis can be given to individuals at high risk of developing S. viridans infections, such as those undergoing invasive dental or surgical procedures, to prevent bacterial colonization and subsequent infection. 

https://en.wikipedia.org/wiki/Viridans_streptococci 

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https://www.ncbi.nlm.nih.gov/books/NBK7611