a-Hemolytic Streptococci

Alpha-hemolytic streptococci, a subgroup of the Streptococcus genus, are characterized by their partial hemolysis on blood agar plates, producing a greenish discoloration around the colonies. 

This group includes Streptococcus pneumoniae and viridans streptococci, both significant in clinical settings. 

S. pneumoniae is a major cause of community-acquired pneumonia and other serious infections such as meningitis and septic arthritis, primarily due to its polysaccharide capsule and various virulence factors. 

Despite advancements in vaccination, emerging drug-resistant strains complicate treatment. 

Viridans streptococci, prevalent in the oral cavity and pharynx, typically exhibit low pathogenicity but can cause severe infections like endocarditis, especially in immunocompromised individuals. 

Effective identification and treatment of these bacteria are crucial, relying on precise diagnostic techniques and tailored antimicrobial therapies.

What is a-Hemolytic Strep? 

Alpha-hemolytic streptococci are a group of bacteria within the Streptococcus genus that exhibit characteristic partial hemolysis on blood agar plates. [5., 11.] This partial hemolysis appears as a green discoloration around bacterial colonies, which is why these organisms are often referred to as "viridans streptococci".

However, alpha-hemolytic streptococci can be difficult to identify using phenotypic methods due to their complex taxonomy and the presence of closely related species. [7.]

Traditional identification methods include hemolysis on blood agar, biochemical tests, and serological grouping, but these are recognized by some as inadequate. [7.] 

Members of a-Hemolytic Streptococcus Group

The members of the alpha-hemolytic streptococci are split into two main groups:

• Streptococcus pneumoniae
• Viridans Streptococci

Some sources report that S. pneumoniae is a subgroup of viridans streptococci, so named for the green halo around bacterial colonies. [11.] 

Streptococcus Pneumoniae [3., 9.] 

Overview of S. pneumoniae

Streptococcus pneumoniae, a gram-positive, lancet-shaped bacterium, is a major cause of community-acquired pneumonia (CAP), with infections peaking in winter and early spring. 

It can also lead to severe conditions like meningitis, septic arthritis, and otitis media. 

Historically the leading cause of pneumonia before antibiotics, S. pneumoniae now accounts for 15% of cases in the U.S. 

Its pathogenicity is primarily due to its polysaccharide capsule, which prevents phagocytosis, and various virulence factors like pneumolysin and IgA1 protease. 

Despite advancements in vaccination, particularly with the PCV13 and PPSV23 vaccines, drug-resistant strains are emerging, complicating treatment. 

Accurate diagnosis often requires a combination of blood cultures, sputum analysis, and urinary antigens, with chest radiography as the primary diagnostic tool.

Management includes timely antibiotic therapy and supportive care, tailored to the severity of the disease. Immunization and risk factor reduction are crucial preventive measures, especially for high-risk groups like young children, the elderly, and those with underlying health conditions. 

Natural Reservoir of Streptococcus pneumoniae in Communities [6.] 

Streptococcus pneumoniae commonly inhabits the respiratory tract and can be found in the nasopharynx of 5% to 90% of healthy individuals, with rates varying by age, environment, and presence of upper respiratory infections. 

Among school-age children, 20% to 60% may be colonized, while 5% to 10% of adults without children carry the bacterium. In military settings, colonization rates can reach 50% to 60%. 

The duration of carriage is generally longer in children than in adults, though the relationship between carriage and the development of natural immunity remains poorly understood.

Transmission of S. pneumoniae [6.] 

Streptococcus pneumoniae is a human pathogen, with its primary reservoir being the nasopharynx of asymptomatic individuals. There is no animal or insect vector for this bacterium. 

Transmission occurs through direct person-to-person contact via respiratory droplets or autoinoculation in those who carry the bacteria in their upper respiratory tract. 

Various pneumococcal serotypes have different propensities for causing asymptomatic colonization, otitis media, meningitis, and pneumonia. 

The spread of the organism within families or households is influenced by factors such as household crowding and viral respiratory infections. 

Pneumococcal infections are more common during the winter and early spring when respiratory diseases are more prevalent. 

While the exact period of communicability for pneumococcal disease is unknown, it is presumed that transmission can occur as long as the bacterium is present in respiratory secretions.

Treatment of S. pneumoniae Infections

Initially, a broad-spectrum antibiotic is typically preferred, although clinicians may choose to use a medication with a more narrow focus. [2.] 

Viridans Streptococcus [1., 4.] 

Overview of Viridans Streptococcus

Viridans group streptococci (VGS) are a diverse group of organisms that inhabit the human gastrointestinal and genitourinary tracts and oral mucosa. 

Typically, VGS are low in pathogenicity for immunocompetent individuals but can cause severe diseases like endocarditis, intra-abdominal infections, and shock in certain populations, including immune compromised individuals.

The infections caused by VGS can manifest in various forms including pulmonary infections, skin and soft tissue infections, bone and joint infections, and meningitis. Intra-abdominal infections were noted as the most common source of bacteraemia in one study. [1.]

VGS are common causes of native valve infective endocarditis (IE) and late prosthetic valve endocarditis. They are also isolated in dental, hepatic, or gastrointestinal abscesses. [1.] 

Diabetes mellitus was the most common comorbidity associated with VGS infections. Other common comorbidities included chronic kidney disease, chronic liver disease, and chronic neurological disease. [1.] 

The VGS group, which includes major groups such as S. mutans, S. salivarius, S. anginosus, S. mitis, S. sanguinis, and S. bovis, exhibits varying antimicrobial resistance patterns. [4.]  

Significant resistance to penicillin and ceftriaxone among VGS species has been reported. [1.]  

Identification of VGS to the species level is challenging due to inconsistent taxonomy and phenotypic variability, but techniques like MALDI-TOF mass spectrometry are emerging as promising methods for accurate identification. 

Pediatric-specific data indicate that S. anginosus group organisms are significant pathogens in children and may play a role in diseases in cystic fibrosis patients. 

Additionally, antimicrobial resistance rates in pediatric VGS are rising, outpacing those in adults, highlighting the need for careful antibiotic use and improved identification methods to manage VGS infections effectively.

Subtypes of Viridans Streptococcus [12.] 

Viridans streptococci are key members of the oral and pharyngeal microbiota, notably the mitis and salivarius groups. They inhibit the colonization of pathogens, including pyogenic streptococci, through two mechanisms: producing bacteriocins and hydrogen peroxide, which also causes α-haemolysis.

Mitis Group

Species like Streptococcus mitis, Str. oralis, and Str. sanguinis inhabit tooth surfaces and mucosal membranes. These bacteria can enter the bloodstream during dental procedures, especially if gingival tissue is inflamed.

In healthy individuals, they are usually cleared from circulation quickly. However, in those with heart valve damage or other predisposing conditions, they can cause subacute bacterial endocarditis, leading to scarring and functional deficiency of heart valves. 

This condition can result in embolic abscesses in organs like the brain. Str. mitis and Str. oralis are also known to cause severe septicaemia in immunocompromised patients.

Mutans Group

Str. mutans and Str. sobrinus colonize tooth enamel and are linked to sugar consumption. They are major contributors to dental caries due to their lactic acid production, even at low pH levels. 

Like other plaque streptococci, they can cause subacute bacterial endocarditis.

Anginosus Group

Str. anginosus and Str. intermedius are found on tooth surfaces, particularly in gingival crevices. They are often isolated from abscesses and other purulent infections.

Bovis Group

Species from this group reside in the human gut and can cause bacteraemia and subacute endocarditis, often linked to colonic carcinoma, which compromises the intestinal barrier. [8., 12.]

In summary, while viridans streptococci are generally low in virulence, they can cause serious infections, especially in immunocompromised individuals or those with underlying health conditions. 

Their role in dental health and disease, as well as their potential to cause endocarditis  highlights their clinical significance.

Natural Reservoir of Viridans Streptococcus in Communities [11.] 

Viridans streptococci are part of the normal flora of the oropharynx and live in close association with the teeth and gingiva. 

They are dominant members of the resident microbiota of the oral cavity and pharynx in all age groups.

Some nonpathogenic species are present in the human gut, where they help maintain a balanced microbiota. [8.] 

Treatment of Viridans Streptococcus Infections [13.]

Treatments depend on the organism involved and antibiotic susceptibility, as well as the severity of the infection.  Combination antibiotic therapy may be recommended.

The treatment of viridans streptococcal endocarditis involves tailored antibiotic regimens based on penicillin susceptibility, patient tolerance, and clinical response. Close monitoring and appropriate adjustments are crucial for successful outcomes.

Lab Testing for a-Hemolytic Strep

Test Information, Sample Collection and Preparation 

A gram stain and culture is a routine test run to determine the infectious agent present in a particular sample.  Also, antibiotic sensitivity should be assessed. [9.]

Samples for culture may include blood, sputum, lung aspirate, cerebrospinal fluid (CSF), joint fluid, stool, or other samples that hold concern for a-hemolytic streptococcal infection. 

Alpha-hemolytic streptococcus levels may also be tested in stool as part of the human microbiome.

Sample collection procedures will depend on the nature of the sample to be used. It is important to consult with the ordering physician prior to sample collection to determine if any special preparation is required. 

Interpretation of Test Results

Assessing for Alpha-Hemolytic Streptococcus Infections

The presence of pathogenic organisms always requires appropriate treatment and followup.  These organisms are usually determined via clinical picture, sample cultures; with concern for S. pneumoniae pneumonia, an x-ray may also be used.

Assessing for Alpha-Hemolytic Streptococcus Levels in the Microbiome

Increasingly, advanced testing is available to assess for levels of particular gut microbiome organisms.  The alpha-hemolytic streptococcus family that naturally resides in the GI tract is considered non-pathogenic.  

Some companies offer the ability to test for the levels of these microorganisms in the GI tract, and to assess for pathogenic variants as well; click here for a sample report.