Viridans Streptococci and Infective Endocarditis
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Direct answer
Viridans streptococci are alpha-haemolytic, optochin-resistant and bile-insoluble streptococci that live in the mouth and are a common cause of subacute infective endocarditis. They colonise damaged or prosthetic valves after dental procedures or other bacteraemic events, and the disease runs over weeks rather than days. Diagnosis rests on the modified Duke criteria, which combine blood culture results with echocardiographic findings.
What you must remember
- Identity: alpha-haemolytic, optochin resistant, bile insoluble and catalase negative, including Streptococcus mitis, S. oralis, S. sanguinis, S. mutans, S. salivarius and the S. anginosus group.
- Dental origin: oral flora enters the blood during extraction or scaling, and S. sanguinis and S. mitis are the species most often linked to native valve disease.
- Subacute course: weeks of low-grade fever, fatigue and weight loss, with splenomegaly, clubbing and a changing murmur.
- Definite endocarditis: two major criteria, or one major and three minor, or five minor criteria under the modified Duke system.
- Major criteria: typical organisms in two separate blood culture sets, or echocardiographic evidence of a vegetation, abscess or new valve regurgitation.
- Minor criteria: predisposing heart condition, fever of 38 degrees Celsius or above, vascular phenomena, immunological phenomena and microbiology that does not meet a major criterion.
- Culture-negative endocarditis: prior antibiotics, HACEK organisms, Coxiella burnetii, Bartonella species, Tropheryma whipplei and nutritionally variant streptococci that need pyridoxal.
A stepwise approach to suspected endocarditis
A patient with a valve lesion, fever for three weeks and a positive blood culture for a viridans streptococcus needs the same workflow as any suspected case. The order of steps matters because antibiotics given too early can hide the organism.
- Blood cultures first: at least two or three sets are taken from separate venepunctures before antibiotics, since typical organisms in separate sets are a major criterion.
- Echocardiography: a transthoracic study is done first, and a transoesophageal study follows when the first study is negative but suspicion remains high.
- Apply the modified Duke criteria: major and minor findings are tallied to classify the case as definite, possible or rejected.
- Look for culture-negative causes: prolonged incubation and specific serology or PCR for Coxiella and Bartonella are used when cultures stay negative.
- Choose therapy by organism: penicillin-susceptible viridans streptococci are treated with penicillin or ceftriaxone for about four weeks per current guidance, with gentamicin added for part of the course in some cases.
- Prevent recurrence: antibiotic prophylaxis before dental procedures is restricted to the highest-risk cardiac conditions, such as prosthetic valves and previous endocarditis.
Where the exam tries to catch you
Viridans streptococci look harmless on the bench, and endocarditis questions reward a clear picture of the timeline.
- Trap: calling viridans endocarditis acute because the patient looks unwell Correct: viridans infection is subacute and progresses over weeks, whereas Staphylococcus aureus causes acute, destructive valve disease within days.
- Trap: using optochin to pick out viridans streptococci as a group Correct: viridans streptococci are optochin resistant, unlike Streptococcus pneumoniae, so optochin sensitivity points away from this group.
- Trap: labelling a sterile culture as ruling out endocarditis Correct: Coxiella, Bartonella and HACEK organisms need specific tests or prolonged incubation before a culture-negative label is applied.
Frequently asked questions
What are the modified Duke criteria for infective endocarditis?
Modified Duke criteria combine major criteria, such as typical organisms in repeated blood cultures and echocardiographic evidence, with minor criteria, including predisposition, fever and vascular or immunological phenomena. A definite diagnosis of infective endocarditis requires two major criteria, or one major and three minor, or five minor criteria.
Which viridans streptococci are most often linked to dental-origin endocarditis?
Streptococcus sanguinis and Streptococcus mitis are the viridans streptococci most often linked to dental-origin endocarditis of native valves. Streptococcus mutans is better known for dental caries, although it can also cause endocarditis.
Why is viridans streptococcal endocarditis called subacute?
Viridans streptococcal endocarditis is called subacute because the infection progresses slowly over weeks, with low-grade fever, fatigue and weight loss. The organisms are of low virulence, so the valve is damaged gradually, unlike the acute destruction caused by Staphylococcus aureus.
What causes culture-negative infective endocarditis?
Culture-negative infective endocarditis is most often caused by prior antibiotic exposure, fastidious HACEK organisms, Coxiella burnetii, Bartonella species and nutritionally variant streptococci. Serology, PCR or prolonged incubation of cultures can identify many of these organisms.
Is antibiotic prophylaxis given before dental work to every patient with a heart murmur?
Antibiotic prophylaxis before dental procedures is not given to every patient with a heart murmur. Current guidance restricts it to the highest-risk cardiac conditions, such as prosthetic valves and previous infective endocarditis, because the benefit is small and resistance is a real concern.