From the Guidelines
Subacute infective endocarditis is most commonly caused by Streptococcus viridans, also known as viridans group streptococci (VGS), as well as other bacteria such as HACEK, and enterococci. These bacteria typically have lower virulence than those causing acute endocarditis, allowing for a more indolent disease course characterized by subtle symptoms developing over weeks to months 1. The most appropriate coverage for patients with a subacute presentation of native valve infection includes S aureus, VGS, HACEK, and enterococci 1. Key characteristics of these bacteria include:
- Being normal inhabitants of the oral cavity or gastrointestinal tract
- Having lower virulence than acute endocarditis-causing bacteria
- Preferentially infecting patients with pre-existing cardiac abnormalities like valve disease or prosthetic valves
- Requiring prolonged antibiotic therapy (4-6 weeks) tailored to culture results, often with a combination of beta-lactams and aminoglycosides for synergistic effect. Some of the significant bacterial causes of subacute infective endocarditis include:
- Streptococcus bovis (associated with colon pathology)
- Enterococcus species
- Coagulase-negative staphylococci (particularly Staphylococcus epidermidis)
- The HACEK group of organisms (Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, and Kingella) 1.
From the Research
Bacteria Causing Subacute Infective Endocarditis
- The most common organisms causing subacute bacterial endocarditis (SBE) are the viridans streptococci 2.
- Viridans streptococci differ in their propensity to cause SBE, which is related to the ability to adhere to damaged heart valves and endothelium 2.
- Streptococcus intermedius is a member of the S. anginosus group and a rare cause of SBE 2.
- Other species of viridans streptococci that can cause infective endocarditis include Streptococcus sanguis, Streptococcus mitis, Streptococcus mutans, and Streptococcus milleri 3.
Characteristics of Viridans Streptococci
- Viridans streptococci are relatively avirulent pathogens in normal hosts, but some strains are inherently more virulent 2.
- The virulence of viridans streptococci is related to their ability to produce extracellular matrix and adhere to damaged heart valves and endothelium 2.
- Some strains of viridans streptococci are resistant to penicillin, which can pose therapeutic challenges 4, 5.
Treatment of Infective Endocarditis Caused by Viridans Streptococci
- The treatment of infective endocarditis caused by viridans streptococci typically involves antibiotic therapy, with penicillin being the drug of choice for susceptible strains 6.
- For penicillin-resistant strains, alternative antibiotics such as ceftriaxone, vancomycin, and gentamicin may be used 4, 5.
- The combination of gentamicin with a beta-lactam antibiotic or vancomycin may be more effective than monotherapy in clearing the infection 5.