Bacterial endocarditis caused by vitamin B6-dependent viridans group Streptococcus

Area of Science:

• Pediatric Infectious Diseases
• Microbiology
• Cardiology

Background:

• Bacterial endocarditis is a serious infection affecting heart valves.
• Viridans group Streptococcus are common causes of endocarditis.
• Some strains exhibit unique growth requirements, such as dependency on vitamin B6.

Observation:

• Two pediatric cases of endocarditis caused by vitamin B6-dependent viridans group Streptococcus are presented.
• These organisms require supplemented media for isolation and accurate susceptibility testing.
• Penicillin tolerance, defined by a high minimum bactericidal concentration (MBC) to minimum inhibitory concentration (MIC) ratio, was observed.

Findings:

• Vitamin B6-dependent Streptococcus can be penicillin-tolerant, leading to treatment failure with single-agent therapy.
• Relapse occurred in one patient treated with a single antibiotic to which the organism was tolerant.
• Susceptibility testing must include both MIC and MBC to identify tolerant strains.

Implications:

• Recognition of vitamin B6-dependent Streptococcus is vital for appropriate diagnosis and management of pediatric endocarditis.
• Initial therapy for suspected cases should include penicillin and an aminoglycoside.
• Monitoring serum bactericidal activity is essential to confirm therapeutic adequacy.