Microbiology
Cardiovascular and Blood Stream Infections
Bloodstream and cardiovascular infections for MBBS: infective endocarditis organisms, Duke criteria, blood culture technique, sepsis and enteric fever, mapped to NMC codes MI2.1 to MI2.7.
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Cardiovascular and Blood Stream Infections
Bloodstream and cardiovascular infections for MBBS: infective endocarditis organisms, Duke criteria, blood culture technique, sepsis and enteric fever, mapped to NMC codes MI2.1 to MI2.7.
This chapter covers the microbes that invade the bloodstream and heart. It deals with infective endocarditis and its causative organisms, bacteraemia and sepsis, enteric fever bacteraemia, and the blood culture techniques and diagnostic criteria used to identify these life-threatening infections.
High-yield: Cardiovascular and Blood Stream Infections
- Infective endocarditis on native valves is most often caused by viridans group streptococci following dental procedures.
- Staphylococcus aureus is the leading cause of acute endocarditis and of endocarditis in intravenous drug users, typically affecting the tricuspid valve.
- Prosthetic valve endocarditis within a year of surgery is commonly due to coagulase-negative staphylococci such as Staphylococcus epidermidis.
- The HACEK group of fastidious Gram-negative organisms causes culture-negative endocarditis and needs prolonged incubation.
- The modified Duke criteria combine blood culture results and echocardiographic findings to diagnose infective endocarditis.
- Three sets of blood cultures drawn from separate sites before antibiotics improve the yield in suspected endocarditis.
- Enteric fever caused by Salmonella Typhi produces a positive blood culture in the first week of illness.
- Bacteraemia is the presence of bacteria in the blood, while sepsis is the dysregulated host response to infection.
- Gram-negative bacteraemia can progress to endotoxic septic shock driven by lipopolysaccharide.
- Automated blood culture systems flag positives and are subcultured for identification and antibiotic sensitivity.
- Catheter-related bloodstream infection is suspected when the same organism grows from the catheter tip and peripheral blood.
- Rheumatic fever is a delayed immunological sequel to group A streptococcal pharyngitis and can damage heart valves.
- Brucella and Coxiella burnetii are important causes of culture-negative endocarditis and need serology to diagnose.
- Candida species cause endocarditis in prosthetic valves and intravenous drug users, producing large friable vegetations.
- The mitral and aortic valves are most often involved in left-sided native valve endocarditis.
Endocarditis by clinical setting
- **Subacute, native valve:** Viridans streptococci, often after dental work on abnormal valves.
- **Acute, native valve:** Staphylococcus aureus, aggressive with valve destruction.
- **Intravenous drug user:** Staphylococcus aureus on the tricuspid valve.
- **Early prosthetic valve:** Coagulase-negative staphylococci such as Staphylococcus epidermidis.
NMC competencies in this chapter
- **MI2.1:** Aetiological agents of cardiovascular and bloodstream infections
- **MI2.2:** Pathogenesis of infective endocarditis, bacteraemia and sepsis
- **MI2.3:** Laboratory diagnosis of endocarditis including blood culture technique
- **MI2.4:** Enteric fever and other systemic bloodstream infections
- **MI2.5:** Antimicrobial approach and sensitivity testing for bloodstream isolates
- **MI2.6:** Prevention of catheter-related and device-associated bloodstream infection
- **MI2.7:** Clinical correlation and interpretation of blood culture reports
Frequently Asked Questions
Which organism most commonly causes subacute endocarditis?
Viridans group streptococci are the classic cause of subacute native valve endocarditis, often seeding an already abnormal valve after a dental procedure.
Why are three sets of blood cultures taken in endocarditis?
Drawing three sets from separate sites before antibiotics improves the chance of capturing the continuous low-grade bacteraemia typical of endocarditis and helps distinguish true infection from contamination.
What causes culture-negative endocarditis?
Prior antibiotics and fastidious organisms such as the HACEK group, Brucella, Coxiella burnetii and Bartonella cause culture-negative endocarditis, which often needs serology to confirm.
How is enteric fever diagnosed early?
Blood culture is the most useful test in the first week of enteric fever, when Salmonella Typhi is circulating, before stool and urine cultures become positive.
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