Positive Blood Cultures, S. aureus Bacteremia & Line Infections
Deciding what is real, what it implies, and how hard to treat it
Interpreting a positive blood culture
Collection. Draw at least two sets, each an aerobic and an anaerobic bottle holding about 20 mL of blood in total, from separate venipunctures after chlorhexidine-alcohol skin antisepsis and before antibiotics. Yield depends on the volume cultured, so underfilled bottles and single sets are the commonest correctable reasons for missed or uninterpretable bacteremia. When a catheter infection is suspected, pair one draw through the catheter with one peripheral draw.
Contaminant or pathogen. Skin organisms growing in only one of two sets are usually contaminants, and the right response is to repeat cultures rather than start therapy. The same organism in two separate sets, or growth in a patient with an intravascular catheter, prosthetic valve, or implanted device, may be true infection. Other organisms are never dismissed, even when they grow in a single bottle.
| Organism | Interpretation | Action |
|---|---|---|
| Coagulase-negative staphylococci | Usually contaminant; real with lines, prosthetic valves, devices | Repeat cultures; treat if ≥2 sets or prosthetic material |
| Corynebacterium, Bacillus (non-anthracis), Cutibacterium, Micrococcus | Usually contaminant | Repeat cultures |
| S. aureus, S. lugdunensis | Always real | Full bacteremia bundle with echocardiography |
| Streptococci, enterococci | Real; a lone viridans streptococcal bottle can be a contaminant | Treat; consider endocarditis |
| Gram-negative rods | Real | Treat; find the urinary, biliary, abdominal, or line source |
| Candida | Real | Remove central lines, echinocandin, dilated eye exam |
S. lugdunensis is coagulase-negative but behaves like S. aureus, causing destructive native-valve endocarditis; a single positive culture warrants treatment and echocardiography.
Organisms that point to a diagnosis
Some isolates reveal the underlying disease or dictate a specific next step.
| Organism | Association | Next step |
|---|---|---|
| Streptococcus gallolyticus (formerly S. bovis) | Colorectal adenoma or carcinoma; endocarditis | Colonoscopy and echocardiography |
| Clostridium septicum | Colorectal (often cecal) cancer; hematologic malignancy with neutropenia; spontaneous gas gangrene | Colonoscopy; urgent debridement of myonecrosis |
| Listeria monocytogenes | Pregnancy, neonates, older adults, impaired cell-mediated immunity (steroids, transplant, TNF inhibitors); soft cheese, deli meats, unpasteurized dairy | Ampicillin; cephalosporins are inactive |
| Candida species | Central lines, parenteral nutrition, broad-spectrum antibiotics | Remove lines, echinocandin, dilated eye exam, daily cultures until clear |
| Streptococcus anginosus group | Occult abscess (liver, lung, brain) | Image for and drain the abscess |
| Nontyphoidal Salmonella, older adult | Infected atherosclerotic aneurysm | CT angiography |
A gram-positive rod in the blood of a pregnant or immunosuppressed patient is Listeria until proven otherwise; do not dismiss it as a diphtheroid. Candida in blood is never a contaminant.
S. aureus bacteremia
A bundle, not a drug. S. aureus seeds valves, bone, joints, and devices early and relapses when a focus is left behind, so every episode of S. aureus bacteremia (SAB) is managed with a set of actions.
- Infectious diseases consultationImproves adherence to the bundle and lowers mortality
- Empiric vancomycinUntil susceptibilities return
- Source controlRemove intravascular catheters; drain abscesses; debride infected tissue
- Repeat blood cultures every 24–48 hUntil negative; the first negative culture starts the treatment clock
- EchocardiographyTTE for all; TEE for persistent bacteremia, prosthetic valve or cardiac device, prior endocarditis, injection drug use, metastatic foci, or an abnormal or nondiagnostic TTE
- Search for metastatic infectionSpine, joints, psoas, lungs; MRI for back pain
| Drug | Role | Cautions |
|---|---|---|
| Cefazolin | MSSA, first line; best tolerated | Poor CNS penetration |
| Nafcillin, oxacillin | MSSA | Phlebitis, interstitial nephritis, neutropenia, hypokalemia |
| Vancomycin | MRSA; empiric therapy | Nephrotoxicity, infusion reaction; inferior to β-lactams for MSSA |
| Daptomycin | MRSA; vancomycin failure (high dose, often with ceftaroline) | CK rise and myopathy (consider holding statins), eosinophilic pneumonia; inactivated by surfactant |
MSSA versus MRSA. Once the isolate is methicillin-susceptible, switch from vancomycin to cefazolin or nafcillin/oxacillin: β-lactams clear MSSA faster, with lower failure and mortality. MRSA is treated with vancomycin, dosed by AUC, or daptomycin; daptomycin cannot treat MRSA pneumonia.
Duration. SAB is uncomplicated only when endocarditis has been excluded, no prosthesis is implanted, follow-up cultures at 2–4 days are negative, fever resolves within 72 hours of effective therapy, and no metastatic infection is found; it then needs at least 14 days of IV therapy counted from the first negative culture. Failing any criterion (endocarditis, osteomyelitis, abscess, persistent bacteremia, retained hardware) makes it complicated, requiring 4–6 weeks.
Persistent bacteremia. Cultures still positive after 72 hours of active therapy signal an undrained focus: endocarditis, a retained line or device, septic thrombophlebitis, abscess, septic arthritis, vertebral osteomyelitis, or epidural abscess. Obtain TEE and targeted imaging, remove hardware, and drain collections.
A confirmed pacemaker or ICD infection requires removal of the entire device and all leads; antibiotics alone fail.
Catheter-related bloodstream infection
Diagnosis. Paired cultures from the catheter and a peripheral vein are drawn before antibiotics. If the catheter culture turns positive at least 2 hours before the peripheral one (differential time to positivity), the catheter is the source; a catheter-tip culture growing the same organism after removal also confirms. Exit-site pus, tenderness along a tunnel, or port-pocket inflammation are local clues. Coagulase-negative staphylococci are the most common cause, followed by S. aureus, enterococci, gram-negative rods, and Candida.
Remove the catheter for:
- S. aureus, Pseudomonas, Candida, or mycobacteria
- tunnel infection or port-pocket abscess
- severe sepsis or septic shock
- endocarditis, septic thrombophlebitis, or other metastatic infection
- bacteremia persisting more than 72 hours despite active therapy
Salvage. Coagulase-negative staphylococcal infection of a long-term catheter that is still needed may be treated through the line with systemic antibiotics plus antibiotic lock therapy for 10–14 days; if the catheter is removed, 5–7 days usually suffices. Replacement catheters go in at a new site whenever possible.
Empiric therapy. Start vancomycin; add gram-negative coverage (cefepime or piperacillin-tazobactam) for sepsis, neutropenia, or a femoral catheter, and an echinocandin when candidemia is likely (parenteral nutrition, femoral catheter, hematologic malignancy, transplant, prolonged broad-spectrum antibiotics).
Prevention. Central line bundles combine hand hygiene, maximal sterile barrier precautions, chlorhexidine-alcohol skin preparation, avoidance of the femoral site in adults, and daily review with prompt removal of catheters that are no longer needed.