What conditions or events cause bacteremia?

What Causes Bacteremia

Bacteremia results from the entry of bacteria into the bloodstream through disruption of mucosal or skin barriers, with the most common mechanisms being trauma to colonized mucosal surfaces, presence of intravascular devices, surgical procedures, and underlying infections that seed the bloodstream. 1

Primary Mechanisms of Bacterial Entry into Bloodstream

Mucosal Trauma and Transient Bacteremia

• Trauma to mucosal surfaces releases endogenous bacterial flora into the bloodstream, particularly from densely colonized sites including the gingival crevice around teeth, oropharynx, gastrointestinal tract, urethra, and vagina 1

• The frequency and intensity of bacteremia correlates with the magnitude of tissue trauma, density of microbial flora at the site, and degree of inflammation or infection present 1

• Dental procedures cause bacteremia in 0-97% of cases depending on the procedure type, with tooth extractions (0-96%), teeth cleaning (0-78%), restorations (16-66%), and dental injections (16-97%) 1

• Routine daily activities cause transient bacteremia more frequently than invasive procedures, with toothbrushing causing bacteremia in 39-46% of cases and chewing potentially causing bacteremia, though the cumulative exposure from daily activities likely accounts for most cases of infective endocarditis 1

• Gingival inflammation leads to thinning and ulceration of the crevicular mucosa, allowing dense bacterial colonies direct access to the increased gingival capillary circulation, resulting in bacteremia from minimal manipulation 1

Intravascular Devices and Foreign Bodies

• Intravascular catheters cause bacteremia through two mechanisms: direct endothelial damage from the device rubbing against tissue, and bacterial entry at the skin insertion site or through the catheter lumen 1

• Central venous catheters, dialysis vascular catheters, and implantable cardiac device leads create sites for bacterial colonization and biofilm formation 1, 2

• More than 92% of blood cultures growing Staphylococcus aureus represent true catheter-related or primary bacteremia rather than contamination 3

• The presence of central vascular catheters is an independent risk factor for persistent bacteremia (defined as positive cultures on different calendar days during the same episode) 4

Surgical and Procedural Sources

• Recent gastrointestinal or obstetric-gynecologic surgery predisposes to anaerobic bacteremia 5

• Abdominal and pelvic surgery is significantly associated with Gram-negative anaerobic bacteremia 6

• Direct infection of indwelling devices can occur at the time of placement (surgical site infection), even despite antibiotic prophylaxis 1

Underlying Infections and Anatomic Sources

• Specific infection syndromes that cause bacteremia include:

  • Pneumonia with positive sputum, BAL culture, or lung biopsy 1
  • Pyelonephritis with positive urine culture, pyuria, and graft or costovertebral angle tenderness 1
  • Intra-abdominal abscesses documented by CT/ultrasound with positive cultures 1
  • Cholangitis with fever, right upper quadrant pain, and abnormal liver function tests 1

• Intestinal obstruction and undrained abscesses serve as sources for bacteremia 5

Host Factors That Predispose to Bacteremia

Immunocompromising Conditions

• Malignant neoplasms are significantly associated with Gram-negative anaerobic bacteremia 6, 5

• Hematologic disorders, organ transplantation, and use of cytotoxic agents or corticosteroids increase risk 5

• HIV/AIDS and other immunocompromised states dramatically increase susceptibility to recurrent Gram-negative sepsis 7

Chronic Medical Conditions

• Diabetes mellitus is significantly associated with Gram-positive anaerobic bacteremia 6 and increases risk for S. aureus bacteremia 2

• Cirrhosis is an independent risk factor for persistent bacteremia 4

• Chronic kidney disease, end-stage renal disease, and conditions requiring immunosuppressive therapy increase susceptibility 7

• Post-splenectomy state increases risk for anaerobic bacteremia 5

Specific High-Risk Populations

• Neonatal bacteremia is associated with prolonged labor, premature rupture of membranes, maternal amnionitis, prematurity, fetal distress, and respiratory difficulty 5

• Pediatric bacteremia occurs with chronic debilitating disorders including malignant neoplasm, hematologic abnormalities, immunodeficiencies, chronic renal insufficiency, or decubitus ulcers 5

• Burns are an independent risk factor for persistent bacteremia 4

• Injection drug use is a major risk factor for S. aureus bacteremia and right-sided endocarditis 2

Pathophysiologic Sequence Leading to Bacteremia

Endothelial Damage and Bacterial Adherence

• Turbulent blood flow from congenital or acquired heart disease traumatizes the endothelium, creating sites for platelet and fibrin deposition (nonbacterial thrombotic endocarditis) 1

• Damaged or denuded endothelium is necessary for initial pathogen colonization 1

• Bacterial surface adhesins (such as FimA protein in viridans streptococci and microbial surface components in staphylococci) facilitate attachment to fibrin-platelet matrix and extracellular matrix proteins 1

Bacterial Characteristics

• The microbial species entering circulation depends on the unique endogenous microflora colonizing the traumatized site 1

• Viridans group streptococci and other oral flora commonly cause bacteremia from dental sources 1

• Gram-negative anaerobic bacteria (mostly Bacteroides fragilis group) account for the majority of anaerobic bacteremia 5

• Polymicrobial infections are an independent risk factor for persistent bacteremia 4

Critical Clinical Pitfalls

• A normal white blood cell count does NOT rule out serious staphylococcal bacteremia and should not delay aggressive antimicrobial treatment 3

• Inappropriate empirical antibiotic treatment is an independent risk factor for persistent bacteremia 4

• A single positive blood culture for coagulase-negative staphylococcus without symptoms is often a contaminant, but multiple positive cultures with compatible clinical signs represent true infection 1, 3

• Klebsiella is NOT a typical contaminant organism and should be considered true bacteremia when isolated from blood cultures 8

• Persistent bacteremia (≥48 hours for S. aureus) is associated with 90-day mortality risk of 39% and requires thorough search for metastatic foci 2