Bloodstream Infections Overview

Bloodstream Infections

• Bloodstream infections (BSIs) are classified into two types:
  • Health care–acquired BSIs (HA-BSIs): Infections occurring in a hospital or healthcare facility after 48 hours of admission.
    • Include device-associated infections (DAIs) like central line-associated bloodstream infections (CLA-BSIs).
  • Community-acquired bloodstream infections (CA-BSIs): Infections occurring outside of healthcare settings or within 2 days of admission.
• BSIs can be caused by various microorganisms, including bacteria, fungi, viruses, and parasites.

Bacteremia

• Bacteremia: Presence of bacteria in the bloodstream
• Can be transient, continuous, or intermittent.
  • Transient bacteremia is often asymptomatic and caused by procedures involving non-sterile sites (e.g., dental procedures).
• Common portals of entry for bacteremia include:
  • Genitourinary tract (25%)
  • Respiratory tract (20%)
  • Abscesses (10%)
  • Surgical wound infections (5%)
  • Biliary tract (5%)

Intravenous Catheter-Associated Infections

• Intravenous catheters (IVs) are commonly used in hospitalized patients for fluid, medication, and nutrition administration.
• Catheter colonization (bacteria or fungi) leads to infections and serious BSIs.
• IV catheter-associated bacteremia (or fungemia) occurs through two routes:
  • Extraluminal spread: Organisms travel from the catheter entry site through the skin to the catheter tip.
  • Intraluminal spread: Organisms migrate along the inside of the catheter to the catheter tip.
• The most common etiologic agents for IV catheter-associated BSIs are skin-dwelling organisms.

Fungal Bloodstream Infections

• Fungemia: Presence of fungi in the blood.
• Commonly occurs in immunosuppressed patients and those with serious illnesses.
• Candida albicans is the most common species causing fungal BSIs.
  • Part of the normal flora in mucous membranes, but can become pathogenic in compromised individuals.

Candida spp.

• Candida albicans:
  • A species within the genus Candida.
  • Can exist as yeast, pseudohyphae, or hyphae depending on the environment.
  • Is a significant opportunistic pathogen, responsible for many fungal infections.
• Pseudohyphae: Elongated yeast cells attached to one another, resembling hyphae.
• True hyphae: Long, tube-like structures with parallel sides, without constrictions.
• Candida spp. are the fourth most common cause of hospital-acquired BSIs in the United States, with a mortality rate of up to 50%.
  • Other species include Candida glabrata, Candida parapsilosis, Candida tropicalis, and Candida krusei.

Pathophysiology of Invasive Candidiasis

• Adherence to surfaces: Candida albicans begins by adhering to surfaces before infection.
• Protease production: Candida albicans produce proteases, enhancing their ability to invade tissues.
• Phenotypic switching: Candida albicans can transition between different forms (yeast, pseudohyphae, and hyphae), promoting infection.
• Biofilm formation: Candida spp. form biofilms (extracellular polysaccharide material) that facilitate hematogenous spread and antibiotic resistance.

Diagnostic Techniques

• Blood culture: The gold standard for diagnosing invasive candidiasis.
• Drug susceptibility testing: Determines the effectiveness of antifungal drugs.

Management of Invasive Candidiasis

• Source control: Eliminating the source of infection (e.g., removing contaminated catheters, draining infected fluids).
• Early effective systemic antifungal treatment: Timely administration of antifungal medication.

References

• Pappas, P.G. et al. (2018). Invasive candidiasis. Nature Reviews Disease Primers doi:10.1038/nrdp.2018.26
• M. Tille, Patricia. "Bailey & Scott’s Diagnostic Microbiology Fourteenth Edition.” (2017), Chapters 62 & 67