Before an effective strategy for prevention and intervention can be developed the etiology or pathogenesis of the "problem" needs to be understood.  Many times this understanding comes through years of research and continues to evolve over time.  This is true for catheter-related bloodstream infection.  Current evidence suggests that the source of most CRBSI is the resident microorganisms on the patient’s skin.  The primary organism is coagulase-negative Staphylococcus spp (CoNS) that normally resides on skin and is now considered a pathogenic organism in the medical world today.  Other sources of organisms come from exogenous microflora from healthcare personnel or contaminated infusates.  Colonization of a catheter can occur rapidly (within 24 hours) and may be a function of the host-produced conditioning films (platelets, plasma, and tissue proteins).

 

Pathogenesis

 

Many microorganisms attach to living and nonliving surfaces. Some of these microorganisms tightly bind to the catheter surface, form biofilms, and are highly resistant to antimicrobial treatment.  There is a strong link between biofilm formation and CRBSI.  Biofilms may be composed of a single species or multiple species, depending on the device and its duration of use in the patient.  Biofilm forms a layer which is known as the slime layer and under scanning by electron microscopy appears either as thin strands connecting the cells to the surface and to one another or as sheets of murky or undefined material on the surface of the catheter.

A distinguishing characteristic of biofilms is the presence of extracellular substances, primarily complex sugars (polysaccharides), surrounding and encasing the cells.  Once these cells irreversibly attach and produce extracellular polysaccharides to develop a biofilm, the rate of growth is influenced by flow rate, nutrient composition of the medium, antimicrobial-drug concentration, and ambient temperature.  All of which means the patient’s bloodstream and catheter serve as an ideal starting place for a CRBSI.

 

Biofilms on catheters and medical devices may be composed of three different types of microorganisms, and the most common are:

 

  • Gram Positive - Enterococcus faecalis, Staphylococcus aureus, Staphylococcus epidermidis, and Streptococcus viridans
  • Gram Negative - Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, and Pseudomonas aeruginosa
  • Yeasts - Candida albicans

 

 

As a rule of thumb, biofilm formation depends on the duration of catheterization:

 

  • Short-term catheters (< 8-10 days) have greater biofilm formation on the external surface
  • Long-term catheters (>30 days) have more biofilm formation on the catheter inner lumen

 

According to the CDC, bloodstream infection (BSI) is considered to be associated with a central line if the line was in use during the 48-hour period before development of the BSI.  Catheter-related BSI is defined as “bacteremia/fungemia in a patient with an intravascular catheter with at least one positive blood culture obtained from a peripheral vein, clinical manifestations of infections, and no apparent source for the BSI except the catheter.  One of the following should be present: a positive semi-quantitative (>15 CFU/catheter segment) or quantitative (>103 CFU/catheter segment catheter) culture whereby the same organism is isolated from the catheter segment and peripheral blood; simultaneous quantitative blood cultures with a 5:1 ratio CVC versus peripheral; or differential period of CVC culture versus peripheral blood culture positivity.

 

References:

CDC guidelines for Prevention of Intravascular Catheter-related Infections.

Biofilms and Device-Associated Infections

Trautner BW., Darouiche RO. Catheter-associated infections. Arch Intern Med 2004:164: 842-850.

 

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