Reliability of the ica, aap and atlE genes in the discrimination between invasive, colonizing and contaminant Staphylococcus epidermidis isolates in the diagnosis of catheter-related infections

Objectives   To evaluate the usefulness of detecting two genes involved in biofilm formation ( icaA and aap ) and one gene involved in initial adhesion ( atlE ) for discrimination between contaminant, colonizing and invasive Staphylococcus epidermidis isolates involved in catheter-related infections.
Patients   The first group contained 29 isolates that were isolated from the skin of healthy volunteers (contaminant isolates). The second group contained 16 isolates recovered from catheters (>1000 CFUs on quantitative catheter culture) from asymptomatic patients without bacteremia. These isolates were considered to be colonizing isolates. The third group contained 34 isolates grown in ≥2 different blood cultures from patients with a systemic inflammatory response. These isolates were considered to be invasive isolates.
Results   The prevalence of atlE did not differ between the three groups. The icaA and aap genes were significantly more prevalent in colonizing isolates (88% aap ; 88% icaA ) than in invasive isolates (68% aap , P  = 0.179; 59% icaA , P  = 0.055) and than in skin isolates (52% aap , P  = 0.02; 38% icaA , P  = 0.002).
Conclusions   The high prevalence of aap and icaA in skin isolates and their higher prevalence in colonizing than in invasive isolates led to a low specificity when these genes were used to differentiate between contamination, colonization and invasive infection. We conclude that, although the prevalence of these genes differs in the three groups, their presence cannot be used for clinical decision-making.