Polyclonal coagulase-negative staphylococcal catheter-related bacteremia documented by molecular identification and typing.

Abstract

Coagulase-negative staphylococci (CNS) form part of the skin flora and commonly cause nosocomial bloodstream and catheter-related infections [l]. The increasing use of intravascular catheters and the high number of immunocompromised patients contribute to the importance of CNS as a cause of catheter-related infection. It is commonly assumed that catheter-related bloodstream infection arises from multiplication of a single infecting clone of CNS, whereas blood-culture contamination by skin flora is polyclonal, reflecting the diversity of CNS clones colonizing the skin [2]. However, there are recent reports of polyclonal endocarditis with multiple species or strains of CNS [3,4]. Different strains of CNS may exhibit the same morphology and since identification tests are usually made from a single colony, the true extent of infection remains to be investigated by identification and typing of different isolates from the same samples. Molecular fingerprinting methods can be used to investigate whether blood-culture isolates of CNS represent bacteremia or contamination. We describe a case of catheter-related bacteremia with polyclonal infection by Staphylococcns epidermidis and Staphylococcus hominis documented by molecular identification based on tRNA gene spacer polymorphism [5] and genotyping based on IS256 spacer polymorphism 161. A 58-year-old women suffering from a common acute lymphocytic leukemia diagnosed 6 months previously was receiving chemotherapy treatment through a totally implanted central venous catheter. This patient had neutropenia (total leukocytes 700/p,L, with 89% neutrophils) and developed fever (39.5C) complicated by septic shock and hypotension (blood pressure: 8 mmHg systolic and 4 mmHg diastolic), which led to her admittance to the intensive care unit. Blood tests showed a CRP level of 26 mg/mL, and a fibrinogen level of 902 mg/mL. On the same day, two quantitative comparative blood samples were taken by reflux from the catheter and from a peripheral vein. As described previously [7], 1.5 mL of blood was sampled on each site and inoculated into Isolator 1 .5 tubes (Oxoid, Basingstoke, UK) containing sodium polymethanesulfonate (SPS) and saponin. All specimens were immediately plated at dilutions of 10-1 and by spreading onto blood agar and incubated at 37°C for 72 h. The remaining blood specimens were inoculated into Bactec Plus Aerobic/F* bottles and incubated in an automated blood-culture system (Bactec 9240, Becton Dickinson, Maryland, USA). In addition, three sets of 10-mL blood cultures were taken on the same day at separate time intervals from a peripheral vein and inoculated into Bactec Aerobic/F* Plus, Anaerobic/F* Plus and Lytic/F* media, and incubated in the Bactec system. Microscopic examination (X 1000) of a Gramstained cytocentrihged preparation of lysed cells fiom the lo-' dilution of the catheter blood sample showed the presence of Gram-positive cocci in clusters typical of staphylococci. Quantitative blood cultures were positive the next day: lo6 CFU/mL of CNS grew in the catheter blood and <10 CFU/mL of CNS in the peripheral blood, suggesting a catheter-related bacteremia (diagnostic threshold, ratio CFU catheter/ blood, 1O:l). The three other peripheral blood cultures taken on the same day also grew CNS from all nine bottles. Phenotypic identification of a single colony per sample from the quantitative blood culture by the ID 32 Staph system (bioMCrieux, La Balme Les Grottes, France) showed Staphylococcus hominis in the catheter blood, while two concomitant species of CNS (Staphylococcus hominis, Staphylococcus epidermidis) were identified in the peripheral blood cultures. The standard blood cultures showed in one set of three bottles the presence of Staphylococcus epidermidis, while two species of CNS (Staphylococcus hominis, Staphylococcus epidermidis) were identified in the two other sets