Can stored MariPOC test swabs be used for culture purpose?

To the Editor Human nasopharynx is colonized by both commensal and pathogenic bacteria like Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis and Staphylococcus aureus (1). Culture is a conventional method for detecting pathogenic bacteria from nasopharyngeal swabs. In recent years, new point of care (POC) tests have become popular, especially in primary health care. MariPOC (ArcDia International Ltd, Turku, Finland) is a novel POC test system for rapid detection of respiratory tract infections (2, 3). It is based on two-photon excitation fluorometry (TPX). The technique applies polystyrene microparticles as solidphase reaction carriers for the immunocomplex formation. As a result of the antibody–antigen interactions, three-component immunocomplexes of monoclonal antibody – antigen – labelled monoclonal antibody are formed on the microspheres in proportion to the analyte concentration (2). In this study, we wanted to investigate whether the stored MariPOC respiratory pathogen test swabs can be used later for research purposes like culturing and microbial genetic studies. We re-examined nasal swabs by culturing after 3 years of preservation in MariPOC test buffer (RTI buffer) at 70 °C. Twenty-nine Finnish families whose child participated in the birth cohort study (the STEP study) (4) were recruited in a substudy examining respiratory viruses and bacteria in the family setting using MariPOC tests. Index children had respiratory infections and were born in 2008–2010. Parents were trained by study nurses to obtain nasal swab samples at home. Eleven families were excluded because none of family members were tested positive for S. pneumoniae by MariPOC test. Altogether 228 samples from 18 families were included in this study. From the onset of the symptoms of respiratory tract infection in any family member, parents took nasal swabs (Copan Diagnostics Inc, Murrieta, CA, USA) from both nostrils of each family member (children and adults) twice a week for 3 weeks. Swabs without transport media were sent by postal mail to the study clinic. In the laboratory, swabs were either analyzed immediately for pneumococcal and viral antigens or stored in transport tubes at 20 °C until the antigen analysis. After the antigen testing, swabs were stored in RTI buffer at 70 °C. Samples used in this study were collected between July 2011 and March 2012. Figure 1 illustrates the workflow for the samples. The study protocol was approved by the Ethics Committee of The Hospital District of Southwest Finland. Parents of all participants gave their written informed consent. MariPOC tests were performed according to manufacturer’s instructions (2, 3) 3–7 days after sample collection. For pneumococcal culture, 50 lL of RTI buffer was inoculated onto a selective sheep blood agar plate, containing gentamicin (2, 5 lg/mL) and spread evenly over the surface with a spreader (5). The semi-quantitative culture was used for detecting other bacteria (5). Plates were incubated in 5% CO2 atmosphere at 35 °C for 24 h. If the first read out was negative, incubation was extended to other 24 h. Alpha-haemolytic bacterial colonies were verified by typical morphology, optochin sensitivity testing and when needed, by bile solubility testing. After verification, S. pneumoniae isolates were serotyped by multiplex PCR and Quelling reaction was used for confirmation when needed (6). Other bacteria including