Laboratory plasticware - Use at your own risk: Suitability of microcentrifuge tubes for spores' analysis of Clostridium difficile.

Clostridium difficile is a Gram-positive spore forming rod-shaped bacterium which causes mild to severe diarrhea. Spores play a key role in transmission of C. difficile in hospital environment. To investigate ability of spores to stay on fomites and to assess levels of contamination it is essential to prevent loss of spores collected for the analysis. Working with C. difficile spores we noticed a significant loss after vortexing of spore suspensions and investigated if it can be prevented by using a specific brand or type of microcentrifuge tubes. 7 types of microcentrifuge tubes from 3 manufacturers were tested. Spores of three types of C. difficile, NAP1/027, NAP4/014 and NAP7/078 (clinical isolates) were used. C. difficile was grown on Brucella Supplemented Agar for 9 days, spores were collected, washed and density of 3 suspensions was normalized to optical density (OD) 550 0.1 or 0.05. These suspensions (OD 0.1) were used in serial dilutions with 3 experimental conditions - pipetting, vortexing or vortexing in 3% albumin solution and in vortexing experiment when 150 μl were vortexed for 1 min at 1500 rpm per tube and loss of spores was measured by a decrease in dipicolinic acid (DPA) concentration measured by time-delayed terbium fluorescence. Inner surface of the tubes was visualized with microscopy to observe adhered spores. In serial dilution experiment, initial concentration of spores would be underestimated by up to 18X in case of vortexing for NAP1 strain and 9X for NAP4 strain. Presence of 3% albumin significantly decreases this effect but does not eliminate it completely. Comparison of 7 types of tubes shows that a single vortexing for 1 min of diluted spore suspension at concentrations of 1.8 × 107 spores per ml leads to a loss of up to 90% of spores in some tubes. Degree of spores' adhesion varied between brands and types of tubes and between tubes of the same type. In some brands there was a significant variability in adhesion between tubes from the same batch. Microscopy after vortexing shows a film of spores attached to the tube's wall. Adherence could be affected by the type of plastic, additives (plasticizers) used in manufacturing and quality of moulds (e.g."diamond polished"). To identify the most appropriate type of tubes for the experiment it is essential to test it beforehand as not every brand is suitable for this purpose. Using tubes with a high degree of adherence could significantly affect measurement of spores' concentration in serial dilutions, e.g. when quantifying spores production by a specific strain or when limits of detection are measured. Also, sensitivity of commercial tests for detection of C. difficile in clinical specimens can be decreased if an unsuitable type of plastic containers and tubes is used.