The inoculated plates were incubated for 18 h and 48 h at 35☌ in a CO 2 incubator. Different ‘fresh’ and whiteboard marker pens were used each time to avoid the dilutional effect of repeated sampling. Each pen was inoculated onto blood agar plates at set times: 0 (immediately after inoculation), 1, 2, 3, 4, 5, 10, and 30 min. The 10 5 CFUs/ml concentration of the same MRSA strain was used to inoculate 8 new permanent marker pens (Pentel W10), 8 new whiteboard (non-permanent) marker pens and 1 ‘dry’ permanent marker pen (cap removed and tip exposed for 2 h). Each pen was used to inoculate two blood agar plates using the methods previously described at 0 h (immediately after inoculation), 4 h, and 24 h and the plates incubated for 18 h at 35☌ in a CO 2 incubator. One pen was used as the control and, therefore, not inoculated. 5 Four permanent marker pens (Pentel N50) were inoculated by dipping into one of the MRSA concentrations for approximately 1 s. aureus concentrations on intact living skin: 10 2–10 3 CFUs/cm 2 on areas such as the forearm and leg, to 10 5 CFUs/cm 2 on the soles of the feet, axillae, forehead, and scalp.
These concentrations were chosen based on different S. aureus NCTC 12493, a methicillin-resistant strain (MRSA) commonly used in clinical laboratories, was used to perform the remaining parts of the experiment, at concentrations of 10 3, 10 4 and 10 5 colony forming units CFUs)/ml. Aerobic culture of the blood agar plates was undertaken at 37☌ in a CO 2 incubator and examined for growth at 24 h and 72 h. The pen tips were sampled by both ‘writing’ with the tip on an agar surface for one set of culture plates and rolling the side of the tip across the agar for the other set. Twenty-six permanent marker pens (of brands Penflex, Pentel, Guilbert, Niceday, and Edding) were collected, 21 from the surgical wards and 5 from surgical doctors within our hospital.
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