Fig. 1

Layout of a the operating room (OR) and b the ultrasound room (UR). Red circles show location of settle plates, black circles location of negative control settle plate and dip slide, and red arrow approximate location for near-patient surface (dip slides) sampled. Distances from the midpoint of the surgical table (OR) to the location of settle plates were: ~ 1.6 m (anaesthesia machine), ~ 2 m (computer table), and ~ 2 m (shelf). Distances from the location for near-patient surface sampling (UR) to location of settle plates were: ~ 2.5 m (computer table) and ~ 1 m (ultrasound machine). Settle plate sampling locations were at a height of ~ 0.9-1.1 m. Equipment (size not according to scale): am anaesthesia machine, cb cupboard, ct computer table, du drawer unit, et examination table, ot operating table, pc pass-through cabinet, sh shelf, um ultrasound machine