Overnight cultures of the isolates were normalised to an optical density at 600 nm (OD600) of 0.1 in 10 ml of LB broth and centrifuged at 4,500 xg for 15 minutes. The supernatant was discarded, and the pellet resuspended in 5ml of phosphate-buffered saline (PBS). The resuspension was then sonicated on ice, for three intervals of 15 seconds, with a 30 second break between each sonication, using a Soniprep 150 plus (MSE centrifuges, UK). 90 μl of the supernatant was added to 10 μl of a 0.5 mg/ml nitrocefin solution in a 96 well clear, flat bottom plate. The absorbance of the plate was read at OD450 every 39 seconds for 9 minutes and 45 seconds, using a FLUOstar OMEGA spectrophotometer (BMG lab systems, Germany). Triplicate overnight liquid cultures were grown from the selection of three distinct colonies, each liquid culture was then assayed in triplicate.