Gel immune diffusion tests: Two dimensional immuno-diffusion tests (Ouchterlony, 2010) were performed in Petri dishes containing 12.5 ml 0.75% agar in PBS (0.02 M, pH 7.6), with 0.02% sodium azide. Fifteen microliter diluted or undiluted antigen and antiserum were charged into 3 mm diameter wells cut into the agar using a cork borer, and spaced 3.5 mm between each well. Unless otherwise stated test antigen consisted sonicated bacteria (Tomlinson and Walkey, 1967) which diffused satisfactorily through the agar gel.