Gram-negative bacteria, which cause a variety of diseases including meningitis, plague, sepsis and food poisoning, are becoming increasingly antibiotic resistant. One reason is their robust outer membrane (OM) which can resist antibiotic penetration. We have been combining our recently developed OM model with neutron science to measure the penetration of an important antibiotic, polymyxin (PMB), into the OM. These have shown the importance of the critical melting of the OM at body temperature in allowing PMB to work. Vancomycin is an important antibiotic which however does not act on Gram negative bacteria, Recently it was shown that it does work on these cells but at 15oC. It is also more effective on cells with complex polysaccharides on their surface, again opposite to PMB. Thus we wish to test the effect of Vancomycin on our models at different temperatures and lipid complexities.