Antibiotic-resistant bacteria, particularly gram negative species, present 
significant health care challenges. The permeation of antibiotics through the 
outer membrane is largely effected by the porin superfamily, changes in which 
contribute to antibiotic resistance. A series of antibiotic resistant E. coli 
isolates were obtained from a patient during serial treatment with various 
antibiotics. The sequence of OmpC changed at three positions during treatment 
giving rise to a total of four OmpC variants (denoted OmpC20, OmpC26, OmpC28 and 
OmpC33, in which OmpC20 was derived from the first clinical isolate). We 
demonstrate that expression of the OmpC K12 porin in the clinical isolates 
lowers the MIC, consistent with modified porin function contributing to drug 
resistance. By a range of assays we have established that the three mutations 
that occur between OmpC20 and OmpC33 modify transport of both small molecules 
and antibiotics across the outer membrane. This results in the modulation of 
resistance to antibiotics, particularly cefotaxime. Small ion unitary 
conductance measurements of the isolated porins do not show significant 
differences between isolates. Thus, resistance does not appear to arise from 
major changes in pore size. Crystal structures of all four OmpC clinical mutants 
and molecular dynamics simulations also show that the pore size is essentially 
unchanged. Molecular dynamics simulations suggest that perturbation of the 
transverse electrostatic field at the constriction zone reduces cefotaxime 
passage through the pore, consistent with laboratory and clinical data. This 
subtle modification of the transverse electric field is a very different source 
of resistance than occlusion of the pore or wholesale destruction of the 
transverse field and points to a new mechanism by which porins may modulate 
antibiotic passage through the outer membrane.