Phenotypic and genotypic characteristics of MPEC strain P4-NR O15:H21H54 and its derivative P4-96. P4-96 has rough colonies (top panel in A) and forms clumps upon mixing with acriflavine (lower panel in A), as opposed to P4-NR that has smooth colonies and does not aggregate. These strains are closely related genetically as demonstrated by PFGE (lanes 1-7 in B; 1-Lambda, 2-P4-96, 3-P4-NR, 4-P4-NR, 5-P333463, 6-K12, 7-Lambda). P4-96 displays reduced proliferation ability in vitro in comparison to ECP4-NR, as indicated by a comparative growth curve (C). P4-NR is serum resistant growing well in normal (N in D) and heat-inactivated (HI in D) bovine serum. P4-96 lack resistance to bovine serum, as indicated by no growth after 60 min of incubation with 80% normal bovine serum (N in image E) and growth in heat-inactivated bovine serum (HI in E). Growth in LB broth were used as controls (C in D–E). Bacterial growth in heat-inactivated (HI) or non-HI (NHI) was also quantified and presented in (F). Figures are of representative results from 3 or more similar experiments.