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Serum antibody levels of the (a) IgM, (b) IgG and (c) IgA isotype against Escherichia coli heat-extracted antigen in rats colonized with E. coli alone (□) or with E. coli and Lactobacillus plantarum (Δ). Antibody titres were determined as the reciprocal of the dilution having an optical density of 0.2 above background level. The controls include germ-free rats matched by age. The error bars represent s.e.m. ○, Control.
