Do antibody-coated bacteria reflect local immune response in the urinary tract?

We examined whether antibody-coated bacteria (ACB) reflect local immune response or passive adsorption of immune globulins (Ig). For this purpose, bacterial subcultures from infected urines and third-party bacteria were incubated with Ig preparations. These were obtained from infected urine either by removal of Ig from ACB (dissociation in alpha-methylmannoside, 0.1 M glycine-HCl, or 3 M NaSCN) or by staphylococcal protein A affinity chromatography from the supernatant of infected urine. Coincubation of either Ig preparation with bacterial substrains of the original urine and with third-party strains (Pseudomonas aeruginosa, Proteus mirabilis, several Escherichia coli strains, enterococci, Serratia marcescens, Klebsiella pneumoniae, Staphylococcus aureus) caused coating of the bacterial surface with Ig. Coating was not measurably influenced by coincubation with various carbohydrates or by preincubation of bacteria with glucosidases. Adsorption of Ig was observed after heat denaturation and formalin treatment of bacteria and was independent of bacterial growth phase. Coating was observed both with intact IgG and with F(ab)2 fragments. We further examined the proportion of bacteria coated with IgA, IgG, and IgM in urines of 200 bacteriuric women with symptomatic urinary tract infection. The frequency histogram plot showed a continuous nonlinear unimodal distribution. Consequently, any selection of a 'normal range' is arbitrary. It is concluded that coating of the bacterial surface with Ig may not necessarily reflect local immune response. A major problem of the ACB test is definition of the normal range.