The amount of secreted IgA may not determine the secretory IgA coating ratio of gastrointestinal bacteria

It is reported that some, but not all, bacteria in human faeces are coated with secretory immunoglobulin A (S-IgA). We evaluated the proportion of S-IgA-coated bacteria to total intestinal bacteria (S-IgA coating ratio) in the gastrointestinal tract of two different strains of mice supplied by two different suppliers. The S-IgA coating ratio was significantly different in each gastrointestinal segment and between mouse suppliers. The amount of non-bacteria-bound IgA (free IgA) in each gastrointestinal segment indicated that this difference in the S-IgA coating ratio might not be due to the amount of secreted IgA. Furthermore, immunoblotting analysis revealed that only a small amount of IgA (<5% to free-IgA) was used for the coating. This indicates that, although sufficient S-IgA was secreted to coat the entire intestinal population of bacteria, only some part of the bacteria were coated with S-IgA. This study suggests that the amount of luminal S-IgA may not determine the S-IgA coating ratio, and that the amount of IgA coating intestinal commensal bacteria is very small.