Timing, localization, and persistence of colonization by segmented filamentous bacteria in the neonatal mouse gut depend on immune status of mothers and pups

As a member of the indigenous gut mucosal microbiota, segmented filamentous bacteria (SFB) colonize the guts of a variety of vertebrates and invertebr ates. They are potent microbial stimuli of the gut mucosal immune system. I n the small intestines of mice and rats, it has been observed that SFB are absent during the suckling period and appear in high numbers shortly after weaning, then quickly retreat to the cecum and large intestine. In this stu dy, we explored whether this microecological phenomenon resulted from the i nteraction between SFB and the passively acquired maternal mucosal immunity and/or the actively acquired mucosal immunity. We set up a mouse model by reciprocal crossings and backcrossings of SFB-monoassociated, formerly germ -free, immunocompetent (+/+) BALB/c mice and immunodeficient (scid/scid) mi ce to produce pups which are either immunocompetent (scid/+) or immunodefic ient (scid/scid) and are born either to immunocompetent (scid/+) mothers or to immunodeficient (scid/scid) mothers. We monitored the number of SFB on the mucosa of the small intestine in the four different groups of mice afte r birth, as well as the level of passively acquired antibodies, the active gut mucosal immune responses, and immunoglobulin A (IgA) coating of SFB in the gut. The results showed that, irrespective of whether the pups were sci d/scid or scid/+, SFB could be found earlier on the mucosa of the small int estine in pups born to scid/scid mothers, appearing from day 13 and rapidly reaching a climax around weaning time on day 28, compared to the significa ntly delayed colonization in the pups of scid/+ mothers, starting from day 16 and peaking around days 28 to 32, After the climax, SFB quickly declined to very low levels in the small intestines of scid/+ pups of either scid/s cid mothers or scid/+ mothers, whereas they remained at high levels in scid /scid pups at least until day 70, the last observation time in this study, The dynamic changes in SFB colonization of the small intestines of the diff erent groups of pups may be related to the dynamic changes in the levels of SFB coated,vith secretory IgA (sIgA), which resulted from the significantl y different levels of sIgA obtained from the mothers' milk during the suckl ing period and, later, of self-produced sIgA in the small intestine. Nevert heless, it is evident that the timing, localization, and persistence of col onization of the neonatal gut by SFB depends on the immune status of both m others and pups.