 After the ED of B. infantis has been identified defined as the dose capable of producing 50 gut colonization by six weeks of age two additional sequential dose escalations will be performed. The purpose of the final two dose escalations is to determine if successively higher doses of B. infantis result in increased gut colonization or barrier protection; or, alternatively, if a Maximum Effective Dose MaxED for B. infantis exists above which there is no further increase in gut colonization or barrier protection. Following the final dose escalation, Hanleys Rule of Three will be applied in order to determine if lower-frequency adverse events are caused by B. infantis. Hanleys Rule of Three states that in order to identify any adverse events occurring at a frequency of 1:10 or greater with a 95 confidence interval, at least 30 subjects must be enrolled. 