Two private investment firms have invested in St. Louis-based Express Medical Transporters Inc., a for-profit provider of non-emergency medical and student transportation. Charlotte, North Carolina-based Mosaic Capital Partners LLC announced a debt and equity investment in EMT this week. Pine Street Capital Partners, based out of Albany, New York, was a co-investor in the deal. Keith Butcher, a founding partner of Mosaic, will take a seat on Express' board. Butcher is also a founding partner…