Putative permease subunit of 142 aas and 4 TMSs, PmpA.  The N- and C-termni are predicted to be outside.  It belongs to the Duf395/COG2391 family.  Its structural gene, pmpA, is adjacent to pmpB (TC# 9.B.102.5.5) which encodes another small 4 TMS protein of opposite orientation in the membrane.  These genes and pmpC, encoding a transporter of the TSUP family (2.A.102) that might function in sulfur or selenium metabolism (Lin et al. 2015), are localized in a gene cluster concerned with prodigiosin (red pigment (Darshan and Manonmani 2015)) production (Gristwood et al. 2011).  Hence, PmpA/PmpB might export prodigiosin, but no direct evidence is available to support such a possibility.  Prodigiosin is an aromatic compound that does not contain sulfur.
Interestingly, The ability of pigmented strains of Serratia marcescens to grow on bread has led to a possible explanation for transubstantiation miracles, in which eucharistic bread is converted into the body of Christ. Such miracles led to Pope Urban IV to institut the feast of Corpus Christi in 1264. This followed celebration of a mass of Bolsena in 1263, led by a Bohemian priest who had doubts concerning transsubstantiation.  During the Mass, the eucharist appeared to bleed, and each time the priest wiped away the blood, more would appear. This event is celebrated in a fresco in the Pontifical Palace in the Vatican City, painted by Raphael.