The present invention relates to vaccines, in particular, to an attenuated gram-negative cell comprising the SP12 gene locus, wherein at least one gene of the SP12 locus is inactivated, wherein the inactivation results in an attenuation/reduction of virulence compared to the wild type of said cell, and to a carrier for the presentation of an antigen to a host, which carrier is the attenuated gram-negative cell, wherein the cell comprises at least one heterologous nucleic acid molecule comprising a nucleic acid sequence coding for the antigen, wherein the cell is capable of expressing the nucleic acid molecule or capable of causing the expression of the nucleic acid molecule in a target cell.