The invention relates to a modified Neisseria lactamica, wherein the modified Neisseria lactamica is transformed with recombinant DNA encoding a heterologous protein; a method of prophylactic treatment for pathogenic infection of a subject comprising nasopharyngeal inoculation of a modified Neisseria lactamica; a method of reducing or preventing meningococcal colonisation of a subject; a method of modifying the microbiome of a subject; a wild-type Neisseria lactamica for use for the prophylactic treatment of meningococcal infection of a subject or reducing colonisation of a subject, wherein the prophylactic treatment comprises nasopharyngeal inoculation of the wild-type Neisseria lactamica; associated nucleic acid for mutagenesis of Neisseria lactamica; methods of mutagenesis; outer membrane vesicle (OMV) vaccines; and associated compositions and methods thereof.