Knockout of the meningococcal mltA homolog gives bacteria that spontaneouslyrelease vesicles that are rich in immunogenic outer membrane proteins and thatcan elicit cross-protective antibody responses with higher bactericidal titresthan OMVs prepared by normal production processes. Thus the invention providesa bacterium having a knockout mutation of its mltA gene. The invention alsoprovides a bacterium, wherein the bacterium: (i) has a cell wall that includespeptidoglycan； and (ii) does not express a protein having the lytictransglycosylase activity of MltA protein. The invention also providescompositions comprising vesicles that, during culture of bacteria of theinvention, are released into the culture medium.
