Research progress on bacterial ghosts as novel fishery vaccines

Outbreaks of fish bacterial diseases have become quite frequent with the intensification of practices in aquaculture. Vaccination has become an increasingly important prevention strategy against infectious agents in farmed fishes. However, only a few licensed fish bacterial vaccines are commercially available worldwide, and the majority of them are conventional inactivated vaccines. The use of bacterial ghosts (BGs) as novel candidate vaccines has attracted increasing attention from researchers. BGs are nonliving empty cell envelopes mainly derived from gram-negative bacteria. Pores in gram-negative bacteria are formed most commonly through the strictly controlled expression of bacteriophage PhiX174 lysis gene E, whereas pores in gram-positive bacteria can be formed through minimally inhibitory gentle chemical protocols. In biological protocols, the temperaturesensitive lambda system has been widely used for quick and efficient lysis without the need for any addition of chemical inducers. To produce BGs with enhanced safety, the incorporation of the staphylococcal nuclease A gene, holin-endolysin system, or antimicrobial peptide genes into gene E-mediated lysis cassettes has been applied to improve lysis efficiency. The ghosts of a few fish pathogenic bacteria, such as Edwardsiella spp., Aeromonas spp., Vibrio spp., and Streptococcus spp., have been successfully developed by gene E-mediated lysis, which maintain the original cell morphology but eliminate genetic contents. Although the lysis efficiency of these BGs does not reach 100%, the lyophilized BGs were free of any living cells. These lyophilized BGs not only have high biological safety for fish, but also can induce stronger humoral, cellular and mucosal immune responses compared to conventional inactivated vaccines. Moreover, BGs are easy to produce in large quantities by fermentation without laborious purification procedures and easy to administer through oral or immersion immunization. Therefore, BGs are good fish vaccine candidates and can be engineered to display heterologous antigens or deliver DNA for use as multivalent vaccines against fish pathogen infection. Yet, completely inactivation was not achieved by the gene E-mediated lysis procedure, how to improve the lysis efficiency of BGs is an important problem for BG production. It is believed that the BG system will play an increasingly important role in the prevention and control of fish diseases with further research and continuous biotechnological development.