A review of vaccine development and research for industry animals in Korea

Lee, Nak-Hyung; Lee, Jung-Ah; Park, Seung-Yong; Song, Chang-Seon; Choi, In-Soo; Lee, Joong-Bok

Live bacterial vaccines consist of a small quantity of attenuated bacteria that elicit good immune responses similar to that provoked by natural infection. Although attenuated bacteria are able to infect and multiply in the vaccinated host, they no longer are capable of causing clinical disease as the result of impairment of biological function of virulent determinants [36] . Immune reactions derived from vaccination with live bacteria last longer than those of immunity generated by inactivated bacteria. The major advantage of live vaccine is a broader scope and duration of protection because the animals are exposed to all stages of the replicating bacteria. However, it is critical to ensure that live bacterial vaccine is neither over-or under-attenuated in target animals. Whereas under-attenuated strains may be pathogenic and consequently causes their natural diseases, over-attenuation would not elicit enough amount of an immune response to be an effective vaccine [19, 36, 48] . Since live bacteria vaccines can elicit good levels of both humoral and cellular immunity, this property per se makes live vaccines highly desirable. Creation of live bacterial vaccine strains followed by a classical method would be achieved by conducting multiple passages of microorganisms in suitable systems and by selecting desirable mutants. This method is readily applicable for most target pathogens to be created as attenuated vaccine strains. However, there has been relatively little success in developing attenuated bacteria by such a classical method. For this reason, relatively mild selective pressure on bacteria during in vitro passage enables bacteria to temporarily suppress virulence determinants and then restart their expression of such determinants in vivo. Once development of such vaccines were not technically feasible for most bacteria due to lack of proper tools for doing so, however, currently advanced genetic techniques make practical the development of attenuated strains. This strategy provides opportunities to create desirable mutants of various types of bacteria, through manipulathttp://www.ecevr.org/ http://dx. doi.org/10.7774/cevr.2012.1.1.18 ing a target gene with various genetic techniques, including gene-insertion, deletion, disruption, replacement, and point mutation. The best targets for a bacterial genome are the genes associated with virulence determinants, biosynthesis, and regulatory genes which are critical for bacterial survival. Interestingly, from the standpoint of creating a vaccine strain, deletion of virulence-associated genes in bacteria may be problematic since protective immunity is sometimes desired against the very virulence-associated protein. In such a case, this strain cannot provide good immunity as a vaccine strain. Chemically altered bacterial vaccine contains modified bacteria that have been grown in media supplemented with the proper level of a chemical that provokes mutation of bacteria, changing the ability of bacteria to cause diseases. This method would be another option for developing weakened bacteria and followed by a proper screening method that is suitable for selecting desirable mutants. Alternatively, temperature sensitive mutants can be generated by selection of the mutants which lost their ability to grow at animal body's temperature but can grow at the temperature present in ocular or nasal cavity. In general, killed vaccines for Salmonella are able to stimulate strong immune response, but offer