Submitted to: Infection and Immunity
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/4/2001
Publication Date: N/A
Citation: Interpretive Summary: Respiratory disease in pigs is currently the most pressing health concern for swine producers. Bordetella bronchiseptica is one of the microorganisms that causes pneumonia and other respiratory diseases in swine. Although B. bronchiseptica vaccines are available and are widely utilized, most users are not satisfied with their effectiveness. Iron is an essential nutrient for nearly all living organisms. Previous studies have shown that an iron-scavenging compound produced by B. bronchiseptica, called alcaligen, is the means by which this bacterium acquires the majority of iron that it needs for growth. In the present study, an alcaligen mutant was constructed from a virulent B. bronchiseptica swine isolate to determine whether the inability to produce alcaligen reduces virulence. Swine infection experiments demonstrated that the mutant is partially attenuated, as compared to the fully virulent strain, based on a reduction in its ability to colonize the respiratory tract and induce turbinate atrophy. We conclude from these results that production of alcaligen is required for maximal virulence in B. bronchiseptica, and that mutants with nonrevertible defects in genes required for alcaligen synthesis may be candidates for evaluation as attenuated, live vaccine strains in conventionally reared pigs.
Technical Abstract: Bordetella bronchiseptica acquires iron through production of the siderophore alcaligin. A nonrevertable alcaligin mutant derived from the virulent strain 4609, designated DBB25, was constructed by insertion of a kanamycin resistance gene into alcA, one of the genes essential for alcaligin biosynthesis. The virulence of the alcA mutant in colostrum- deprived, Caesarean-derived piglets was compared with that of the parent strain in two experiments. At one week of age, piglets were inoculated with PBS, 4609, or DBB25. Two piglets in each group were euthanized on day 10 post-infection. The remainder were euthanized 21 days post-infection. Clinical signs, including fever, coughing, and sneezing were present in both groups. Nasal washes performed 7, 14, and 21 days after inoculation demonstrated that DBB25 colonized the nasal cavity, but at levels significantly less than those achieved by 4609. Analysis of colonization based on the numbers of CFU/g of tissue recovered from the turbinate, trachea, and lung also demonstrated significant differences between DBB25 and 4609, both at day 10 and day 21 post-infection. Mild to moderate turbinate atrophy was apparent in pigs inoculated with 4609, while turbinates of those infected with DBB25 developed no or mild atrophy. We conclude from these results that siderophore production by B. bronchiseptica is not essential for colonization of swine, but is required for maximal virulence. B. bronchiseptica mutants with nonrevertible defects in genes required for alcaligin synthesis may be candidates for evaluation as attenuated, live vaccine strains in conventionally reared pigs.