Phenotype evaluation of Bordetella bronchiseptica cultures by urease activity and Congo red affinity.

AIMS: The present study shows that Congo red binding and urease activity assays are useful for selection of virulent (Bvg+) Bordetella bronchiseptica cultures. METHODS AND
RESULTS: Congo red binding and urease activity of Bvg+ B. bronchiseptica cultures in different liquid media were compared with the expression of virulence markers such as filamentous haemagglutinin and some outer membrane proteins (OMP). The correlation with the reference virulence markers allowed the establishment of cut-off values for the proposed markers to assure the virulent phenotype (> or = 26 nmol ml-1 of CR and < or = 2.6 U). Using both assays, modulated cultures with avirulent phenotype (Stainer-Scholte broth, with MgSO4 20 mmol l-1 and brain heart infusion broth) and semi-modulated cultures with intermediate phenotypes (tryptose phosphate broth and 83% Stainer-Scholte with MgSO4 5 mmol l-1 cultures) could be distinguished.
CONCLUSION: CR binding assay and urease activity are specific and sensitive enough to detect intermediate phenotypes that could only be detected by subtle changes in OMP profiles. SIGNIFICANCE AND IMPACT OF THE STUDY: The production of effective veterinary vaccines is hampered by reversible B. bronchiseptica antigenic modulation. The proposed assays are technically suitable for selection of virulent cultures to optimize vaccine production.