Natural competence in Histophilus somni strain 2336

Abstract

Histophilus somni is an etiologic agent of shipping fever pneumonia, myocarditis, and other systemic diseases of bovines. Virulence factors that have been identified in H. somni include biofilm formation, lipooligosaccharide phase variation, immunoglobulin binding proteins, survival in phagocytic cells, and many others. However, to identify the genes responsible for virulence, an efficient mutagenesis system is needed. Mutagenesis of H. somni using allelic exchange is difficult, likely due to its tight restriction modification system. Mutagenesis by natural transformation in Haemophilus influenzae is well established and shows a strong bias for fragments containing specific uptake signal sequences (USS) within the genome. We hypothesized that natural transformation may also be possible in H. somni strain 2336 because its genome is over-represented with H. influenzae USS (5′-AAGTGCGGT-3′) and contains most of the genes necessary for competence. H. somni strain 2336 was successfully transformed and mutated with genomic linear DNA from an H. somni mutant (738Δlob2a), which contains a kanamycin-resistance (KanR) gene and the USS within lob2A. Although most of the competence genes found in H. influenzae were present in H. somni, comD and the 5′ portion of comE were absent, which may account for the low transformation efficiency. The transformation efficiency of strain 2336 was greatest during mid-log growth phase and when cyclic adenosine monophosphate was added to the transformation medium. However, mutants were not isolated when strain 2336 was transformed with genomic DNA containing the same KanR gene from H. somni luxS or uspE mutants, which lack the USS in these specific genes. Shuttle vector pNS3K was also naturally transformed into strain 2336, though at a lower efficiency. However, natural transformation with either H. somni linear DNA (2336Δlob2A) or pNS3K was unsuccessful with H. somni commensal strain 129Pt and several other disease isolates.