Two genes from the capsule of Aeromonas hydrophila (serogroup 0:34) confer serum resistance to Escherichia coli K12 strains

The Escherichia coli DH5 alpha strain as well as other K12-derived strains are unable to produce O-specific lipopolysaccharide and are thus rough and serum-sensitive. One representative recombinant clone (COS-SR1) containing Aeromonas hydrophila (serogroup O:34) chromosomal DNA conferred serum resistance to E. coli K12 strains. Genetic, biochemical, and immunological studies suggested that the two genes (orf1 and wcaJ) identified in a subclone (pAC-SR9) of COS-SR1 are necessary for the production of the colanic acid capsule at 37 degrees C on E. coli DH5a, rendering the strain serum-resistant. A. hydrophila strains from serogroup O:34 are able to produce capsule when they grow both in synthetic medium and in an autolysate of fish viscera. However, defined weal insertion mutants of A. hydrophila 1051-88 (serogroup O:34) are unable to produce capsule on these media. This strongly suggests that both genes belong to the gene cluster responsible for capsule production (wca) of A. hydrophila 1051-88 (serogroup O:34). (C) Elsevier, Paris.