Morphological and biochemical comparison of virulent and avirulent isolates of Haemophilus pleuropneumoniae serotype 5.

Capsular structure and biochemical composition varied between two isolates (virulent and avirulent) of Haemophilus pleuropneumoniae serotype 5. The presence of capsule was determined by transmission electron microscopy with glutaraldehyde-osmium, ruthenium red, alcian blue, and phosphotungstic acid staining procedures. The virulent isolate of H. pleuropneumoniae had a distinct, adherent capsule. The avirulent isolate had a fragile, easily removed capsule. Capsular material (CM) and a lipopolysaccharide (LPS) were isolated from each bacterial isolate and were compared biochemically and biologically. CM from both isolates contained carbohydrates, no detectable protein, and no detectable to trace amounts of lipid A. Each LPS contained heptose, hexose, galactose, glucosamine, 2-keto-3-deoxyoctonate, and lipid A. Biological responses to CM and LPS from both isolates were demonstrated in the proclotting enzyme of Limulus polyphemus amebocyte lysate activation and in serological cross-reactions by immunofluorescence and immunodiffusion precipitation. The virulent isolate contained approximately 10 mg of LPS per g more on an original dry weight basis than the avirulent isolate. LPS from the virulent isolate contained approximately 13 times more galactose than LPS from the avirulent isolate. The differences of capsular structure and biochemical composition may contribute to the role of CM in porcine H. pleuropneumoniae infections.