SUMMARY. The pathogenicity of the predominantly non-capsulated, V-factor requiring haemophili that are commonly recovered from oral infections has been explored by studies of their endotoxins and infectivity as compared with those of . Similar yields of endotoxin (2.40-2.59% w/w) were obtained from all the haemophili examined except (1.61% w/w). The lipopolysaccharide (LPS) extracts all contained heptoses but not 2-keto-3-deoxyoctonate (KDO). The fatty acid compositions of the lipid-A fractions were essentially similar but comprised 76% of the LPS in the type d strain tested and only 20% in the type b strain and some strains of . All extracts contained arachidic acid, which may be unique to haemophili. The endotoxins from all strains produced characteristic pyrogenic and polymorph effects in rabbits. The endotoxins from the pharyngeal X-and V-factor-requiring strains had LD50 values for actinomycin-D-sensitised mice of 2.4-2.7 μg/kg, and were about eight times more potent than those from the oral V-factcr-requiring strains (LD50 values 17.2-22.4 μg/ml). Approximately ten thousand times more free endotoxin was detected in broth cultures of type b than in those of oral haemophili, and this greater endotoxin release was not associated with a greater degree of autolysis. Endotoxin release from viable cells may be important in the pathogenicity of this organism. type b was much more potent in producing infection in chambers implanted subcutaneously in guinea pigs than were oral strains of haemophili; only 10 type b organisms were required, compared with 9 x 10 of . However, in infections maintained for 90 days, the numbers of haemophili—c. 10/ml of chamber fluid—were similar for all the test strains. Thus, although the oral haemophili lack special attributes of invasiveness and resistance to host defences, they are not devoid of pathogenic potential and, if allowed to proliferate, may become an important element in an infection.


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