We have shown that the endotoxin content of standardized dust mite allergenic extracts appears to be significantly higher in Dermatophagoides farinae (Df) than in D. pteronysinnus (Dp) extracts by limulus amebocyte gel clot (LAL) assay (J Allergy Clin Immunol 2003;111:777-783). We now examine whether this can be explained by differences in the endosymbiont bacteria residing in the mite species, or by the presence of LAL-inhibitory substances in the Dp extracts.

We tested multiple samples of cultured washed mites obtained from three different sources. The mites were washed free of media in either ethanol or water (depending on the source) and were cultured for gram-negative bacteria. We also performed the LAL on mixtures of the extracts (Df alone, Dp alone, Df+Dp, and Df+endotoxin-free water) to detect inhibitory substances in Dp.

We detected no gram-negative organisms in the 3 Df samples and 2 Dp samples that were washed in ethanol. Among 7 Df samples washed in water, 2 were positive for Ralstonia picketii; among 7 Dp samples washed in water, 2 were positive for Ralstonia picketii; and another 2 were positive for Acinetobacter species. The LAL results for the extracts (in EU/mL): Df, 6.8×10⁵; Dp, 1.7; Df +water, 6.8×10⁵; Df+Dp, 3.4×10⁵.

Both Df and Dp cultures seem to carry gram-negative organisms. There is minimal if any inhibition of LAL by the Dp extracts. The observed differences in endotoxin concentration may be due to differences in bioburden, or to fastidious species that we were unable to culture.