Bactericidal activity of hemolymph from the horseshoe crab, Limulus polyphemus

Bactericidal activity was found in Limulus serum, with great individual variation in titers toward different bacteria and also among individual horseshoe crabs toward the same bacterial species. These titers varied between monthly determinations of activity. There were crabs with zero activity toward each bacterial species tested. Although environmental factors are likely influences on the bactericidal activity of Limulus serum, the marked variability within similarly treated groups indicates large individual differences in the horseshoe crab population. The highest titers were recorded against those Gram-negative bacilli found normally in the environment. Lower titers were found against those species found normally in warmblooded animals and present in water as contaminants. The serum bactericidal factor is probably released from the circulating amoebocytes during clotting since there was no activity in the “plasma” portion of the blood. Exposure to heat (56°C, 30 min) destroyed the bactericidal activity.