Entomopathogenic nematodes (EPNs) of the genus Steinernema are pathogenic to the insects and well known as ideal models for understanding parasite-host interaction. EPNs harbor a number of bacterial symbionts in their gut belonging to the noble genus Xenorhabdus which are capable of killing insects by themselves or by combination with nematodes by suppressing insect immune defense. Here, we report host range of Steinernema monticolum and its symbiont Xenorhabdus hominickii. S. monticolum has a diverse host range including lepidopteran and coleopteran insects although they showed higher pathogenicity to the lepidopteran insects. Especially, X. hominickii suppressed insect immune responses. A target insect, Spodoptera exigua, exhibited both cellular and humoral immune responses by expressing antimicrobial peptides and forming nodules in response to heat-killed X. hominickii. However, live bacteria significantly suppressed the immune responses. An addition of arachidonic acid to the bacterial infection significantly rescued the immune responses, suggesting eicosanoid biosynthetic pathway as a pathogenic target of X. hominickii.

An entomopathogenic bacterium, Xenorhabdus sp., symbiotic to Steinernema monticolum was investigated in its insecticidal activity. The bacteria induced septicemia of two lepidopteran insects (Plutella xylostella, Spodoptera exigua), a coleopteran insect (Tribolium castaneum), and a hemipteran insect (Riptortus clavatus) when they were injected into hemocoel. The bacterial culture broth contained immunosuppressive factor(s) that inhibited hemocyte nodulation in response to heat-killed bacteria. The immunosuppressive activity appeared to be caused by inhibition of two main immune-associated enzymes, phospholipase A2 (PLA2) and phenoloxidase (PO). HPLC analysis of the bacterial culture broth contained several PLA2 inhibitors. The bacterial culture broth significantly enhanced Bt pathogenicity. There results support a novel insect pest control strategy using eicosanoid-biosynthesis inhibitors.

2005년 제주도 30지역에서 곤충병원성 선충 분포 조사를 실시하였다. 미끼곤충 인 꿀벌부채명나방 유충을 이용하여 토양 내 곤충병원성 선충을 조사하였는데 1차 적으로 치사유충의 색깔 변화로 Steinernema속과 Heterorhabditis속을 구별하였 다. 30곳의 토양 시료들 중 곤충병원성 선충이 검출된 지역은 5지역으로 검출율은 16.7%였다. 분리 된 곤충병원성 선충은 모두 Steinernema spp.였는데 형태학적, 분 자학적, 생물학적 특징으로 분류, 동정 한 결과 Steinernema monticolum과 S. longicaudum으로 동정 되었다. Steinernema monticolum은 조천읍, 어리목, 성판 악, 1100도로 주위 산림에서 발견 되었고, S. longicaudum은 성산읍 한 곳에서 발견 되었다. 곤충병원성 선충의 서식처로는 산림에서 28.6%, 해안가에서 12.5% 발견 되었고, 토성은 모두 사양토였다. S. longicaudum은 공주와 논산지역에서 발견 된 바 있고, S. monticolum은 지리산에서 발견 되었었는데 제주도에서 발견 된 것은 처 음이다. 분리한 곤충병원성 선충의 계통과 접종 농도는 병원성에 영향을 주었다.