Paragonimus westermani possesses aerobic and anaerobic mitochondria in different tissues, adapting to fluctuating oxygen tension in microaerobic habitats.

We previously showed that adult Paragonimus westermani, the causative agent of paragonimiasis and whose habitat is the host lung, possesses both aerobic and anaerobic respiratory chains, i.e., cyanide-sensitive succinate oxidase and NADH-fumarate reductase systems, in isolated mitochondria (Takamiya et al., 1994). This finding raises the intriguing question as to whether adult Paragonimus worms possess two different populations of mitochondria, one having an aerobic succinate oxidase system and the other an anaerobic fumarate reductase system, or whether the worms possess a single population of mitochondria possessing both respiratory chains (i.e., mixed-functional mitochondria). Staining of trematode tissues for cytochrome c oxidase activity showed three types of mitochondrial populations: small, strongly stained mitochondria with many cristae, localised in the tegument and tegumental cells; and two larger parenchymal cell mitochondria, one with developed cristae and the other with few cristae. The tegumental and parenchymal mitochondria could be separated by isopycnic density-gradient centrifugation and showed different morphological characteristics and respiratory activities, with low-density tegumental mitochondria having cytochrome c oxidase activity and high-density parenchymal mitochondria having fumarate reductase activity. These results indicate that Paragonimus worms possess three different populations of mitochondria, which are distributed throughout trematode tissues and function facultatively, rather than having mixed-functional mitochondria.