Anaerobic metabolism of primary and secondary forms of Photorhabdus luminescens

An oxygen electrode inserted into a dead Munducu sex~u larva infected with Heterorhabditis nematodes carrying the bacterium Photorhabdus luminescens showed barely detectable levels of oxygen in a 1 to 2 mm zone below the cuticle, and virtual anaerobiosis deeper in the carcass. This observation indicates that the bacteria in this habitat, where they are actively growing, are probably carrying out a fermentative metabolism. Therefore, the anaerobic metabolism of the primary and secondary form variants of P. luminescens Hm and NC1 was compared. Amino acids were not fermented by either strain, either singly or in mixtures. Glucose was fermented by both forms of both organisms, forming products typical of mixed acid fermentation by Enterobacteriaceae. The fermentation patterns were the same in the primary and secondary forms. Growth rates of the secondary form cells were higher in defined medium with glucose as energy and carbon source. Growth yields of the primary and secondary forms of strain Hm were nearly identical, whereas the growth yield of secondary form cells of strain NC1 was slightly higher than that of the primary form. The results of this study indicate that the observed predominance of primary form cells in infected insect larvae cannot be explained by an advantage over the secondary form cells related to the efficiency of anaerobic growth or fermentative metabolism.