IN-VITRO AND IN-VIVO CHARACTERIZATION OF A SMALL-COLONY VARIANT OF THE PRIMARY FORM OF PHOTORHABDUS-LUMINESCENS MD (ENTEROBACTERIACEAE)

A small-colony variant (Vsm) of the primary form (Vp) of Photorhabdus luminescens MD from in vitro and in vivo cultures is described. Unlike the primary form, Vp, the Vsm variant is not the preferred diet of it s nematode symbiont, a Heterorhabditis sp., does not support developme nt and reproduction of the nematode, and is less pathogenic than Vp to Galleria mellonella larvae, Vsm cells were carried by 25% of infectiv e juveniles, but they comprised a very low percentage (-0.4%) of the t otal cells carried by the juvenile. In vitro subculture and in vivo in jection into the larvae with either Vp or Vsm always produced a mixtur e of both Vp and Vsm, In nematode-bacterium-infected G. mellonella lar vae, the Vp population in the hemocoel was high (4 x 10(9) to 5 x 10(9 ) CFU/g of wet insect tissue) at 24 h after infection, decreased about 10-fold by 48 h, and then regained a high level at day 5 before decre asing at day 7 and then remaining relatively constant through day 15 p ostinfection, The Vsm population, under the same conditions as those o f Vp, increased gradually to a high level (9 x 10(8) CFU/g of wet inse ct tissue) at day 5 postinfection and then declined gradually through day 15.