Photorhabdus

Abstract

Pho.to.rhab' dus . Gr. n. photo light; Gr. fem. n. rhabdus rod; M.L. masc. n. Photorhabdus bioluminescent rod‐shaped bacterium. Proteobacteria / Gammaproteobacteria / Enterobacteriales / Enterobacteriaceae / Photorhabdus Asporogenous rod‐shaped cells 0.5–2 × 1–10 μm. Cell size is highly variable within and between cultures with occasional filaments up to 30 µm long. In the last stage of exponential growth and during stationary growth period, spheroplasts may occur with an average of 2.6 μm in diameter (10–20% of cell population), resulting from the partial disintegration of the cell wall. Proteinaceous protoplasmic inclusions are synthesized inside a high proportion of cells (50–80%) during the stationary period. Gram negative, motile by means of peritrichous flagella. Facultatively anaerobic, having both a respiratory and a fermentative type of metabolism. Optimum growth temperature usually ∼28°C; some strains grow at 37–38°C. Most strains produce pink, red, orange, yellow, or green pigmented colonies on nutrient agar, and especially on rich media (tryptic soy agar, egg yolk agar). Bioluminescent, usually detectable by the dark‐adapted eye; intensity varies within and between isolates and may only be detectable by photometer or scintillation counter in some isolates; only very few nonluminescent isolates have been reported. Spontaneous phase shift occurs in subcultures inducing the appearance of phase II clones characterized by the loss of neutral red adsorption on MacConkey agar, of production of antibiotics, and of some other properties usually exhibited by wild clones freshly isolated from the natural environment and named phase I variants. Catalase positive. Do not reduce nitrate . Negative for oxidase, o‐nitrophenyl‐β‐ D ‐galactopyranoside (ONPG), Voges–Proskauer, arginine dihydrolase, lysine and ornithine decarboxylase tests. Proteolytic for gelatin. Most strains hemolytic for sheep and/or horse blood, some producing an unusual annular hemolysis on sheep blood at 25°C (Figure 1). Lipolytic activity on Tween 20; many strains lipolytic for Tweens 40, 60, 80, and/or 85. Acid production from glucose without gas. Acid produced from fructose, D ‐mannose, maltose, ribose, and N ‐acetylglucosamine; weak for acid production from glycerol. Fumarate, glucosamine, L ‐glutamate, L ‐malate, L ‐proline, succinate, and L ‐tyrosine are utilized as sole carbon and energy sources. Biochemical identification of Photorhabdus within the family Enterobacteriaceae is summarized in Table 1. Sequence analyses of 16S rDNA show that all Photorhabdus strains branch deeply within the radiation of the family Enterobacteriaceae , and have a specific TGAAAG sequence at positions 208–213 ( E . coli numbering). The natural habitat for most strains is the intestinal lumen of entomopathogenic nematodes of the genus Heterorhabditis and insects infected by these nematodes. However, some nonsymbiotic strains have been identified as opportunistic pathogens for humans, not nematodes. The mol % G + C of the DNA is : 43–45. Type species : Photorhabdus luminescens (Thomas and Poinar 1979) Boemare, Akhurst and Mourant 1993, 254 Xenorhabdus luminescens Thomas and Poinar 1979, 354.)