Isolation, identification, and molecular characterization of strains of Photorhabdus luminescens from infected humans in Australia

Abstract

We describe the isolation of Photorhabdus (Xenorhabdus) luminescens from four Australian patients: two with multiple skin lesions, one with bacteremia only, and one with disseminated infection. One of the patients had multiple skin lesions following the bite of a spider, while the lesions in the other patient were possibly associated with a spider bite. The source of infection for the remaining two patients is unknown. As a member of the family Enterobacteriaceae, P. luminescens is unusual in that it fails to reduce nitrate and ferments only glucose and mannose. It gives negative reactions for lysine decarboxylase, arginine dihydrolase, and ornithine decarboxylase (Moeller). The species is motile, utilizes citrate, hydrolyzes urea, and usually produces a unique type of annular hemolysis on sheep blood agar plates incubated at 25 degrees C. A weak bioluminescence is the defining characteristic. P. luminescens is an insect pathogen and is symbiotically associated with entomopathogenic nematodes. Its isolation from human clinical specimens has been reported previously from the United States. Restriction fragment length polymorphism-PCR analysis of the 16S rRNA gene demonstrated a high level of similarity among the Australian clinical strains and significant differences between the Australian clinical strains and the U.S. clinical strains. However, numerical analyses of the data suggest that the two groups of clinical strains are more similar to each other than they are to the symbiotic strains found in nematodes. This is the first report of the isolation of P. luminescens from infected humans in Australia and the second report of the isolation of this species from infected humans worldwide.

FIG. 1

Gel electrophoresis of restriction endonuclease digests of 16S rDNAs of P. luminescens strains. Lanes: 1, 100-bp molecular size marker; 2, negative control; 3, Hb/1 (ATCC 29999^T) symbiotic strain; 4, negative control; 5 through 9, U.S. clinical strains 3265.86, 2617.87, 1216.79, 3105.77, and 2407.88, respectively; 10, negative control; 11 through 14, Australian clinical strains 4, 2, 3, and 1, respectively; 15, negative control; 16 through 18, symbiotic strains D1/1, C1/1, and Meg/1, respectively; 19, negative control; 20, P. vulgaris ATCC 49132. Restriction patterns for the Australian clinical strains are similar to one another, as are those for the U.S. clinical strains, but differences between the two groups are evident with restriction enzymes AluI and CfoI.

FIG. 2

Dendrogram derived from unweighted group mean average analysis of RFLP-PCR analysis data for 16S rDNAs of P. luminescens strains showing clustering of the Australian clinical strains (strains 1 through 4) with the U.S. clinical strains (strains 2407.88, 3105.77, 2617.87, 3265.86, and 1216.79) before they joined symbiotic strains C1/1 and Meg/1 and then symbiotic strains Hb/1 and D1/1.

FIG. 3

Dendrogram derived from L&W flexible analysis of the RFLP-PCR analysis data for 16S rDNAs of P. luminescens strains showing clustering of the Australian clinical strains with the U.S. clinical strains before they joined the clustered symbiotic strains.

FIG. 4

Dendrogram derived by ISS variable analysis of RFLP-PCR analysis data for 16S rDNAs of P. luminescens strains showing clustering of the U.S. clinical strains with the symbiotic strains before they joined the Australian clinical strains.