J. Med. Microbiol. - Vol. 46 (1997), 565-570 0 1997 The Pathological Society of Great Britain and Ireland BACTERIAL PATHOGEN IClTY Ribotype differences between clinical and environmental isolates of Burkholderia pseudomallei S. TRAKULSOMBOON*T, D. A. B. DANCES, M . D. SMITH§**II, N. J. WHlTEglI and T. L. PITT* *Laboratory of Hospital Infection, Central Public Health Laboratory, 61 Colindale Avenue, London NW9 5HT, 7 Division of Infectious Disease, Department of Medicine, Faculty of Medicine, Siriraj Hospital, Mahidol University, Bangkok 10700, Thailand, $Public Health Laboratory, Derriford Hospital, Plymouth PL6 8DH, §Faculty of Tropical Medicine, Mahidol University, Bangkok 10400, Thailand and ][Centrefor Tropical Medicine, Nuffield Department of Clinical Medicine, John Radcliffe Hospital, Headington, Oxford OX3 9DU Burkholderia pseudomallei is isolated frequently from the soil in regions where the disease melioidosis occurs. However, recent surveys in Thailand have shown that the frequency of isolation of the organism from soil samples is not directly related to the incidence of melioidosis in an area. To determine whether strain populations of B. pseudomallei prevalent in soil are gentypically related to strains causing clinical disease, rFWA Bam HI restriction fragment length polymorphisms (RFLP) of 139 soil environmental isolates and 228 human isolates were compared. Two groups of ribotype patterns were found. Group I comprised 37 different ribotype patterns which were characterised by five to eight hybridisation bands of 2.8-> 23 kb. All of these ribotypes were identified among the clinical isolates, and 18 of them were also found in 59 environmental isolates. Group I1 was represented by 12 ribotypes found only in environmental strains. These ribotype patterns comprised one to five bands in the size range 9-> 23 kb. All but one of the 73 isolates in this group grew on a minimal medium supplemented with L-arabinose. In contrast, only 3% of the 66 isolates from the environment with group I ribotype patterns could utilise this sugar as their sole energy source. These findings suggest that B. pseudomallei strains that utilise arabinose constitute a population that is genetically distinct from other environmental and clinical strains. Introduction Melioidosis is an infection caused by Burkholderia pseudomallei. Despite the apparent ubiquity of B. pseudomallei in the soils of endemic areas of Southeast Asia, there is great variability in the incidence of melioidosis in different regions [l]. This may be due to host susceptibility related to conditions such as diabetes mellitus, a well recognised risk factor [2], and differences in occupational exposure to the organism. Differences in bacterial virulence might also contribute to the geographic distribution of melioidosis. A recent study by Wuthiekanun et al. [3] suggests that Received 15 Aug. 1996; accepted 14 Nov. 1996. Corresponding author: Dr T. L. Pitt. **Present address: Public Health Laboratory, Musgrove Park Hospital, Taunton TA1 5DB. although clinical and environmental isolates of B. pseudomazlei from Thailand are morphologically similar and antigenically indistinguishable, two pop- ulations of the species are revealed by extended biochemical testing. They identified two different biotypes of B. pseudomallei that were characterised by two phenotypes based on their ability to utilise the sugar L-arabinose as a sole energy source for growth. This study suggested that the biotype differences are associated with the disease-producing potential of the bacteria for man as arabinose positive (ara+) isolates were found exclusively in the environment and not from patients with melioidosis, while ara- isolates were isolated from both man and the environment. The question arises as to whether these varieties of B. pseudomallei are genomically related or show suffi- cient differences to warrant further investigation of species identity. Therefore, the variation in rRNA gene loci of isolates of both phenotypes was examined to determine their degree of genetic relatedness.