Bacteriological Classification

Burkholderia, formerly called Pseudomonas in homology group II, was transferred to a new genus (Burkholderia) including B. cepacia, B. pseudomallei, B. mallei, and Burkholderia pickettei in 1992 by Yabuuchi et al.[5] using

16S rRNA sequence, DNA-DNA homology values, cellular lipid and fatty acid composition, and phenotypic character. Recently, a new species, B. thailandensis, is also classified in the Burkholderia genus.[6]

B. pseudomallei is gram-negative rod, nonacid-fast, non-spore-forming, and has bipolar flagella when stained with Sudan black. The organism is a causative agent of melioidosis. For a recent review, see Ref. [1].

B. cepacia, which is placed within the genus Burkholderia, has emerged as an increasingly important opportunistic pathogen, particularly in relation to patients suffering from cystic fibrosis.[7] There are several distinct biochemical characters to separate B. cepacia from B. pseudomallei. However, there are some evidences indicating the similarities of these two species such as the same fatty acid pattern and the same pH optimum for their phosphatase activity.[5] Diagnostic bacteriology concerning B. pseudomallei and B. cepacia is attracting much attention in melioidosis areas.

B. mallei is a nonmotile, gram-negative rod-shaped bacterium that stains irregularly with methylene blue.[8] In contrast to B. pseudomallei, growth of B. mallei is more slowly on nutrient agar. Like other Burkholderia genus, B. mallei is an obligate aerobe. The colonial appearance of B. mallei overlaps in appearance with 24-hr B. pseudo-mallei colonies and it is difficult to distinguish them. B. mallei, a causative agent of glanders disease, is a seriously infectious disease in equine.[8] Human glanders is rare but can be found primarily in veterinarians, horse and donkey caretakers, abattoir workers, and laboratory workers.[9] B. mallei and B. pseudomallei are very similar with respect to their nutritional, biochemical, and genetic properties, and in addition, glanders bears a striking resemblance to melioidosis both clinically and pathologically. However, they are epidemiologically dissimilar.

B. thailandensis is a new species which is found in soil isolates (environmental isolates) from the northeastern Thailand. This species was classified in the Burkholderia genus in 1998 by Brett et al.[6] There are some evidences which support the significant genotypic dissimilarities exhibited between this organism and the true B. pseudo-mallei strains. Although this organism is almost identical to B. pseudomallei in terms of morphology and anti-genicity, some differences could be demonstrated. The significant differences in the sequences of 16S rRNA

gene,[10] the exoenzyme production,[10] the flagellin gene,[11] and the hamsters virulence are seen when compared between these two species.

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