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597 member of the microflora of soil and water in the tropics. However, the closely t. related species P. mallei is a true parasite, causing a disease of horses known as glanders.P.mallei is unable to survive in nature in the absence of its specific animal � host. It is the only aerobic pseudomonad that is permanently immotile: its inclu- sion in the group is based on its close genetic relationship and its phenotypic ; (; similarity to P. pseudomallei. ; The pseudomallei group also contains several species that occur in soil and www are occasionally pathogenic for plants; they are exemplified by P. cepa(ia, notable [ r" for its extreme nutritional versatility; it can use over 100 different organic com- pounds as carbon and energy sources. These compounds are listed in Table 2.3 3 The soil bacteria of the acidovorans group,comprising the species P.acidovorans and P.testosteroni(Table 19.7)are nonpigmented;they accumulate poly-p-hydroxy- ..d butyrate and do not require growth factors. The nutritional spectrum of these bacteria is a distinctive one. Many of the organic substrates commonly utilized by the fluorescent and pseudomallei groups cannot support their growth; these include glucose and other aldose sugars, polyamines such as putrescine and spermine,and several amino acids,notably arginine and lysine.Nevertheless,they t t can grow at the expense of several organic adds and amino acids rarely, if ever, utilized by members of other groups: these include glycollate, muconate, and norleucine. Another interesting property of the acidovorans group is the possession of _ metabolic pathways for the dissimilation of widely utilized substrates that are ; unlike those found in other aerobic pseudomonads. For example, p-hydroxy- F" benzoate, dissimilated through the ,6-ketoadipate pathway by all other aerobic pseudomonads capable of utilizing this substrate,is dissimilated through a special „ F " (meta cleavage) pathway by the acidovorans group (Figure 19.4). Certain plant pathogenic pseudomonads have long been placed in a special ; =- genus, Xanthomonas, distinguished by the production of yellow cellular pigments. Nucleic add homology studies have shown that these xanthomonads are a geneti- cally isolated subgroup, although distantly related to one other aerobic pseudo- _ monad, Pseudomonas maltophilia. In contrast to the groups so far discussed, both f. xanthomonads and P. maltophilia require organic growth factors, including methi- onine and (for xanthomonads) a few of the B vitamins. It was long assumed that the yellow cellular pigments of xanthomonads are carotenoids, but chemical studies have shown that these pigments are of unique chemical composition,being brominated derivatives of aryl octanes,of the general structure shown in Figure 19.5. Pigments of this type are not known to occur r" in any other bacteria;thus,the chemical nature of the cellular pigments is a highly x" distinctive property of the xanthomonas group. tive The Gram-negative aerobic chemoheterotrophs with rod-shaped cells include .genera many representatives in which flagellar insertion is not polar,and which are hence C excluded by definition from the aerobic pseudomonads. These bacteria normally and bear few(1 to 4)flagella and the flagellar insertion,which is not easy to determine um unambiguously,is often described as"subpolar"or as"degenerately peritrichous." Apart from the practical difficulty of determining this character,there are consid- erable grounds for doubting whether or not it really possesses the taxonomic importance that has traditionally been ascribed to it. An extension of the study Of genetic interrelationships by nucleic add hybridization, which has been so _ successful in revealing the internal relationships of the aerobic pseudomonads, "W ,... n.. .tN•h'�:i � :*?.�fi92CP+isS�-:Mi.`*'.::�+*.Mts!.n�i< . �!Fi3. v. v,�-wed+wdwaa.x�.k'i�3'U'w'"m'�5nti.e.