Whole cell fatty acid analysis as a tool for classification of phytopathogenic pseudomonas bacteria

In this thesis some members of the plant pathogenic bacterial genus Pseudomonas have been studied. Conventional morphological, biochemical, physiological and pathogenitcity tests as well as a 'finger-print' technique, viz. automated whole cell fatty acid analysis, were used. The taxonomy of the plant pathogenic Pseudomonas bacteria is in many cases unsettled. Not in the last place this is because plant pathogenicity is difficult to assess and its value in taxonomy is differently estimated. The genus Pseudomonas has been subdivided on the basis of DNA-r(ibosomal) RNA homology studies into four rRNA groups. Fluorescent plant pathogens are found in group I, non-fluorescent species in group II (Chapter 1.2.).Whole cell fatty acid analysis has been found very useful in the classification of phytopathogenic bacteria. Especially the development of the Microbial Identification System (MIS) of Microbial ID. Inc. Newark, USA enables rapid and reliable determination of fatty acid patterns and has enhanced its use in bacterial taxonomy (Chapter 1.3.). The MIS has been used in all my studies on fatty acids of plant pathogens.At first a bacterium was studied which was well known in our laboratory, viz. the fluorescent P.syringae subsp. savastanoi, causing excrescences on Oleaceae and Nerium oleander. Based on pathogenicity, host range and plant hormone production three pathogenic varieties of this bacterium could be distinguished (Chapter II.1.). With fatty acid analysis (FAA), the three pathovars could also be distinguished, leading to the notion that pathogenicity of primary plant pathogens is not just one phenotypic factor, but it is also reflected in the bacterial membranes, where most fatty acids are present (Chapter II.2.).Subsequently the important non-fluorescent plant pathogen P.solanacearum was investigated. From this bacterium, which causes a devastating vascular disease on many different food crop plants, both biochemical and pathogenic varieties have been described. The occurrence of biochemical varieties could not be confirmed by FAA. Apparently differences in the ability to metabolize a few carbon compounds have no effect on fatty acid composition. As was the case with P.s. subsp. savastanoi, pathogenic varieties of P. solanacearum could be discriminated by FAA. Fatty acid patters of P. solanacearum were also studied in relation to those of other members of rRNA-group II, such as P.cepacia , P. gladioli , P. caryophylli and P. pickettii. The taxonomic patterns found were in good congruence with those determined in DNA-DNA homology studies by other authors. This once more confirms that FAA is a powerful additional tool in the classification of bacteria (Chapter II.3.).Finally the very complex group of the fluorescent, oxidase positive soft rot Pseudomonas bacteria was studied. These bacteria are opportunistic plant pathogens, especially important in post harvest situations. They have been found to be biochemically indistinguishable from saprophytic pseudomonads such as P.fluorescens biovars, P.putida and P.chlororaphis (incl. P.aureofaciens). On the basis of their ability to hydrolyze pectin and to cause soft rot, they have been named P. marginalis. In this study soft rot strains were biochemically similar to biovars of P. fluorescens or intermediates of these biovars, unknown forms of P. fluorescens, or similar to P. putida and P.chlororaphis. With FAA, oxidase positive sof rot pseudomonads were all found in a heterogeneous super cluster with saprophytic strains biochemically identified as P. fluorescens biovars or intermediates, P.chlororaphis and P.putida.Therefore it is suggested to abandon the use of ' P.marginalis ' and to name oxidase positive fluorescent soft rot bacteria ' P. fluorescens ', with some additional information between brackets, e.g. P.fluorescens (pectolytic, soft rot strain).P. aeruginosa strains from plants, animals and men were found in a very homogeneous cluster, well separated from the P.fluorescens supercluster. A supposed plant pathogenicity of P.aeruginosa could not be confirmed. This bacterium can multiply and cause some necrotic action only occasionally on plant material under special (unknown) circumstances. Certain non-pectolytic, non-soft rot strains of P. fluorescens are described which cause bacterial stripe symptoms on Iris sp. The pathogenicity factors of the Iris strains have not been substantiated (Chapter II.4.).Fatty acid analysis has been shown to be a welcome and useful tool in elucidation of natural relations between plant pathogenic Pseudomonas bacteria. Fatty acid analysis in combination with other methods such as conventional phenotypic tests and DNA-and protein fingerprinting may lead to a better understanding of this interesting group of bacteria, not in the last place to achieve a better disease control (Chapter III).