The name Pseudomonas was proposed initially by Professor W.E.F.A. Migula of the Karlsruhe Institute of Germany at the end of the 19th century (Migula, 1894, 1900; Palleroni, 2010) and it was reported for the first time in the Bergey’s Manual of Determinative Bacteriology in 1923. The choice of name seems to be due to its similarity in size and motility to the nanoflagellate Monas (from the Greek: “pseude”5false). The first description of Pseudomonas was inaccurate: Professor Migula described Pseudomonas as “. . . cells with polar organs of motility. Formation of spores occurs in some species, but it is rare. . .”. Pseudomonas pyocyanea (now P. aeruginosa) was proposed as the type species. In 1926, the extreme versatility of Pseudomonas was highlighted by L.E. den Dooren de Jong in his thesis (den Dooren de Jong, 1926; Palleroni, 2010) which focused on soil bacteria. By the middle of 1900, more than 800 species had been ascribed to Pseudomonas, creating a confusing background for researchers interested in the genus. The major cause of this erroneous classification was the trend to categorize any Gram-negative, strictly aerobic, nonsporulating, motile bacillus as a representative of the genus Pseudomonas (Scales et al., 2014). The turning point in this view was the development of first biomolecular approaches that, alongside classical microbiology, unraveled the difficult classification of the genus strains. Around the beginning of the third quarter of the 20th century, DNA/DNA hybridization revealed deep differences among phenotypically similar strains (Pecknold and Grogan, 1973). Subsequently, RNA/DNA hybridization showed the presence of 5 different rRNA groups (rRNA group I, II, III, IV, and V; Palleroni et al., 1972). Pseudomonas rRNA group I (called Pseudomonas sensu stricto) comprised P. aeruginosa, all the fluorescent Pseudomonas and some nonfluorescent Pseudomonas (such as P. stutzeri, P. alcaligenes, P. pseudoalcaligenes, and P. mendocina). A more in-depth analysis of genetic differences among Pseudomonas species was conducted with the study of 16S sequence homologies: despite the low discriminatory power of rRNA, the study
allowed the identification of distinct phylogenetic groups (Laguerre et al., 1994; Anzai et al., 2000).
In 2000, great advance in the study of Pseudomonas was made with a siderophore study by Meyer and colleagues that provided an excellent characterization of several species (Meyer, 2000; Meyer et al., 2002).
At the present time (December 2015) the genus comprises 244 species, as reported at http://www.bacterio.net/, having different characteristics.
In this chapter, the importance of the genus Pseudomonas and related genera as food spoilers is described. Taxonomic organization, identification methods, spoilage mechanisms, and control plans are reported, with the goal of highlighting the extreme complexity of the spoilage potential of the genera Pseudomonas, Xanthomonas, and Shewanella
