The <i>Pseudomonas</i> Story (2010)

I wrote myself and in collaboration with colleagues a substantial number of reviews on the taxonomy of Pseudomonas (Palleroni et al., 1973; Doudoroff and Palleroni, 1974; Palleroni, 1984; 2005; 2008) and, inevitably, the present one repeats details that were described before elsewhere. The main difference between this ‘Pseudomonas Story’ and previous reviews will be in the designation of its chapters, which carry titles that have been borrowed from episodes of the history of mankind, and soon I shall explain some arguments in support of this decision, which I adopt with the proviso that I am not trying to depreciate History or to make fun of a respectable group of bacteria. Our story will fit comfortably within the span of the twentieth century plus the few years of the present one, and it will be divided into the following stages: (1) Beginning; (2) Middle Age; (3) Renaissance; (4) Modern Times and (5) Contemporary Outlook. These names became familiar to us as milestones in the history of Western civilization, but their meaning is wide enough to be applied to periods in the evolution of our knowledge on Pseudomonas species. Therefore, they will be used here as subtitles of a story that, thank God, has nothing to do with human political affairs. In the absence of a reliable fossil record, a safe assumption is that the origin of the pseudomonads and other groups of bacteria may have occurred many millions of years ago, but their scientific study started when we became aware of their existence by means of microbiological techniques. The conventional ‘Beginning’ of the story is followed by the ‘Middle Age’, a name that sounds more compassionate than ‘Dark Age’, which has the meaning of a blackout. The name of the third stage could be ‘Restoration’, which is less presumptuous than ‘Renaissance’, and this is followed by ‘Modern Times’, which will have nothing in common with Constantinople or with the beautiful Chaplin's movie. The last or ‘Contemporary’ stage is not very rich in traditional taxonomic studies, because it is directed mostly towards nucleic acids and proteins and in exploring the relationships among the structures and functions of these macromolecules. In a way, this exciting stage, which shall be mentioned very succinctly, has its roots in previous taxonomic studies that gave us a frame of phylogenetic relationships among the species of Pseudomonas and of some of the related genera. In any case, the following pages are not meant to be just an inventory of reminiscences, in spite of the predominance of episodes taken from my personal experience, and as far as it was possible, I did my best to restrain the subjective element to an acceptable proportion. The name Pseudomonas occurred to professor Migula of the Karlsruhe Institute in Germany at the very end of the nineteenth century (Migula, 1894; 1900). His description of the new genus was short and inaccurate but, even so, it was accepted for publication. It reads: ‘Cells with polar organs of motility. Formation of spores occurs in some species, but it is rare (for instance: Pseudomonas violacea)’. That was all. We now know that Pseudomonas strains do not produce spores, and it seems possible that Migula may have been observing refractile granules of reserve materials, which often look like spores, but even with this stipulation, Migula's definition seems rather useless to be of help in the identification of members of this genus. However, the odds were on his side when shortly afterwards he decided to propose as type species Pseudomonas pyocyanea, later renamed P. aeruginosa. This was indeed a most fortunate decision, and therefore, this species will be described in some detail in a later section. As far as I know, Migula never clarified the etymology of the word Pseudomonas in any of his writings, but it suggested to later taxonomists a direct derivation from the Greek monas or unit. Thus, we find in the 7th edition of Bergey's Manual of Determinative Bacteriology of 1957, the formal, somewhat telegraphic definition: ‘Gr. pseudes false; Gr. monas a unit, monad; M.L. fem. n. Pseudomonas false monad’. This etymology was transcribed without further comments by Michael Doudoroff and myself for the description of the genus in the 8th edition of the manual in 1974, and I inserted it unchanged in the first and second editions of Bergey's Manual of Systematic Bacteriology (Palleroni, 1984; 2005 respectively). Migula's description does not mean much, and the above etymology elicited some bad taste humorous comments. Therefore, I thought that it may be of interest to revive now one other possibility for which I could never find any written confirmation from Migula himself, and that it may have only the value of hearsay. Some years before I met Professor Santos Soriano, my microbiology mentor at the University of Buenos Aires, Argentina, he had spent some time in Beijerinck's laboratory in Delft, Holland, and he also had visited the Karlsruhe Institute in Germany, and from both places, he brought the information that Migula had not traced directly the Greek ancestry of the name, but had simply created the name Pseudomonas for the resemblance of the cells to those of the nanoflagellate Monas in both size and active motility. Migula's primary intention may have been to call the bacteria ‘false Monas’, and not ‘false units’. In any case, the point is no longer of much importance. Even though the idea of ‘Middle Age’ in History is usually perceived as a sort of hibernation period, some truly remarkable events took place at this time in our story. These facts did not have an immediate impact, and their significance was appreciated much later, but at the time of their conception, for one reason or another, members of the scientific community interested in Pseudomonas biology and taxonomy seemed to have been indifferent to them. The most important event was the description of the remarkable nutritional versatility of Pseudomonas species, which was published in 1926 (that is, during the core of our Middle Age) in the thesis of L.E. den Dooren de Jong (1926). He was the last student of the famous microbiologist Beijerinck, and he had been assigned the project of examining the soil microflora with respect to the degradation of organic compounds as part of the process of carbon mineralization. The results were quite dramatic, in the sense that bacteria of the genus Pseudomonas appeared to be endowed with the ability of decomposing a large variety of organic molecules, including many that are often toxic to microorganisms of other groups and to higher organisms. As a consequence of these results, one of the important conclusions of the Delft thesis was a recommendation to use the high diversity in nutrition properties of the strains for their description and taxonomy, but both the findings and the recommendations only reached a small circle of scientists, in part because the thesis was written in Dutch, a language that was not familiar to many bacteriologists of other countries. Therefore, it is indeed a remarkable fact that some of the results presented in the thesis were transcribed extensively a few years later in the superb book on bacterial metabolism by Marjory Stephenson (1939), although she was more attracted by the nutritional diversity itself from the biochemical point of view than by the possible taxonomic applications of the screenings. In spite of the fact that the American science was quite rich in outstanding names of pioneer bacteriologists, few of them seem to have manifested much interest in Pseudomonas. An exception was Erwin Frank Smith, who had a strong background in botany and microbiology and worked in the Laboratory of Plant Pathology of the U.S. Department of Agriculture. He recorded in several important monographs (Smith, 1904; 1911) his observations on bacterial diseases in plants, the starting point of our knowledge on the many variants of Pseudomonas syringae, which are specifically pathogenic towards plants of different species. Finally, it is important to point out that during the early decades of the twentieth century took place the foundation of a society by the American bacteriologists that, under the heading of David H. Bergey, decided to start the publication of a manual of determinative bacteriology with periodic updated records of bacterial taxonomy. Unfortunately, some important contributions from foreign laboratories were excluded from the first editions of the manual, but the insularity of the manuals was corrected in later editions. However, the momentum imparted by the activity of the first two decades of the twentieth century was eventually lost during the ‘Middle Age’ of this story. In the meantime, the number of species assigned to the genus was constantly growing to reach quite unmanageable numbers. In the particular case of Pseudomonas taxonomy, the corresponding chapters of later editions of the Manual were written by Haynes and Burkholder (1957), who performed a laborious and most remarkable job of creating determinative keys that included the great majority of nomenspecies, but the properties used in the keys, borrowed from the more familiar sanitary bacteriology and the enteric organisms, were of very limited usefulness for the description and taxonomic determination of new isolates in the laboratory. The chaotic condition of Pseudomonas taxonomy was much aggravated by the enormous number of species assigned to the genus. A rough estimate showed that there were more than 800 names of species assigned to Pseudomonas towards the middle of the twentieth century. At the time, I was involved in the study of the carbohydrate metabolism by a strain of Pseudomonas saccharophila (Palleroni, 1956), a species now transferred to a different genus. The single strain of this species had been isolated from a mud close to the Berkeley branch of the University of California by Dr Michael Doudoroff. Mike and I became close friends, and his enormous enthusiasm and biological vocation were for me a constant source of scientific inspiration. Therefore, in a relatively short time, I had been able to come out with very interesting findings in the biology and carbohydrate metabolism of this hydrogen bacterium, which was a handy supplement to the information already available on this subject. Consequently, it was simply inconceivable for me to switch to any other field. Once in a while, however, both Mike and myself were involved in discussions that had their origin in Dr Roger Stanier's concern for the chaotic situation of Pseudomonas taxonomy and nomenclature (Stanier and van Niel, 1962). Of course, the main objective of these talks among the three of us was Roger's effort to convince Mike and myself of the convenience of doing something to correct the situation. Unfortunately, simple negative objections did not work against Roger's arguments, and the discussions took place periodically until, half convinced by the convenience of such a project, I decide to give it a try. None of us, unfortunately, realized at the time that the hook that attracted us had barbs that prevented any decision of moving to an altogether different field, or that taxonomy could give us a day the pleasure of a boisterous celebration to welcome some satisfactory conclusions. However, Roger was very pleased with our response, and soon rescued from his large book collection a beat-up copy of den Dooren de Jong's thesis, and I remembered my nightmares to try to disentangle parts of the text, using my poor knowledge of German, of which Dutch is actually a dialect. Of course, I had to carry out most of the experiments, which included many biochemical reactions, and in particular it involved the job of testing many strains for utilization of a large number of organic compounds, using solid media dispensed into regular glass Petri plates. A pile of 10 or more plastic dishes of the modern brands of plates makes an almost perfectly vertical column, but in contrast, the landscape of several tables covered with of the glass Petri plates was an to my The are very described in the enormous that from our and from those of the Dutch thesis in the use of the by and a welcome of the of poor den Dooren de A of properties of the Pseudomonas strains to this was and the results were used in the of tables mostly on the nutritional and on some details of interest by the on the other were included in the studies to supplement the (Stanier et al., At present the and job of nutritional properties has been by the use of which are by of but the for these is not very In I the of a description of the type species P. strains as one of the many in the Pseudomonas is the type species for the genus. of the of the species, many strains may in fact the of has suggested that of this species are by the number of of any species on the and one of the of its and of its activity as a or as a human is the enormous of scientific and that is available on its and et al., In the on some of the findings by biology on this species will be The name had occurred years before to because of the of the on a that of or the name to the genus and Migula did was to the species to the new genus Pseudomonas of his The cells are isolated or in small and most of them have a single inserted Some of are presented in the second edition of Bergey's Manual of Determinative The cells are but of or and are or and human with some to A study of the has the of 10 but in spite of the fact that P. indeed a very in with in of it has been by with in some laboratories to the study of this species without any I spent some time in the laboratory of Professor in the was for many years by and correct without any help of Pseudomonas in the as in some in the absence of is able to the source for motility. Therefore, the cells in when is produce several among them and The type strain is a of properties that are in different in the other species of the genus. The nutritional properties of P. are to be mentioned here and they be in other in the study by and colleagues The of compounds a respectable of molecules, and the is following the The a taxonomic of our Pseudomonas species many pages of one of the of in the the of using for the of the The was and in a few it reached the of a an that is not very common for of taxonomic However, at the of the Roger and van had the of the taxonomic to support conclusions of phylogenetic the was in I had to the to some in Argentina, and a I to when the was out and I was pleased to that it was a handy to some of the for the of studies of groups of isolated strains of Pseudomonas species that were as a supplement of our my absence from the laboratory I had the of two with on the of the of et al., and and some on the of this to taxonomic studies started in my although I did not the to this to our A number of strains of the Pseudomonas groups were for in experiments, which I had from the at the University of in and The gave results of two different the one the results the on properties among strains of the but on the other the to any between of strains of different among groups of species that had been assigned to the genus. Mike and I these findings without much of a but at the to my the results of the two on the high of among et al., and and I suggested to to switch to the study of these As my was not with much enthusiasm by because of our However, in spite of this negative response, I decided to do some The was from and I used the nucleic from them in As the results were very they were and by the able collaboration of and In the very interesting was that the species that had been under the name Pseudomonas (that is, Pseudomonas were to one of which included the type P. the species P. P. and some P. P. P. The name Pseudomonas was for this group I or Pseudomonas In our which out in we suggested that the other groups be assigned to other of the or different but we did not any as this process was in other I very much our and the only that I thought we have corrected was the of simply to the of our In part this may have the interest of the but the to this work were soon to a point of this of work into As I we did not by the of new names to of the and this may have that we were not as regular the was somewhat more the who had not in the stage of the had his to a at the Institute in Mike was a with and the Berkeley Pseudomonas as I decided to to a different place or to for to this was the first of the use of as a taxonomic in and, of course, it was soon followed by contributions from other the first of which was work on and and the results those on Pseudomonas. The work was described in a the of in taxonomic studies was welcome and a few the Pseudomonas results were by the group by and in his laboratory they were with the able help of by including strains of many other in the and de were in other and they included the determination of of and among the of the The usefulness of to taxonomic at the genus or higher became and this often with a of was by taxonomists et al., Of course, the high of the the for species and but the possibility of groups corresponding to or was a and very to bacterial taxonomy. In to the studies on several of a less were included to for more of species and, for an of their phylogenetic The number of species assigned to the Pseudomonas group of pseudomonads has already to a number and it is which is a consequence of the remarkable of pseudomonads to the of different I mean mostly the which has to the of and and a rather limited of and hydrogen pseudomonads are less to wide in these last and this results in the absence of truly or In the the names assigned to new species usually to some such as but the possible in details are and the names from those of or which makes them very but does not help very much the identification of strains years ago, names like or the name of the plants for the many variants of P. syringae, were identification The description of new species for publication has now to by the have to be by Even so, the usually more than properties such as and and carbon compounds for and The use of has the time and job of the determination of the nutritional versatility and some related among which is the of identification of some However, I have some for publication in which the description of some new species did not the number and of the or a of the cells under the or under in or for or reserve a familiar taken from not is in the of The work that has been on of species has been quite for identification However, of the species, the that have are P. P. P. A of work to P. to the remarkable versatility of this important species. a description in the of Pseudomonas by we that the important strain of P. designation that from was described as the type strain of the species and assigned the number in the Berkeley The of this has the in the of that are of a in to a part with of within the of in the and a of has of the as with the or with the an to and to of the have no to any A of work was to P. and This strain from a as P. and later renamed to P. performed on its in a recommendation to use this as a of in the of The book by Professor P. on to Pseudomonas is an source of information on the biology of P. and P. The P. strains have not much and the will A large number of P. strains of our collection were into a decision by a thesis work by at Delft under the of Professor strains of the species were divided by into on the of two and from and our work was in fact an of conclusions to our collection that was None of the strains of the that was to P. or P. of the P. and had species but they were later under one of them because of These two species have a of in the that is between P. and the of the other species and included one strain that was P. my to Professor in I was my on the best strains of P. that could be from the point of view using the that was very with P. strains and course, P. on the of and in the of the my the P. names that I could had already been but they seemed to the In to the case of P. an interesting species with very and P. has been the of a great of very interesting work under the able of of the University of the in et al., The studies on this species have the new of and of the already some have been to the species The work of and his at the University of et al., has a new and very interesting for species which is in the determination of the type of by pseudomonads of the and and it has been with very results on a large number of species. In are in the taxonomic and the information in this may as a phylogenetic background of the and in the Pseudomonas of are now available for the study of to information for taxonomic among Pseudomonas species. It without that this may be reached quite far in the but it is that studies to those above may be applied soon to other that they may the that they In our however, we have to in as has that history makes us but not