In Memoriam: Paul Baumann

Paul Baumann (1939 - 2026) had a lifelong association with the University of California, earning his BA, MA, and Ph.D. from UC Berkeley and spending 38 years on the faculty as a professor of microbiology at UC Davis from the time the current Department of Microbiology and Molecular Genetics was the Department of Bacteriology until his retirement in 2005. Considering his abysmal early education, this was quite an accomplishment and a testament to his intellectual curiosity and willingness to expend whatever effort needed to reach his goals.

He was born in Shanghai, China, in 1939 to an Estonian father and Russian Cossack mother and lived there until the political situation forced the family to emigrate to Venezuela in 1947. Paul was a lifelong autodidact, some of it by necessity. His early schooling in Venezuela was a patchwork reflecting the location of his engineer father’s work: Spanish elementary schools in his early years, high school by correspondence course when his family lived at the delta of the Orinoco River, accessible by floatplane or curiara and finally two years in an unaccredited “boarding school” where Paul was one of about a dozen students, an effort unacknowledged by a diploma. There was little improvement in the academic instruction at the Baptist junior college in Missouri that followed. Paul always claimed that his true education began in UC.Berkeley, where he was plunged into academics that were way beyond him. Many hours in the library were his eventual salvation.

As a graduate student, he found himself in the joint laboratories of Michael Doudoroff and Roger Stanier, where the appreciation of the microbial world was instilled in him. He emerged not only with a degree, after imposing order on chaos in a group of organisms randomly assigned to Acinetobacter/Moraxella, but with a fascination with the microbial world. It was through these mentors that he learned that one could discern relationships among organisms by determining their phenotypic characteristics, particularly their metabolic capabilities, as the Berkeley lab had done with the vast genus Pseudomonas. More in depth analyses based on genetic characteristics had yet to be devised. Ironically, in time, his efforts were rewarded by the honor of having Acinetobacter baumannii named after him (colloquially known as Iraqibacter for its multi-drug resistance and prevalence among troops serving in Iraq).

Fate ensured that our paths crossed when he was two weeks away from leaving for Boston: I, Linda Michelson, an incoming UC Berkeley graduate student; he, a recent graduate heading for a post-doctoral position in the laboratory of Barbara Wright at the Massachusetts General Hospital. Uncannily, I shared his Estonian heritage, childhood in Venezuela, and pursuit of a career in bacteriology. As Paul knew he was leaving for the east coast in two weeks, he proposed marriage four days after we met. We married a year later and pursued research together for 42 years, 38 of them at UC Davis.

Paul’s first Assistant Professorship at the University of Hawaii, Manoa campus, almost preordained his decision to study marine bacteria. In particular, the question of whether marine organisms are confined to their habitat because of a salt requirement intrigued him. Before this was possible it would be necessary to investigate the resident microorganisms for meaningful comparison to terrestrial species. In those days, little was known of marine bacteria. To Paul, this was a field in dire need of the type of research that had been his graduate work and Hawaii was ideal for such a project: inexpensive with the added benefit that hard work would guarantee results. 

Seawater samples sourced from tidepools to ocean depths of 1,300 meters were readily available and yielded organisms belonging to many different species by simple direct isolation or by varying the carbon source in enrichment cultures. As hypothesized, all had a specific requirement for sodium ion for optimal growth.

This was an era concentrated on the study of individual organisms and their relatives, at the genus and species level. With the advent of tools permitting the analysis of organisms at the genomic level, the emphasis has shifted to communities of organisms better known as the microbiome. As the modern era of studying noncultivables expanded, they were found to predominant in the ocean. While the Baumann lab would also end up in the world of the noncultivables, it wasn’t in the marine environment.

Nevertheless, the work with marines left its trace, as Paul, himself, summarized in the bacteriologist’s Bible, Bergey’s Manual of Systematic Bacteriology, Volume 1 [N.R. Krieg & J.G. Holt, 1984] summarizing the lab’s work on large numbers of bacteria residing in marine environments: Alteromonas, p 343-352; Vibrionaceae; p516-517; Vibrio, p518-538; and Photobacterium, p539—545. In time, a newly described aerobic marine bacterium, Oceanimonas baumannii, was named in our honor. It seems fitting that Paul, who was so intrigued by the microbial world should end up having organisms named after him.

By 1984 Paul decided the lab should concentrate on research more relevant to the UC Davis agricultural campus. Paul couldn’t decide whether to delve into biological pest control via Bacillus sphaericus or explore noncultivable endosymbionts of sap-sucking insects. Eventually he decided that it made more sense to first hone our molecular biological skills by studying the mosquitocidal toxin of Bacillus sphaericus. The work was interesting as, in addition to the cloning and sequencing, it was necessary to maintain a mosquito colony, purify and analyze proteins, employ bioassays and tissue culture in the study of the binary toxin derived from B. sphaericus. 

By 1989 the time seemed right to shift the lab’s emphasis to the noncultivable endosymbionts of sap-sucking insects such as aphids, mealy bugs, psyllids, and white flies. Bacterial phylogenetic relationships were beginning to be elucidated by a comparison of 16S ribosomal genes and Paul reasoned that, as procaryotic and eucaryotic organisms can be distinguished by their ribosomal genes, this would be a means of probing DNA extracted from the whole insect to find out the lineage of its resident endosymbiont(s). PCR machines were not yet commercially available and Brian Unterman, the first to tackle the project, did the first amplifications manually (25 cycles)! The results indicated that both the primary and secondary endosymbionts of Acyrthosiphon pisum were found to be members of the Enterobacteriaceae, with the secondary more closely related to E. coli.

None of the previous research in the Baumann lab compared with the appeal of the subsequent 16 years of research. Sap-sucking insects have recruited free-living bacteria into their special partnership by offering them a cozy environment in return for favors (most often by provisioning essential amino acids lacking in the insect’s plant sap diet). This has proved such a successful relationship that not only has each type of sap-sucking insect found its own distinct partner but some employ plasmids to amplify the production of enzymes charged with producing essential amino acids such as trpEG, the rate-limiting step in tryptophan biosynthesis. What started as Brian Unterman’s first publication in the Baumann lab has burgeoned into a huge field of study, with over 450 publications by 2018. There is still so much fascinating science to be gleaned from these creatures.

In the end, Paul Baumann, the lover of microbes, left behind 154 publications in three different fields. Ironically, while he didn’t remain faithful to any one project, he occasionally bequeathed them to scientists whom he had mentored. Several have done very well for themselves.

Ed DeLong, who has a distinguished career in the field of metagenomics developed his interest in marine bacteria while a 199 student for several years in the Baumann lab. 

Andrew Broadwell, who studied Bacillus sphaericus while a graduate student in the Baumann lab, moved to New Zealand where he cofounded “BioDiscovery” , continuing his interest in using microbes to promote biological solutions to improving agriculture. In 2014, BioConsortia Inc., headquartered in Davis, became the parent company of BioDiscovery, NZ.

Nancy Moran, though never a student in Paul’s lab, owes her adoption of the study of endosymbionts to Paul. Their collaboration began several years after the Baumann lab turned to the study of endosymbionts. Paul felt that her finding that an American and Asian aphid species had been separated 48 million years ago could be used to determine the rate of sequence change of primary endosymbiont DNA based on the time of aphid divergence. Additionally, the minimum age of the endosymbiotic association could be estimated to be 160-280 million years ago. Paul predicted that their individual expertise could complement each other: one a microbiologist, the other an entomologist. It certainly has been fruitful. 

Nancy’s final tribute to Paul was to name the Sharpshooter endosymbiont after the Baumanns: Baumannia cicadellinicola.     

— Linda Baumann
Davis, CA, May, 2026