The research activities of Kolter's laboratory at Harvard Medical School from 1983 to 2018 encompassed several major parallel lines of investigation and spanned many interrelated subfields of microbiology.[5][7] The overarching theme of the laboratory was to use genetic approaches to study physiological processes (and associated emergent properties) that bacteria have evolved to respond to stressful conditions in the environment, like starvation or limited nutrients, or as a result of ecological interactions with other living organisms.[7][13] The eclectic nature of Kolter's research program was also a result of his policy of encouraging postdoctoral scientists to explore independent interests.[5] In an interview with Nature in 2015, Kolter was quoted on this mentorship style: "I let postdocs explore what they want to explore, as long as it is within the sphere of my interest."[5]
In total, Kolter has co-authored over 250 research and other scholarly articles which together have been cited over 50,000 times.[7][14][15] Kolter's research group was influential in the study of bacterial transport systems known as ABC exporters, published some of the earliest examples of experimental evolution through investigations of the stationary phase of bacterial growth,[7][16][17][18] and was foundational in genetic studies of bacteria adhered to surfaces (living within communities called biofilms).[19][20] The lab popularized the concept of bacterial biofilm formation as developmental or multicellular microbial processes,[21][22][23] and pioneered genetic studies of cellular differentiation, signaling,[24] and division of labor in bacteria.[25][26][27] In addition, his group has worked on other aspects of bacterial physiology,[28] the domestication of lab strains of bacteria,[29] microbiome ecology,[30][31][32][33] interactions between plants and bacteria,[34][35][36] bacterial respiration processes,[37] and bioactive compound discovery.[38][39][40][41]
Some of Kolter's significant scientific contributions are categorized below in chronological order.
Major topics of investigation
Regulation of DNA replication
As a graduate student, Kolter's research provided early evidence for what was called the "replicon hypothesis," proposed by Jacob, Brenner and Cuzin in 1962.[42] His work defined an origin of DNA replication that led to the development of many suicide cloning vectors still in use today.
Kolter, R; Helinski, DR (1978). "Construction of plasmid R6K derivatives in vitro: characterization of the R6K replication region". Plasmid. 1 (4): 571–80. doi:10.1016/0147-619X(78)90014-8. PMID372982.
Kolter, R; Inuzuka, M; Helinski, DR (Dec 1978). "Trans-complementation-dependent replication of a low molecular weight origin fragment from plasmid R6K". Cell. 15 (4): 1199–208. doi:10.1016/0092-8674(78)90046-6. PMID728998. S2CID20082813.
Kolter, R; Helinski, DR (1982). "Plasmid R6K DNA replication. II. Direct nucleotide sequence repeats are required for an active gamma-origin". J Mol Biol. 161 (1): 45–56. doi:10.1016/0022-2836(82)90277-7. PMID6296394.
Peptide antibiotic biosynthesis and ABC exporters
As a new faculty member at Harvard Medical school in the 1980s, Kolter's research group made use of Escherichia coli as a model organism for understanding the molecular genetics of antibioticbiosynthesis. During the course of this work the group was among the first to characterize ABC exporters, today known to be one of the most important membrane protein systems that move molecules across the cell membrane.
In the late 1980s, Kolter's research group became interested in bacteria living in the stationary phase of the growth cycle, a state more like the natural conditions that bacteria experience in environments outside of the laboratory.[43] The group discovered regulatory systems exclusive to cells in this non-growing state and found that mutants with greater fitness in stationary phase evolved and rapidly took over the cultures.[16][17][44] The Zambrano et al. paper in 1993 which published this finding was one of the earliest examples of evolution occurring in the laboratory, or experimental evolution.[18]
In the 1990s, Kolter's group began to focus on the regulation and genetic components of surface-associated communities of bacteria called biofilms. Before then, biofilms had been discovered and were studied in the context of biofouling and in engineering solutions to prevent biofouling,[45][46][47] but the genetics of biofilm formation was unexplored and most microbiologists did not view biofilm formation as a physiological process of bacterial cells.[48][49][50] The lab went on to discover major regulatory systems underpinning biofilm development[51][52] and characterized key materials within the extracellular matrix of biofilms using model species like Pseudomonasaeruginosa,[53][54][55]Escherichia coli,[56]Vibrio cholerae,[57][58] and Bacillus subtilis.[59][60][61][62] Microbial biofilms have since become a major field of microbiology, recognized as a predominant lifestyle of microbes in nature, with relevance to medicine and infections caused by pathogenic bacteria.[63][64]
O'Toole, GA; Kolter, R (1998). "Initiation of biofilm formation in Pseudomonas fluorescens WCS365 proceeds via multiple, convergent signaling pathways: a genetic analysis". Mol Microbiol. 28 (3): 449–61. doi:10.1046/j.1365-2958.1998.00797.x. PMID9632250. S2CID43897816.
Microbial intraspecies interactions, cell differentiation & division of labor
Another body of research stemmed from work on biofilms in the Kolter group in collaboration with the laboratory of Richard Losick: the discovery that subpopulations of different functional cell types develop within single-species biofilms of the bacterium Bacillus subtilis. Some cells were found to express genes for motility, others for sporulation, cannibalism, surfactant production or the secretion of extracellular matrix.[26] Some cell types were found localized in clusters in different physical locations and time points during biofilm development.[25] Another study from the group in 2015 showed that collective behaviors like group migration across a surface can emerge due to interactions between multiple cell types.[27]
Much of Kolter's most recent work focused on interactions between several species in mixed communities, as they typically exist in natural environments. This work has produced several influential studies of the emergent properties and social behaviors of microbes while interacting with other species.
Kolter is an advocate and participant in the communication of microbial science to early career microbiologists and non-scientific audiences.[7] His work in this area began during his term as Co-Director of the Harvard Microbial Sciences Initiative from 2003 to 2018. In this role, Kolter organized an annual public lecture in Cambridge, Massachusetts on topics of general relevance, such as microbial foods and drinks like cheese, sake and wine.[65] His work in science communication then intensified in the years leading up to his retirement and now as an Emeritus professor through invited lectures, writing and museum projects.[8][66]
^Seyedsayamdost, Mohammad R.; Traxler, Matthew F.; Clardy, Jon; Kolter, Roberto (2012). "Old meets new: using interspecies interactions to detect secondary metabolite production in actinomycetes". Natural Product Biosynthesis by Microorganisms and Plants, Part C. Methods in Enzymology. Vol. 517. pp. 89–109. doi:10.1016/B978-0-12-404634-4.00005-X. ISBN9780124046344. ISSN1557-7988. PMC4004031. PMID23084935.
^Jacob, François; Brenner, Sydney; Cuzin, François (1963-01-01). "On the Regulation of DNA Replication in Bacteria". Cold Spring Harbor Symposia on Quantitative Biology. 28: 329–348. doi:10.1101/SQB.1963.028.01.048. ISSN0091-7451.
^Geesey, G. G.; Richardson, W. T.; Yeomans, H. G.; Irvin, R. T.; Costerton, J. W. (December 1977). "Microscopic examination of natural sessile bacterial populations from an alpine stream". Canadian Journal of Microbiology. 23 (12): 1733–1736. doi:10.1139/m77-249. ISSN0008-4166. PMID340020.
^Kolter, Roberto (March 2010). "Biofilms in lab and nature: a molecular geneticist's voyage to microbial ecology". International Microbiology. 13 (1): 1–7. doi:10.2436/20.1501.01.105. ISSN1618-1905. PMID20890834.