David Holden (microbiologist)

Holden was born in Newcastle-upon-Tyne in 1955 to Bronwen and John Holden. When he was 15, the family moved to Edinburgh, where he completed his secondary education at George Watson’s College. He obtained a B.Sc. Hons degree from Durham University in 1977, and received his PhD in microbiology from University College London in 1981. After post-doctoral work in Canada the US and at the National Institute for Medical Research, he joined the Royal Postgraduate Medical School (which later merged into Imperial College London) as a lecturer in 1990, and was promoted to Professor of Molecular Microbiology in 1995.^[3]

Holden invented signature-tagged mutagenesis (STM; also called mutant barcoding) for identification of mutants with altered growth in mixed populations ^[4][5] This involves the creation of mutant cells labeled with unique identifying DNA sequence tags (barcodes), which enable the fates of large numbers of different mutants to be followed simultaneously. The technique represents the conceptual basis for subsequent multiplexed mutant screens that involve (a) the generation of mutants carrying unique DNA barcodes (b) combining them to create an ‘input’ pool, (c) subjecting the pool to selection, (d) collection of an ‘output’ pool and (e) comparison of barcode abundance in ‘input’ and ‘output’ pools to identify mutants of interest. This approach and its derivatives (such as Transposon insertion sequencing, or TnSeq) have proved extremely popular in genetic research since 1995. They have been used in studies of virtually all bacterial pathogens that are amenable to genetic analysis, many fungi, parasites and in mammalian cells, where it is frequently combined with CRISPR-mediated mutagenesis. His invention therefore transformed high-throughput functional genetics. Several patents on the barcoding technology were granted and licensed to pharmaceutical companies.

Holden’s group first applied STM to Salmonella in a mouse model of typhoid fever.^[6] This led to his team’s discovery of a pathogenicity island, SPI-2,^[7] which is required for systemic growth of this pathogen in its mammalian hosts and encodes a type III secretion system (injectisome) that delivers virulence proteins into host cells from the intracellular Salmonella-containing vacuole (SCV). Subsequently, his group revealed how the assembly of the secretion system and translocation of virulence proteins^[8] are regulated. His team elucidated the biochemical functions of several SPI-2 virulence proteins,^[9][10] provided a molecular understanding of how two of them anchor the SCV to a host cell organelle^[11][12] and showed that another maintains the integrity of the SCV^[13] while preventing it from maturing into a phagolysosome.^[14] His group characterised processes by which other Salmonella proteins subvert both innate and adaptive^[15] immunity. The discovery of the SPI-2 T3SS stimulated many other research groups to study its function; collectively these advances have provided an understanding of the physiological basis of systemic pathogenesis of Salmonella.