Natural Sciences

What are bacteria?

Editors’ Note: This is Part 2 of Dr. Aswin Sai Narain Seshasayee’s series ‘Bacterial Expressions

Very naively speaking, bacteria are a bag of molecules of varying colours and shades and sizes that is capable of asexual reproduction. However, that does not quite do justice to the structures that make up a bacterium. Let us use the great model organism Escherichia coli, popularly referred to as E. coli, as a case study to describe the principal molecules that are its building blocks, while shying away from the complexities and pedagogy that are best left to textbooks and research papers.

Schematic of E Coli morphology, based on transmission electron micrography. Image via University of California Museum of Paleontology, links to source.

Schematic of E Coli morphology, based on transmission electron micrography. Image via University of California Museum of Paleontology, links to source.

Let us assume a small particle, a millionth of a millmetre in size, but with the ability to see the world around it. If this particle were to encounter an E. coli cell, it will countenance a rod-shaped object one thousand times its size along its long axis. If our curious particle were to get closer, it would see the E. coli cell covered by polymers of molecules, whose building units are puckered rings – these are sugar molecules, which are connected to the rest of the cell body by long chains of carbons, called lipids. This continuous structure of lipids and sugars, called lipopolysaccharides, make the cell boundary. Our particle sees large pores built into the lipopolysaccharide layer through which nutrients flow inwards, and our particle joins these nutrient molecules and somehow gets past this barrier. Inside, it sees a space within which is a second barrier, also comprising sugars linked with a new type of molecular linkage, which the particle will later realise is also part of proteins. This cell wall is closely associated with a bi-layered inner membrane, which is composed of what are basically detergents. The particle is astounded by the levels of activity associated with this inner membrane – many proteins doing their job, sensing signals including the presence of this foreign particle (which somehow is considered benign and let through unharmed), transporting molecules in and out, synthesising molecules that make up life’s energy currency.

Somehow, our particle bungles past the wall and the membrane and comes face to face with the inner belly of the cell, a water-filled space crowded with a variety of molecules, including proteins, sugars and nucleic acids, which make up DNA and RNA. It is not simply a static collection of large and small objects, but a dynamic space, involving processes – interconversions, construction and destruction among these. One molecule entering a protein machine, and coming out as another; large DNA molecules passing through larger proteinaceous structures, releasing more DNA or RNA; and RNA snaking through humungous protein architectures, producing more protein; amd lots more. All these seemingly coordinated beautifully such that the E. coli cell looks like an impossibly complex but an exceptionally well-organised machine.

About the author

Aswin Sai Narain Seshasayee

I am a junior faculty working at the National Centre for Biological Sciences (

I did my undergraduation in Industrial Biotechnology from the Centre for Biotechnology, Anna University, 2005. Following a one-year stint as an Inlaks Foundation visiting scientist in Nick Luscombe's lab (then) at the EMBL-European Bioinformatics Institute, I continued my work as a PhD student in the same lab, under the aegis of the Faculty of Biology at the University of Cambridge and the International PhD Programme at EMBL. My PhD thesis has (what I think is) a self-explanatory title: "A computational study of bacterial gene regulation and adaptation on a genomic scale". Following a short bridging post-doctoral stint (rather un-adventurously in the same lab), I moved back to India - to NCBS, Bangalore - to establish a wet (experimental, in every sense of the word) / computational research lab investigating bacterial gene regulation and adaptation on a genomic scale.

My work in Cambridge was funded over the years by The Tucker Price Research Fellowhip at Girton College, Cambridge; The St. John's College Research Scholarship, Cambridge; The Developing World Education Fund and Dharam Hinduja Scholarship, Cambridge; University of Cambridge ORS award (now defunct); EMBL; The Inlaks Foundation; and BBSRC (through a grant awarded to Nick and Gillian Fraser, our collaborator).

I currently hold the Ramanujan Fellowship from the Department of Science and Technology, Government of India. I receive core funding from NCBS, our host institute. Extramural students are currently funded by Junior Research Fellowships from the Council for Scientific and Industrial Research and the University Grants Commission, and INSPIRE Fellowships from the Department of Science and Technology.

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