How does oxygen kill bacteria in the body?


The key to understanding oxygen activation is the conversion of this molecule into a reactive singlet species within neutrophil cells in the blood. This process leads to light emission, which can be used to monitor in real time how the immune system functions.

Based on over 40 years of research, Professor Robert C Allen proposes an exquisitely detailed model of how oxygen becomes an aggressive bactericidal agent in the body.

Read more in Research Features:

Read the original research:



Image Credit: Adobe Stock / Katerina Yevtekhova




Hello and welcome to Research Pod! Thank you for listening and joining us today.


In this episode we look at the work of Professor Robert C Allen. His research investigates the extremely potent but short-lived singlet oxygen generated by neutrophils, which effectively kills bacteria in the human body. Oxygen plays a crucial role in our immune system. With modification of oxygen’s reactive frontier orbital electronic structure, it acts as a powerful bactericide in white blood cells. However, the mechanism explaining this anti-pathogen activity is  poorly understood. Allen shows that the conversion of oxygen to the singlet multiplicity quantum state is the key.


Oxygen plays a crucial role in sustaining life on Earth, supporting the existence of a multitude of organisms. It is an indispensable component of cellular respiration and drives the metabolic processes that enable living beings to extract energy from nutrients. Aerobic organisms, from the tiniest microorganisms to complex multicellular life forms, depend on oxygen for the efficient use of the metabolic reducing equivalents that drive cell function.


Oxygen is also a critical element in the immune system. In the human body, neutrophils, a type of white blood cell, are responsible for orchestrating a frontline defence against microbial invaders. These cells realise the reactive potential of oxygen by changing oxygen’s spin multiplicity through a process collectively referred to as the ‘respiratory burst’. This altered spin state of oxygen is antimicrobial, and the mobile nature of the neutrophils makes them available in any area of the body affected by pathogens through the blood flow.


Although the role of oxygen in supporting a healthy immune system has long been recognised, the details of how oxygen interacts with the complex biochemical machinery of the immune system remain poorly appreciated. The chemistry and biochemistry of oxygen exhibit very peculiar features, setting it apart from other common molecules that interact with our bodies. Professor Allen has devoted more than 50 years to exploring what makes oxygen so special for the immune system, and why this molecule could not be replaced by any other. Drawing from an understanding of oxygen’s chemistry, Allen provides an elegant model as to how neutrophils fight microbes. The key to understanding why oxygen is so important for life and its sustenance, says Allen, relates to its frontier orbital electrons responsible for reactivity and is best considered at the quantum mechanical level.


Molecules are groups of atoms that exist as individual stable species because of the presence of chemical bonds between those atoms. When molecules interact with each other, for instance during chemical reactions, bonds are broken and new bonds formed, to give rise to new molecular species. Chemical bonds are established when two atoms share one electron each, and these electrons pair close to each other in the region between the two atoms – providing the glue that keeps a molecule together. Sometimes atoms contribute more than one electron to create bonds, which leads to the formation of multiple bonds. For instance, within each molecule of nitrogen, which is the major constituent of the atmosphere, three chemical bonds are established between the two N atoms. Oxygen itself contains a double bond. There is, however, a very important difference in the way reactive frontier orbital electrons are arranged in oxygen compared to those of other molecules, like nitrogen. This is what makes oxygen a unique chemical species.


Electrons are quantum particles that possess a fundamental physical property, or ‘quantum number’, known as ‘spin’. The spin of an electron can only assume two values, which are usually labelled ‘spin-up’ and ‘spin-down’. When two electrons pair to form a chemical bond, they must have different spins. This is what happens, for instance, in nitrogen; each one of the three bonds of this molecule contains exactly one spin-up and one spin-down electron. Oxygen, however, is different. In the oxygen molecule, and in accordance with Hund’s maximum multiplicity rule, two electrons with the same spin value occupy two equivalent energy levels, rather than pairing with opposite spins. The oxygen molecule is therefore a di-radical, that is, a species with two unpaired electrons. In quantum mechanics terms, this peculiar electronic state is called a triplet.


Whereas oxygen has unpaired electrons, many of the molecules with which it interacts on Earth, including most of the organic molecules that constitute living organisms and materials derived from them, possess a singlet state, with no unpaired electrons. Chemically, singlets and triplets do not successfully react with each other. This explains why flammable materials, such as wood or fuels, do not burn spontaneously when in contact with the atmosphere, which contains about 20% of triplet oxygen. However, supplying a modest amount of energy, for instance by ignition with a lighter or a match, is sufficient to convert the singlet flammable material to two paramagnetic doublet molecules capable of reacting with triplet oxygen and selfsustaining chemical oxidation, or combustion. During combustion, energy is released in the form of heat and light. According to Allen, the generation of singlet oxygen allows direct reaction with singlet biological molecules and is the most important step in the bactericidal action of neutrophils.


Allen has studied how bacteria trigger the response of the neutrophils in the body, by inducing the controlled formation of singlet oxygen. This process is catalysed by enzymes such as NADPH oxidase, and plays an important role in the respiratory burst by promoting the oxidation reactions that kill the bacteria. Singlet oxygen is extremely potent but short-lived. The respiratory burst metabolism of neutrophils drives production of singlet oxygen and microbe killing. These reactions, like those involved in the combustion of burning, emit energy in the form of light. Allen reasoned that light emission, or chemiluminescence, would follow if spin was conserved. Spin conservation is a fundamental law of quantum mechanics. Chemiluminescence provides information with regard to the nature of reaction, and measuring such luminescence provides a wealth of information on the exergonicity of the oxygenation reactions that occur in our bodies. Such measurements and the additional application of chemiluminigenic probes have allowed for the development of fast and non-invasive diagnostic tools to monitor in real time how the body responds to microbial attack or to potentially stressful environments.


We asked Professor Allen about what prompted him to study how oxygen acts as an anti-pathogen agent in the body.


He revealed that his journey in science has been relatively unique. In high school, college, and later in military service his interests were directed to the interrelatedness of systems and unusual phenomena – and he was attracted to philosophy and epistemology, especially the work of Kant.

However, his  long-lasting interest in oxygen and combustive action began in high school.

Allen explained that none of his teachers were able to adequately answer his questions as to why combustion is not spontaneous. After graduation from college, he began two years of military service starting in the infantry but having a college degree in biology-chemistry, was later assigned to technical work in clinical laboratory medicine. It was in that capacity that the ubiquitous polymorphonuclear leukocyte, typically referred to today as the neutrophil leukocyte, or neutrophil, captured his attention.


On completion of military service in 1970, and with the encouragement of Randolph M Howes, a schoolmate in college, Allen began graduate work in biochemistry at Tulane University School of Medicine in his hometown of New Orleans, Louisiana. He was especially interested in the research of Richard H Steele who had spent several years as a post-doctoral fellow with Albert Szent-Györgyi.  Allen had enormous respect for the research and writings of Szent-Györgyi. Professor Steele accepted him as a student and he began work on several projects related to riboflavin redox action and microsomal mixed function oxidase action. His interests in quantum chemistry and spectroscopy were rekindled and he began in-depth studies into these areas, especially the works of Gerhard Herzberg and Paul Dirac.


That’s all for this episode – thanks for listening. Links to the original research can be found in the shownotes for this episode. And, as always, stay subscribed to Research Pod for more of the latest science.


See you again soon.


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