The Sun Light plays an important role in regulating life (microbes,plants and animals including human beings). During earlier times light energy was used to kill pathogens. After the discovery that.white light is composed of 7 different components and each one has different energy levels, scientists wanted to know the interaction of life with each component of the white light;and found that some components are having positive effects whereas others  have negative effects. Here we have reviewed the cross talk between various frequencies of light and microbiota.The length of the light exposure to the human body changes the intestinal microbiota. This information is useful in therapeutics in addition to management of microbiota within the human body and outside too. 

The light is not the only sole factor that regulates intestinal microbiota; other factors such as, host genetics,  diet , age, mode of birth,  and presence or absence of antibiotics in the body also play an equally important role in regulating intestinal microbiota. The imbalance of gut   microbiota due to whatever reason can cause serious body disorders including cancer.

Way back in 1903 Finsen received the Nobel Prize for Physiology or Medicine for his work in treating cutaneous tuberculosis with UV light and smallpox with red light [1].   Since then many ups and downs came in this study mainly because of the introduction of new bactericidal and antibiotics drugs [2-5]. The microbiota directly affects the body and the body also influences the microbiota and since both are affected by light it was interesting to know whether “Light” affects both the systems in the similar manner or opposite way [6]. 

Another Nobel prize winning research by Sir C V Raman where he showed that white light  when passage through a prism, the white light is separated into its component colors - red, orange, yellow, green, blue and violet with different frequencies [7].  It is well known that different frequencies of light [8] has different effects on microbiota: Violet: 380–450 nm (688–789 THz frequency)Ultraviolet light inactivates microorganisms by forming pyrimidine dimers in RNA and DNA, which can interfere with transcription and replication  . Blue: 450–495 nm.Blue light can be sensed by numerous bacteria and can induce physiological responses elicited by the blue light receptors. As a result of this, blue light can regulate bacterial motility, suppress biofilm formation, and subsequently potentiate light inactivation of bacteria (9-11). Green: 495–570 nm.and Yellow: 570–590 nmn green and white color were found to get more bacterial cells or less phototoxic effect, while the cell number significantly higher than starting number. That means improving growth rate during the incubation of patient samples, exactly in case of low growing bacteria. Red: 620–750 nm (400–484 THz frequency)the treatment with red light can be potentially employed as an therapeutic method to inactivate certain pathogenic strains of porphyrin producing bacteria without the use of external photosensitizers [12]. Photobiomodulation (PBM) employs red or near-infrared (NIR) light (600-1100nm) to stimulate healing, protect tissue from dying, increase mitochondrial function, improve blood flow, and tissue oxygenation. PBM can also act to reduce swelling, increase antioxidants, decrease inflammation, protect against apoptosis, and modulate microglial activation state

Now it is well established that with constant light exposure it is possible to alter the gut microbiota which is clearly evident with recently published data [13, 14]. Constant light exposure impacts gut microbiota and its metabolic products, impairs gut barrier function and gut-liver axis, due to which the host suffers with Non-alcoholic fatty liver disease progression in HFD rats. (Fig. 1)

Fig. 1 Effects of 24h light exposure.

Recent findings have shown altered intestinal microbial communities and dysbiosis of the gut microbiota probably directly or indirectly due to exposure of light in shift workers during night. This in turn disrupts circadian rhythms. The sleep and circadian disruption in humans alters the gut microbiota, contributing to an inflammatory state and metabolic disease associated with shift work. There is a close relationship between insufficient sleep, circadian misalignment and the gut microbiota [15].     

With the advancement in optic technology Laser lights were produced which inhibit growth of the bacteria. The PBM can influence the microbiome and the known effect of PBM on cytokines, transin recent years, increasing attention has been paid to the total microbial population that colonizes the human body, chiefly in the gut and the mouth, called transcription factors, and the metabolome we introduce the term “photobiomics” to represent the combined effects of PBM on metabolomic factors, the microbiome, and the interaction between the two. Photobiomics most probably has a wider application than simply PBM. As is now generally understood, light has an effect on a wide range of living organisms in multiple biological kingdoms.  Light in general may affect the microbiome as a downstream effect.

 It is known that the composition and health of the gut microbiome affects many diseases related to metabolism, obesity, cardiovascular disorders, autoimmunity, and even brain disorders.    In consideration of the known effects of PBM on microbiota, as well as other effects of light on circadian rhythms, the present perspective introduces a new term "photobiomics" and looks forward to the application of PBM to influence the human microbiota. 

It is most amazing that the well-known light sensing organ is the pineal gland and it is deeply sitting at the base of the brain, though it is said that it receives light through the eyes but it is also a well-known fact that even blind also sense light through the pineal only. Similar to this intestinal microbiota are deeply placed yet they are regulated by external light. Thus, it is of considerable interest and importance to understand the crosstalk between circadian rhythms and microbiota and especially the microbial influences on the host. It seems both the key regulators (pineal and microbiota) are in turn regulated by light.

In addition to light other factors such as, host genetics [17], diet [18], age [19], mode of birth [20] and presence or absent antibiotics in the body [21]. Exercise regularly [22] sleep well [23].    Also play an equally important role in regulating intestinal microbiota. The imbalance   of  gut   microbiota due to whatever reason can cause serious body disorders including cancer.