National Centre for Earth Science Studies

The presence of bacterial biofilm on the tooth or root surface is a major cause of gingivitis and periodontitis. Mechanical removal of biofilm, adjunctive use of antibiotics and surgery are conventional methods of periodontitis therapy. In the light of recent reports about bacterial strains becoming resistant to frequent doses of antibiotics researchers are currently investigating the use of antimicrobial photodynamic therapy (aPDT) as an alternative to antibiotic drugs in the treatment of periodontal infections. During aPDT, intense light irradiation of affected lesion after application of photosensitizing dyes induces phototoxic reactions, which release oxygen free radicals that selectively destroy the bacterial cells.

As part of this Indo-Bulgarian collaborative project it is proposed to develop technologies and treatment modalities for photo-inactivation of periodonto-pathogenic bacteria. The study involves identification of pathogenic bacteria (aerobic and anaerobic) present in periodontal pockets of local population, their culture and photo-inactivation in the presence of suitable non-toxic photosensitizers both under in vitro and in vivo conditions (Fig. 1).

A clinical trial has been initiated at the Govt Dental College to test the potential of using diffuse reflectance imaging detect and monitor periodontal infection by comparison with pocket depth and gingival inflamation. In patients diagnosed with aggressive periodontitis, PDT is performed (Fig. 2) with methylene blue as sensitizer at 10mg/mL and a laser (655nm) irradiation dose of 60 mW/cm2.

Fig. 1. Antimicrobial PDT of E. Fecalis bacteria in vitro in the presence of riboflavin

Fig. 2. Clinical trial for treatment of aggressive periodontitis using aPDT. The inset a close-up of the gingiva after application of the methylene blue.

Fig. 1. LIF spectra of different species recorded in situ from corals in Palk Bay normalized to chlorophyll fluorescence peak intensity at 680 nm

Coral reefs are one of the most productive ecosystems on Earth. They are also the most sensitive among ecosystems to long term climate change. Around the world corals are under threat from a multitude of sources. Depending on their location, reefs have been damaged directly through harmful practices such as coral mining, overfishing, pollution, haphazard coastal development and even by careless pleasure diving by tourists. Lakshadweep archipelago, which consists of 12 atolls and 3 reefs, is also threatened by the change in environmental condition due to alterations in salinity, rise in seawater temperature, rise in carbon dioxide content of the seawater, pollution, overfishing etc. Understanding the impact of environmental changes on coral reefs by fluorescence imaging enables us to obtain knowledge on coral growth and their adaptation under ambient conditions, without disturbing their natural habitat.

In situ LIF measurements of corals such as Porites lutea, Acropora digitifera, Montipora foliosa and Goniastrea rectiformis were carried out using the LIFRS system from a boat in the Palk Bay near Rameswaram. The LIF spectra of corals was found to consist of peaks at 485, 550, 575, 685 and 730 nm that varies in intensity and position among coral species (Fig. 1). Change in fluorescence emission intensity and peak position with stress points to the role of host pigments in coping with stress. Our study has shown that coral species could be identified and stress induced bleaching and mortality could be detected based on fluorescence spectral signatures.

Laser-induced multi-spectral fluorescence imaging system (LIMFIS) was developed to record fluorescence images of corals (Fig. 2). Diode-pumped solid-state (DPSS) laser (457 nm) operated in the pulsed mode at 20 kHz is used as the excitation source. The expanded laser beam illuminates corals on the ocean floor and an intensified CCD camera records the fluorescence images through a liquid crystal tunable filter (LCTF) and camera lens. The LCTF is tuned to the coral emission peaks sequentially to record the fluorescence images and coral health-sensitive information can be obtained from the image intensity ratios (Fig. 3).

Fig. 2. Laser-induced Multi-spectral fluorescence Imaging System (LIMFIS) developed for remote sensing of corals

Fig. 2. Laser-induced Multi-spectral fluorescence Imaging System (LIMFIS) developed for remote sensing of corals

Fig. 1. Sunlight-induced multi-spectral fluorescence imaging system for vegetation stress studies

Plant growth and development depend on photosynthetic efficiency, which in turn is related to the availability of sufficient water, mineral nutrients, carbon dioxide and light. Insufficient availability of these vital inputs due to natural or anthropogenic stress can affect the photosynthetic performance directly or indirectly, thereby altering the optical fluorescence properties. Photosynthesis is particularly sensitive to stresses, such as drought, nutrient deficiencies, acid rain, suspended particulates and air pollutants.

A multi-spectral imaging system was developed for fluorescence imaging of sunlight induced plant fluorescence from different types of vegetation (Fig. 1). The inverse dependence of plant fluorescence on photosynthetic activity is used to assess the impact of stress on plants. The small component of emitted fluorescence on absorption of solar energy is extracted from the reflectance spectra of plants by using the Fraunhofer discrimination technique. It is hoped that this instrument could aid in passive remote sensing of vegetation through the measurement of fluorescence at selected Fraunhofer lines located close to the fluorescence emission peaks, followed by digital processing of the recorded images. The study covers monitoring of vegetation health in different plant types exposed to varying climatic conditions.

Fig. 2. Variation in chlorophyll ratio of a senescent to dry leaf on dark background.

Vegetation fluorescence reflects the physiological status of plants and this principle has been applied in classification of plants, identification of vegetation characteristics, physiological and nutrient stress detection, weed infestation identification and related developments. It is well established that the intensity ratios of these emission bands, in particular the chlorophyll bands, are good indicators of photosynthetic function. In Fig. 3 the F685/F720 fluorescence intensity ratio image of a partially dry leaf is displayed in false colours with value of 0.66 for blue colour (green area of leaf) , 0.829 for green colour (senescent leaf) and 0.918 for red colour (dry leaf).

Fig. 3. False colored NDVI image of a tree 100 meter away from camera.

Reflectance images of distant trees were recorded at 720 and 687 nm with the imaging camera. From the visible and NIR light reflected by vegetation, the Normalized Difference Vegetation Index (NDVI) that describes plant productivity was computed using the relation NDVI= (NIR-VIS)/ (NIR+VIS). For the green denser region of vegetation, the NDVI ranges from 0.5 to 0.7. The false coloured NDVI image of a tree is shown in Fig. 3; the area with vegetation is shown as red to yellow in colour with the NDVI varying from 0.2 to 0.6. Healthy vegetation absorbs most of the visible light that falls on it and reflects a large portion of the NIR light. Unhealthy or sparse vegetation reflects more visible light and less NIR light, yielding a lower NDVI. The fluorescence ratio F687/F720 shows increasing trend with senescence from 1.22 to 1.35 while the NDVI from the corresponding image area shows a decreasing trend from 0.27 to 0.13.