Pritam Chattopadhyay

Microbial transformation of xenobiotics for environmental bioremediation

The accumulation of recalcitrant xenobiotic compounds is due to continuous efflux from population and industrial inputs that have created a serious impact on the pristine nature of our environment. Apart from this, these compounds are mostly carcinogenic, posing health hazards which persist over a long period of time. Metabolic pathways and specific operon systems have been found in diverse but limited groups of microbes that are responsible for the transformation of xenobiotic compounds. Distinct catabolic genes are either present on mobile genetic elements, such as transposons and plasmids, or the chromosome itself that facilitates horizontal gene transfer and enhances the rapid microbial transformation of toxic xenobiotic compounds. Biotransformation of xenobiotic compounds in natural environment has been studied to understand the microbial ecology, physiology and evolution for their potential in bioremediation. Recent advance in the molecular techniques including DNA fingerprinting, microarrays and metagenomics is being used to augment the transformation of xenobiotic compounds. The present day understandings of aerobic, anaerobic and reductive biotransformation by co-metabolic processes and an overview of latest developments in monitoring the catabolic genes of xenobiotic-degrading bacteria are discussed elaborately in this work. Till date, several reviews have come up, highlighting the problem of xenobiotic pollution, yet a comprehensive understanding of the microbial biodegradation of xenobiotics and its application is in nascent stage. Therefore, this is an attempt to understand the microbial role in biotransformation of xenobiotic compounds in context to the modern day biotechnology.

Virulence plasmid of Aeromonas hydrophila induces macrophage apoptosis and helps in developing systemic infection in mice.

Pathogenic Aeromonas hydrophila Strain AO1 bears a 21kb plasmid encoding several virulence determinants. Infection studies revealed that this isolate induced cytotoxicity in BALB/c mice splenic macrophages involving reactive oxygen species generation. DNA gel, Hoechst 33342, annexin-V and TUNEL assay documented macrophage death induced by 21kb plasmid bearing isolates to be apoptotic in nature. Apoptosis induced by the plasmid bearing isolates involved initiator caspase-8 and caspase-9 and executed by effector caspase-3. ELISA revealed the wild-type isolate as weak inducer of pro-inflammatory cytokine IL-1beta. Oral infection with wild-type isolates caused systemic infection in BALB/c mice. With plasmid curing the isolate looses several virulence attributes including cytotoxic potential. The cured isolate induced significant amounts of IL-1beta from infected macrophages, disseminated into Peyers patches, spleen and liver but never attained the bacterial loads recorded with wild-type isolates and were rapidly cleared. Transformation of 21kb plasmid helped the cured bacteria regain wild-type virulence attributes, apoptotic potential and ability to cause systemic infection in mice. Thus the 21kb plasmid is a virulence factor in mice. It helps in suppressing the production of pro-inflammatory cytokine IL-1beta and induced apoptosis of host macrophages enabling A. hydrophila to evade host immune responses and establish systemic infection in mice.

On the sub-band gap optical absorption in heat treated cadmium sulphide thin film deposited on glass by chemical bath deposition technique

The sub-band gap optical absorption in chemical bath deposited cadmium sulphide thin films annealed at different temperatures has been critically analyzed with special reference to Urbach relation. It has been found that the absorption co-efficient of the material in the sub-band gap region is nearly constant up to a certain critical value of the photon energy. However, as the photon energy exceeds the critical value, the absorption coefficient increases exponentially indicating the dominance of Urbach rule. The absorption coefficients in the constant absorption region and the Urbach region have been found to be sensitive to annealing temperature. A critical examination of the temperature dependence of the absorption coefficient indicates two different kinds of optical transitions to be operative in the sub-band gap region. After a careful analyses of SEM images, energy dispersive x-ray spectra, and the dc current-voltage characteristics, we conclude that the absorption spectra in the sub-band gap domain is possibly associated with optical transition processes involving deep levels and the grain boundary states of the material.

Microbial Degradation of Recalcitrant PAHs-Microbial Diversity Involving Remediation Process

Domestic pollutants, largely due to population explosion and industrial inputs lead to the accumulation of various types of recalcitrant xenobiotic compounds (Hadibarata et al. 2009; Igwo-Ezikpe et al. 2010). As majority of them persist for longer period of time and are carcinogenic in nature, their disposal is a matter of global concern (Jain et al. 2005). Primarily, xenobiotic compounds are anonymous to living organisms and also have a tendency to get accumulated in the environment (Sinha et al. 2009). They encompass pesticides, fuels, solvents, alkanes, synthetic azo dyes, polyaromatic, nitroaromatic, chlorinated and polycyclic hydrocarbons. Amongst them, the presence of polycyclic aromatic hydrocarbons (PAHs) in the environment causes acute health hazard with their intrinsic chemical stability, high recalcitrance ability against different types of degradation and high toxicity to living organisms for their mutagenic or carcinogenic properties (Zhang et al. 2006). Apart from it, they are ubiquitous and prevail as persistent bioaccumulative toxins (PBT) (NiChadhain et al. 2006). For instance, phenanthrene, a lipophilic and relatively insoluble in water, is skin photosensitizer and mild allergenic to human (Hafez et al. 2008). It is also found as an inducer of the sister chromatid exchange process (Popp et al. 1997) and a potent inhibitor of gap-junction intercellular communications (Blaha et al. 2002).

Effects of annealing and structural phase transformation on the Urbach absorption in thin silver sulphide films

The sub-band gap optical absorption in silver sulphide thin films prepared by chemical conversion technique has been studied in the light of Urbach absorption law at different annealing temperatures. The study reveals the Urbach absorption parameters and band gap of the material are sensitive to annealing temperature. Interestingly, these parameters undergo a sudden change as the annealing temperature exceeds a certain critical value. The observed changes in the values of absorption parameters have been attributed to the structural phase transition and the modification of grain boundary interfaces and/or changes in the crystallographic orientation due to annealing.