Abstract
Haloarchaea are salt-loving halophilic microorganisms that inhabit marine environments, sea water, salterns, and lakes. The resistance of haloarchaea to physical extremities that challenge organismic survival is ubiquitous. Metal and antibiotic resistance of haloarchaea has been on an upsurge due to the exposure of these organisms to metal sinks and drug resistance genes augmented in their natural habitats due to anthropogenic activities and environmental pollution. The efficacy of silver nanoparticles (SNPs) as a potent and broad spectrum inhibitory agent is known, however, there are no reports on the inhibitory activity of SNPs against haloarchaea. In the present study, we have investigated the antimicrobial potentials of SNPs synthesized using aqueous leaf extract of Cinnamomum tamala against antibiotic resistant haloarchaeal isolates Haloferax prahovense RR8, Haloferax lucentense RR15, Haloarcula argentinensis RR10 and Haloarcula tradensis RR13. The synthesized SNPs were characterized by UV-Vis spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, dynamic light scattering, X-ray diffraction and Fourier transform infrared spectroscopy. The SNPs demonstrated potent antimicrobial activity against the haloarchaea with a minimum inhibitory concentration of 300–400 μg/ml. Growth kinetics of haloarchaea in the presence of SNPs was studied by employing the Baranyi mathematical model for microbial growth using the DMFit curve fitting program. The C. tamala SNPs also demonstrated cytotoxic activity against human lung adenocarcinoma epithelial cell line (A540) and human breast adenocarcinoma cell line (MCF-7). The mechanism of inhibition of haloarchaea by the SNPs was investigated. The plausible mechanism proposed is the alterations and disruption of haloarchaeal membrane permeability by turbulence, inhibition of respiratory dehydrogenases and lipid peroxidation causing cellular and DNA damage resulting in cell death.
Highlights
Antibiotic resistance has been an emerging phenomenon in recent times due to the rampant use of antibiotics and spread of multi-drug resistance genes amongst microorganisms
The biosynthesis of silver nanoparticles (SNPs) was performed by using leaf extract of C. tamala and the color change in the reaction mixture from pale to dark brown indicative of the bioreduction of silver nitrate was observed at regular time intervals (1 h) using UV-Vis spectrophotometry (Figure 1A)
The analysis revealed them to be spherical in shape with an average size ranging from ∼25–50 nm (Figure 1B) and the energy-dispersive X-ray spectroscopy (EDS) confirmed the presence of silver in ionic composition of the SNP (Figure 1C)
Summary
Antibiotic resistance has been an emerging phenomenon in recent times due to the rampant use of antibiotics and spread of multi-drug resistance genes amongst microorganisms. The drug resistance has disseminated from pathogenic bacteria to non-pathogenic environmental isolates due to the release of hospital effluents and sewage in marine habitats like the sea, estuaries, and aquatic systems (Vaidya, 2011; Da Costa et al, 2013). Extreme haloarchaea are halophilic archaea that occur in such marine and aquatic environments and thrive in the presence of 1.5–5 M NaCl (Oren, 2008; Thombre et al, 2016a). Their resistance to copious stress factors like oxygen limitation, salinity, temperature, and perchlorate is well documented (DasSarma, 2006; DasSarma et al, 2012). It is essential to explore antimicrobial agents for inhibition and control of haloarchaea
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