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As a part of our drug discovery program, ursolic acidwas chemically transformed into six semi-syntheticderivatives, which were evaluated for their antibacterialand drug resistance reversal potential in combinationwith conventional antibiotic nalidixic acid against thenalidixic acid-sensitive and nalidixic acid-resistantstrains of Escherichia coli. Although ursolic acidand its all semi-synthetic derivatives did not showantibacterial activity of their own, but in combination,they significantly reduced the minimum inhibitoryconcentration of nalidixic acid up to eightfold. The3-O-acetyl-urs-12-en-28-isopropyl ester (UA-4) and3-O-acetyl-urs-12-en-28-n-butyl ester (UA-5) deriva-tives of ursolic acid reduced the minimum inhibitoryconcentration of nalidixic acid by eightfold againstnalidixic acid-resistant and four and eightfold againstnalidixic acid-sensitive, respectively. The UA-4 andUA-5 were further evaluated for their synergy potentialwith another antibiotic tetracycline against the multi-drug-resistant clinical isolate of Escherichia coli-KG4.The results showed that both these derivatives in com-bination with tetracycline reduced the cell viability inconcentration-dependent manner by significantly inhib-iting efflux pump. This was further supported by the insilico binding affinity of UA-4 and UA-5 with effluxpump proteins. These ursolic acid derivatives may findtheir potential use as synergistic agents in the treat-ment of multidrug-resistant Gram-negative infections.Key words: ester analog, multidrug-resistant, nalidixic acid,non-antibiotics, tetracycline, triterpenoidsReceived 27 May 2014, revised 1 December 2014 andaccepted for publication 1 December 2014

The purpose of present investigation was to understand the drug resistance reversal mechanism of 4-hydroxy-α-tetralone (1) isolated from Ammannia spp. along with its semi-synthetic derivatives (1a-1e) using multidrug resistant Escherichia coli (MDREC). The test compounds did not show significant antibacterial activity of their own, but in combination, they reduced the minimum inhibitory concentration (MIC) of tetracycline (TET). In time kill assay, compound 1 and its derivative 1e in combination with TET reduced the cell viability in concentration dependent manner. Compounds 1 and 1e were also able to reduce the mutation prevention concentration of TET. Both compounds showed inhibition of ATP dependent efflux pumps. In real time polymerase chain reaction (RT-PCR) study, compounds 1 and 1e alone and in combination with TET showed significant down expression of efflux pump gene (yojI) encoding multidrug ATP binding cassettes (ABC) transporter protein. Molecular mechanism was also supported by the in silico docking studies, which revealed significant binding affinity of compounds 1 and 1e with YojI. This study confirms that compound 1 and its derivative 1e are ABC efflux pump inhibitors which may be the basis for development of antibacterial combinations for the management of MDR infections from inexpensive natural product.

Antibacterial activity of lysergol (1) and its semi-synthetic derivatives (2-14) and their synergy with the conventional antibiotic nalidixic acid (NA) against nalidixic acid-sensitive (NASEC) and nalidixic acid-resistant (NAREC) strains of Escherichia coli were evaluated. Lysergol (1) and derivatives (2-14) did not possess antibacterial activity of their own, but in combination, they significantly reduced the minimum inhibitory concentration (MIC) of NA. All the derivatives showed two- to eightfold reduction in the MIC of NA against NAREC and NASEC. Further, lysergol (1) and its derivatives 10 and 11 brought down eightfold reductions in the MIC of tetracycline (TET) against multidrug-resistant clinical isolate of E. coli (MDREC). Treatment of these strains with the combinations of antibiotics and lysergol and its derivatives 10 and 11 (at reduced concentrations) significantly decreased the viability of cells. In an another observation, lysergol and its derivatives 10 and 11 inhibited ATP-dependent efflux pumps, which was evident by ATPase inhibition and down-regulation of multidrug ABC transporter ATP-binding protein (yojI) gene. These results may be of great help in antibacterial drug development from a very common, inexpensive, and non-toxic natural product.

DbMDR is non-redundant reference database of multidrug resistance (MDR) genes and their orthologs acting as potential drug targets. Drug resistance is a common phenomenon of pathogens, creating a serious problem of inactivation of drugs and antibiotics resulting in occurrence of diseases. Apart from other factors, the MDR genes present in pathogens are shown to be responsible for multidrug resistance. Much of the unorganized information on MDR genes is scattered across the literature and other web resources. Thus, consolidation of such knowledge about MDR genes into one database will make the drug discovery research more efficient. Mining of text for MDR genes has resulted into a large number of publications but in scattered and unorganized form. This information was compiled into a database, which enables a user not only to look at a particular MDR gene but also to find out putative homologs based on sequence similarity, conserved domains, and motifs in proteins encoded by MDR genes more efficiently. At present, DbMDR database contains 2843 MDR genes characterized experimentally as well as functionally annotated with cross-referencing search support. The DbMDR database (http://203.190.147.116/dbmdr/) is a comprehensive resource for comparative study focused on MDR genes and metabolic pathway efflux pumps and intended to provide a platform for researchers for further research in drug resistance.