The ability of microorganisms to generate resistance outcompetes with the generation of new and efficient antibiotics; therefore, it is critical to develop novel antibiotic agents and treatments to control bacterial infections. An alternative to this worldwide problem is the use of nanomaterials with antimicrobial properties. Silver nanoparticles (AgNPs) have been extensively studied due to their antimicrobial effect in different organisms. In this work, the synergistic antimicrobial effect of AgNPs and conventional antibiotics was assessed in Gram-positive and Gram-negative bacteria. AgNPs minimal inhibitory concentration was 10–12 μg mL-1 in all bacterial strains tested, regardless of their different susceptibility against antibiotics. Interestingly, a synergistic antimicrobial effect was observed when combining AgNPs and kanamycin according to the fractional inhibitory concentration index, FICI: <0.5), an additive effect by combining AgNPs and chloramphenicol (FICI: 0.5 to 1), whereas no effect was found with AgNPs and β-lactam antibiotics combinations. Flow cytometry and TEM analysis showed that sublethal concentrations of AgNPs (6–7 μg mL-1) altered the bacterial membrane potential and caused ultrastructural damage, increasing the cell membrane permeability. No chemical interactions between AgNPs and antibiotics were detected. We propose an experimental supported mechanism of action by which combinatorial effect of antimicrobials drives synergy depending on their specific target, facilitated by membrane alterations generated by AgNPs. Our results provide a deeper understanding about the synergistic mechanism of AgNPs and antibiotics, aiming to combat antimicrobial infections efficiently, especially those by multi-drug resistant microorganisms, in order to mitigate the current crisis due to antibiotic resistance.

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Some of these mutants, albeit inactive, were still able to interact with the negative regulator GrlR, indicating that loss of activity was not a consequence of protein misfolding. Additional residues in the vicinity of the HTH domain, as well as at the end of the protein, were also shown to be important for GrlA activity as a transcriptional regulator, but not for its interaction with GrlR. In summary, GrlA consists of at least two functional domains, one involved in transcriptional activation and DNA binding and the other in heterodimerization with GrlR.

A Rhizobium etli Tn5 insertion mutant, LM01, was selected for its inability to use glutamine as the sole carbon and nitrogen source. The Tn5 insertion in LM01 was localized to the rsh gene, which encodes a member of the RelA/SpoT family of proteins. The LM01 mutant was affected in the ability to use amino acids and nitrate as nitrogen sources and was unable to accumulate (p)ppGpp when grown under carbon and nitrogen starvation, as opposed to the wild-type strain, which accumulated (p)ppGpp under these conditions. The R. etli rsh gene was found to restore (p)ppGpp accumulation to a ⌬relA ⌬spoT mutant of Escherichia coli. The R. etli Rsh protein consists of 744 amino acids, and the Tn5 insertion in LM01 results in the synthesis of a truncated protein of 329 amino acids; complementation experiments indicate that this truncated protein is still capable of (p)ppGpp hydrolysis. A second rsh mutant of R. etli, strain AC1, was constructed by inserting an ⍀ element at the beginning of the rsh gene, resulting in a null allele. Both AC1 and LM01 were affected in Nod factor production, which was constitutive in both strains, and in nodulation; nodules produced by the rsh mutants in Phaseolus vulgaris were smaller than those produced by the wild-type strain and did not fix nitrogen. In addition, electron microscopy revealed that the mutant bacteroids lacked poly-␤-hydroxybutyrate granules. These results indicate a central role for the stringent response in symbiosis.Rhizobia are soil bacteria able to colonize the roots of compatible legumes under conditions of nitrogen limitation (31,39,46). This symbiotic interaction leads to the formation of organelle-like structures called nodules in the plant roots, in which these bacteria differentiate into N 2 -fixing forms known as bacteroids (31,46). Bacteroids in the nodules are surrounded by the plant cell membrane, called the peribacteroidal membrane (31,42). Bacteroids, the peribacteroidal space, and the peribacteroidal membrane are also referred to as symbiosomes (38). In the process of symbiosome formation, freeliving rhizobia move from a variable environment to a more controlled one inside the plant cells by adapting in succession to three different environments: the rhizosphere, the infection thread (IT), and the plant cell cytoplasm (31, 39, 42). Bacteroid differentiation is accompanied by the loss of bacterial cell division and by a switch from a metabolism dedicated to ammonium assimilation to one dedicated to nitrogen fixation (33, 44).The study of rhizobial metabolic networks that lead to productive nodules is clearly of importance for understanding the symbiotic process. Diverse studies have implicated amino acid metabolism in the bacterial adaptation to nodule conditions, as well as in the metabolic interchange between plant and bacteroids in fully developed nodules (7,16,20,33,34). The rhizobial metabolic adaptations required for using amino acids inside the IT and ammonium excretion in these circumstances could function as a signal to uncouple ammonium assimilation a...