The misuse of antibiotics combined with a lack of newly developed ones is the main contributors to the current antibiotic resistance crisis. There is a dire need for new and alternative antibacterial options and nanotechnology could be a solution. Metal-based nanoparticles, particularly silver nanoparticles (NAg), have garnered widespread popularity due to their unique physicochemical properties and broad-spectrum antibacterial activity. Consequently, NAg has seen extensive incorporation in many types of products across the healthcare and consumer market. Despite clear evidence of the strong antibacterial efficacy of NAg, studies have raised concerns over the development of silver-resistant bacteria. Resistance to cationic silver (Ag+) has been recognised for many years, but it has recently been found that bacterial resistance to NAg is also possible. It is also understood that exposure of bacteria to toxic heavy metals like silver can induce the emergence of antibiotic resistance through the process of co-selection. Acinetobacter baumannii is a Gram-negative coccobacillus and opportunistic nosocomial bacterial pathogen. It was recently listed as the “number one” critical level priority pathogen because of the significant rise of antibiotic resistance in this species. NAg has proven bactericidal activity towards A. baumannii, even against strains that display multi-drug resistance. However, despite ample evidence of heavy metal (including silver; Ag+) resistance in this bacterium, combined with reports of heavy metal-driven co-selection of antibiotic resistance, little research has been dedicated to assessing the potential for NAg resistance development in A. baumannii. This is worrisome, as the increasingly indiscriminate use of NAg could promote the development of silver resistance in this species, like what has occurred with antibiotics.

Background Treatment of bacterial biofilms are difficult and in many cases, expensive. Bacterial biofilms are naturally more resilient to antimicrobial agents than their free-living planktonic counterparts, rendering the community growth harder to control. The present work described the risks of long-term use of an important alternative antimicrobial, silver nanoparticles (NAg), for the first time, on the dominant mode of bacterial growth. Results NAg could inhibit the formation as well as eradicating an already grown biofilm of Pseudomonas aeruginosa, a pathogen notorious for its resilience to antibiotics. The biofilm-forming bacterium however, evolved a reduced sensitivity to the nanoparticle. Evidence suggests that survival is linked to the development of persister cells within the population. A similar adaptation was also seen upon prolonged exposures to ionic silver (Ag+). The persister population resumed normal growth after subsequent passage in the absence of silver, highlighting the potential risks of recurrent infections with long-term NAg (and Ag+) treatments of biofilm growth. The present study further observed a potential silver/antibiotic cross-resistance, whereby NAg (as well as Ag+) could not eradicate an already growing gentamicin-resistant P. aeruginosa biofilm. The phenomena is thought to result from the hindered biofilm penetration of the silver species. In contrast, both silver formulations inhibited biofilm formation of the resistant strain, presenting a promising avenue for the control of biofilm-forming antibiotic-resistant bacteria. Conclusion The findings signify the importance to study the nanoparticle adaptation phenomena in the biofilm mode of bacterial growth, which are apparently unique to those already reported with the planktonic growth counterparts. This work sets the foundation for future studies in other globally significant bacterial pathogens when present as biofilms. Scientifically based strategies for management of pathogenic growth is necessary, particularly in this era of increasing antibiotic resistance. Graphic abstract

The increasing biological use of graphene-based materials has prompted research inquiries on their effects on microorganisms. The work herein reported different types of microbiological activity of reduced graphene oxide (RGO). At relatively high concentrations (200 and 400 μg/mL), RGO exhibited antibacterial activity on the model bacterium Escherichia coli, while at lower concentrations (10 and 50 μg/mL), interestingly, no antibacterial effect was observed. Instead, an increase in the viable population after exposure at lower concentrations was observed, verified by colony counting and fluorescence microscopy. Further investigation ruled out the possibility of nutrient release from RGO being responsible for this growth-enhancing effect, whereby a comparable number of viable cells were found in the particle-free RGO leachate systems relative to the control. A before and after exposure X-ray photoelectron spectroscopy (XPS) analysis of the RGO detected less presence of C-C bond on the particle surface, suggesting the ability of the bacterium for the use of the carbon-based materials for growth. This potential RGO-cell interaction is further supported by the observed emergence of C-N bond on the particle surface, the nitrogen moieties most likely of bacterial (cell envelope) origins. Although still an early evidence, such RGO-cell interactions could explain the viable cell increase observed at the lower concentration RGO systems. The present study highlights the concentration-dependent microbiological effects of RGO, clarifying the contradicting reports on the growth enhancing versus antibacterial effect of graphene-based materials. The knowledge is important not only for the antibacterial formulation of carbon-based materials but also when assessing their environmental impact.

Several recent studies have reported on the development of bacterial resistance to broad-spectrum antimicrobial silver nanoparticles (nanosilver; NAg). NAg is currently one of the most important alternative antimicrobial agents.