Nanoscale Titanium Surface Engineering via Low-Temperature Hydrothermal Etching for Enhanced Antimicrobial Properties.

Bioinspired nanotopography artificially fabricated on titanium surfaces offers a solution for the rising issue of postoperative infections within orthopedics. On a small scale, hydrothermal etching has proven to deliver an effective antimicrobial nanospike surface. However, translation to an industrial setting is limited by the elevated synthesis temperature (150 °C) and associated equipment requirements. Here, for the first time, we fabricate surface nanostructures using comparatively milder synthesis temperatures (75 °C), which deliver physicochemical properties and antimicrobial capability comparable to the high-temperature surface. Using a KOH etchant, the simultaneous formation of titania and titanate crystals at both temperatures produces a one-dimensional nanostructure array. Analysis indicated that the formation mechanism comprises dissolution and reprecipitation processes, identifying the deposited titanates as hydrated layered tetra-titanates (K2Ti4O9·nH2O). A proposed nanospike formation mechanism was confirmed through the identification of a core and outer shell for individual nanostructures, primarily comprised of titanates and titania, respectively. Etching conditions dictated crystalline formation, favoring a thicker titanate core for nanorods under higher synthesis temperatures and etchant concentrations. A bactericidal investigation showed the efficacy against Gram-negative bacteria for a representative low-temperature nanosurface (34.4 ± 14.4%) was comparable to the higher temperature nanosurface (34.0 ± 17.0%), illustrating the potential of low-temperature hydrothermal synthesis. Our results provide valuable insight into the applicability of low-temperature etching protocols that are more favorable in large-scale manufacturing settings.

Development of Nanoparticle Adaptation Phenomena in Acinetobacter baumannii: Physiological Change and Defense Response.

The present work describes the evolution of a resistance phenotype to a multitargeting antimicrobial agent, namely, silver nanoparticles (nanosilver; NAg), in the globally prevalent bacterial pathogen Acinetobacter baumannii. The Gram-negative bacterium has recently been listed as a critical priority pathogen requiring novel treatment options by the World Health Organization. Through prolonged exposure to the important antimicrobial nanoparticle, the bacterium developed mutations in genes that encode the protein subunits of organelle structures that are involved in cell-to-surface attachment as well as in a cell envelope capsular polysaccharide synthesis-related gene. These mutations are potentially correlated with stable physiological changes in the biofilm growth behavior and with an evident protective effect against oxidative stress, most likely as a feature of toxicity defense. We further report a different adaptation response of A. baumannii to the cationic form of silver (Ag+). The bacterium developed a tolerance phenotype to Ag+, which was correlated with an indicative surge in respiratory activity and changes in cell morphology, of which these are reported characteristics of tolerant bacterial populations. The findings regarding adaptation phenomena to NAg highlight the risks of the long-term use of the nanoparticle on a priority pathogen. The findings urge the implementation of strategies to overcome bacterial NAg adaptation, to better elucidate the toxicity mechanisms of the nanoparticle, and preserve the efficacy of the potent alternative antimicrobial agent in this era of antimicrobial resistance. IMPORTANCE 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. However, no studies have yet established whether Acinetobacter baumannii, a globally prevalent nosocomial pathogen, can develop resistance to the nanoparticle. The study herein describes how a model strain of A. baumannii with no inherent silver resistance determinants developed resistance to NAg, following prolonged exposure. The stable physiological changes are correlated with mutations detected in the bacterium genome. These mutations render the bacterium capable of proliferating at a toxic NAg concentration. It was also found that A. baumannii developed a "slower-to-kill" tolerance trait to Ag+, which highlights the unique antimicrobial activities between the nanoparticulate and the ionic forms of silver. Despite the proven efficacy of NAg, the observation of NAg resistance in A. baumannii emphasises the potential risks of the repeated overuse of this agent on a priority pathogen.

Evolution of biofilm-forming pathogenic bacteria in the presence of nanoparticles and antibiotic: adaptation phenomena and cross-resistance.

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.

Emerging Concern for Silver Nanoparticle Resistance in Acinetobacter baumannii and Other Bacteria.

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.