The Effect of Silver Nanoparticles on the Gut Microbiota and Rat Digestive Tract

Silver nanoparticles (AgNPs) that have antimicrobial properties are gradually playing a role in many aspects of consumer products such as dietary health supplements, topical wound dressing, and food-related products, to prolong the shelf life and lower the potential growth of bacteria and coatings for medical devices (Franci). Regarding that, there are growing concerns about their potential adverse health consequences and environmental contamination.

Due to the large surface area to volume ratio of nanoparticles, they show high reactivity towards cells, besides, to easily attaching to cellular surfaces. Upon attaching themselves, the nanoparticles undergo endocytosis which can lead to fragmentation resulting in different ionic charges or forming aggregates; therefore, culminating in compositions with varying cellular interaction properties (Mysara). The size, surface coating, and shape largely determine the cellular adhesion and interaction properties that dictate their toxicity. Apart from affecting the host, ingested AgNPs can impact the gut microbiome.

A small proportion of rodent studies have yielded minimal evidence as to whether AgNPs and other nanoparticles affect the gut microbiome. In one study, there was the suggestion that rats which got orally exposed for a long duration to various doses and sizes of AgNPs showed a decline in the population of Lactobacillus and Firmicutes (Li and Dongyao). Conversely, there was a greater proportion of gram-negative bacteria that tended to be more pathogenic in the gastrointestinal system (Antonopoulos and Dionysios). An apparent limitation is selectively screening bacteria groups with real-time PCR analysis. Other researchers undertook a global methodology in their approach by using 16rRNA sequencing of gut bacteria from mice exposed to the same duration of AgNPs, doses, and size (Donaldson). They, however, reported no effect on gut microbiome diversity or communities.

Another study undertaken on the rodents that gauged cecal bacteria phyla found young rats exposed to different doses of AgNPs for a month never showed any variation relative to controls (Li and Dongyao). A recent experiment that used next-generation sequencing revealed mice orally subjected for a month to varying doses of AgNPs showed dose-dependent interferences (Loeschner and Katrin). It concerns measures on bacterial sequence and population. In the analysis, the exposure to AgNPs escalated the ration between Bacteriodetes and Firmicutes phyla that got mainly occasioned by changes in the S24-7 family and Lachnospiraceae.

The effects of AgNPs on the gut microbiome received an examination in other species (Frohlich, Esther and Eleonore). An example is in pigs. The conclusion was AgNPs may be a possible antimicrobial additive. Exposure of fruit flies to AgNPs leads to less diverse gut microbiota, overgrowth of Lactobacillus brevis and a decline in Acetobacter in comparison to those exposed to copper NPs. Examination of the effects of AgNPs on a determined bacterial community established from a suitable human donor discloses that the particles negatively influence the bacterial communities. It is as determined by alterations in fatty acid methyl ester profiles and gas production. The research also highlighted AgNPs stimulate a change in the bacterial community determined by various methods.

In vivo and in vitro assessments propose that AgNPs can encourage non-coding RNA and transcriptomic changes in brain regions, neurons, and hippocampus (Hans and Katrine). The studies are, however, not consistent. After fourteen days of exposure to AgNPs in diet, no effects got detected in pigs, but there was significant body weight increase. In mice and rats, AgNPs lowered the animals growth rate in addition to affecting intestinal microvilli. There was no difference in body weight, and detection got made in toxicity and inflammation in the kidney. There were also behavioral disruptions particularly in altered activity level, depression-like behavior, anxiogenic effects and cognitive functions reported after openly exposing AgNPs to rodents and fish (Hans and Katrine). Other evident neurotoxicological effects include astrocyte swelling, amyloid plagues, edema formation, and synaptic degradation, an increase in reactive oxygen species, blood - brain barrier and tight junction disruption.

The observable changes in the EPM are due to possible ultrastructural and molecular changes. The other possible cause of the neurobehavioral disruptions is brought about by consistent exposure to AgNPs may be mediated by gut microbiome modifications (Behra). Evidence supporting the finding from research is the existence of gut microbiome axis. Findings supporting the spindle originate from germ-free mice lacking a gut microbiome. It is in addition to the demonstration of some behavioral changes to counterparts that possess the microbiome particularly intensified anxiety-like traits. In adjusting the gut microbiome, AgNPs could affect intestinal epithelial barrier that may heighten the possibility of bacterial pathogens penetrating the systemic circulation and reach the brain causing CNS dysfunction (Eckburg). Besides, the AgNPs might get spread to the blood-brain barrier which would directly impact brain segments.

From current studies undertaken, short-term and low-dose exposure of rats to AgNPs increases anxiety-like traits and possible stereotypical actions that effect is more evident to those in contact with AgNS. Both AgNPs and AgNS demonstrate antimicrobial activity by lowering select bacteria made up of the gut microbiome (Frohlich, Esther and Eleonore). The bacterial variations were in unison with some of the traits gauged in the EPM. There was no proof on overt histopathological alterations identified in the brain or gastrointestinal system. There is, however, a knowledge gap that exists on whether the detected variations are brought about by ultrastructural changes in the brain and AgNP-induced transcriptomic against the contributions that take place in the gut dysbiosis (Javurek). Due to the growing use of AgNPs, present findings raise concern about the possibility of exposing humans.

Intestinal microbiota plays a significant role in gut physiology and homeostatic. Imbalances in the bacterial community can lead to chronic diseases and transient intestinal dysfunctions (Manichanh). In the mice model, deep sequencing revealed intestinal bacterial diversity exceeding that of the human. The transplantation leads to an increase in microbial diversity of the recipients brought about by capturing of new phylotypes and surge in their numbers (Manichanh and Chaysavanh). Nevertheless, when transplantation took place after antibiotic intake, the result combined the reshaping effects of individual treatments (Manichanh and Chaysavanh). It, therefore, lowers the recipient bacterial load intake before transplantation that never increased establishment of donor phylotypes. In general, the original microbial composition is more plastic than anticipated. Nonetheless, antibiotic treatment interferes with the creation of exogenous community (Ji and Shou). It is likely conditioned more by altered microbiome caused by antibiotics rather than the first bacterial loss.

The gut microbiome is significantly involved in many aspects of health. Nevertheless, it can be promptly perturbed by environmental toxicants. Interaction among host, microbiome, and gut is complex (Campbell and James). There has been a demonstration on how gut microbiome phenotypes get driven by bacterial infection and genetics and the response to arsenic exposure. The issue raises the question on the role sex plays in exposure induced microbiome response (Markle and Janet). Results indicate that arsenic exposure disconcerted the trajectory and function of the gut microbiome in a particular manner.

Gender differences in silver accumulation were evident with regards to certain organs and tissues with accumulation being higher in female rats particularly on the liver, colon, and kidney (Kim and Yong). The study was meant to evaluate particulate and ionic forms of silver and particle size for alterations in silver accumulation, toxicity, and morphology (Boudreau and Mary). In conclusion, controlled usage of AgNP doses relevant for human dietary intake induces microbial alterations which can lower the possible risk of microbial changes. Care is, however, necessary to minimize the impact of the nanoparticles on human and the environment.

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Kozich, James J...