Calcium phosphate nanoparticles as intrinsic inorganic antimicrobials: mechanism of action

Abstract

This is the final report of the study aimed at assessing the antimicrobial activity of calcium phosphate (CP) nanoparticles delivered in the form of hydroxyapatite (HAp) or amorphous CP (ACP) and understanding the fundamental principles behind their mechanisms of action. Not responding to propidium iodide and causing no gross morphological changes except moderate stress-induced filamentation in Escherichia coli (E. coli), CP nanoparticles were shown to be bacteriostatic, not bactericidal. Also, the lack of expression of genes involved in DNA repair indicated no genotoxic activity. In contrast, the softening of amide infrared bands and the partial dissociation of lipopolysaccharide structures comprising the membrane of Gram-negative Pseudomonas aeruginosa (P. aeruginosa) was detected in a vibrational spectroscopic analysis of the nanoparticle/bacterium interaction. Similarly, the inhibition of the growth of Staphylococcus aureus (S. aureus) was paralleled by a reduced integrated intensity and the softening of the C = O ester carbonyl stretch in lipoteichoic acid, a major component of the Gram-positive cell membrane. Electron microscopy analyses confirmed that changes to the cell membrane are a major mode of action of CP nanoparticles. While HAp got internalized by E. coli significantly more than ACP, the membrane damage was more pronounced in ACP-treated bacteria, which was explained by the higher surface reactivity of ACP. HAp nanoparticles decreased the activity of overexpressed efflux pumps in methicillin-resistant S. aureus, suggesting that they may hijack these pumps and use them to enter the cell without producing any visible damage to the membrane, thus acting on the cell from the inside out, as opposed to ACP, whose action is mostly external in mechanism. This may explain why HAp, unlike ACP, suppresses the mechanisms of resistance in methicillin- and multidrug-resistant S. aureus and P. aeruginosa, respectively. The findings of this study will be essential in the optimization of these nanoparticles for becoming an alternative to less biocompatible inorganics and small molecule antibiotics in the global effort to curb the rising resistance of bacterial pathogens to the existing therapies.

Altering Microbiomes with Hydroxyapatite Nanoparticles: A Metagenomic Analysis

“…This means that even the evident increase in the relative population of phyla present at under 1% of the total microbiome could be a simple artifact of the removal of more prominent phyla from the system by HAp, primarily Proteobacteria. Overall, these results are also in agreement with the earlier detected greater antibacterial activity of HAp against Gram-negative bacteria as compared to the activity with respect to Gram-positive ones [ 20 , 21 , 22 ]. These results are summarized in Table 1 , demonstrating the ability of HAp to not only remove but also inhibit planktonic Gram-negative microorganisms from aqueous systems.…”

“…When both the cells and the particles adopt elongated morphologies, the surface area of their contact will be higher on average than when at least one of them adopts the shape of a sphere. As for (ii), this evokes the aforementioned Vroman effect as the explanation why HAp is highly effective against Pseudomonas in monoculture [ 20 , 21 , 22 ], but it does not bind it under the microbial community conditions measured here. According to this effect, more mobile entities tend to reach a foreign surface first and adsorb on it, but only to be later displaced by the less mobile, if not also bulkier, entities, which have a greater thermodynamic affinity for binding.…”

“…Despite being one of the oldest biomaterials in research and clinical application, HAp continues to attract the attention of researchers to this day, and new studies on it do not abate [ 17 , 18 , 19 ]. Recently, our group demonstrated that HAp displays finite antibacterial properties, which appear to become intensified when the bacterial species are converted from regular lab strains to clinical, multidrug-resistant ones [ 20 , 21 , 22 ]. Interestingly, when it comes to mammalian organisms, HAp, such as that comprising the dental tissues, is in contact with a rich microbiome of the oral cavity, while many infective pathologies of the maxillofacial area, such as periodontitis [ 23 ] or dental caries [ 24 ], are increasingly being seen as diseases of the microbiome.…”

Nanomedicine for The Poor: A Lost Cause or An Idea Whose Time Has Yet to Come?

“…The use of CaP, for example, as a means for bridging the gap between the rich and the poor has aesthetic connotations that may resonate with cultures other than the western and thus go beyond sheer economic considerations. The idiosyncrasy of CaP among similar bioceramics [10], its biogenic proliferation despite the intrinsic structural weaknesses [11], and its chemical resemblance to the skeletal foundations of our bodies [12] can carry such aesthetic connotations. CaP is also a material whose popularity is evenly distributed across the globe, meaning that the sole act of engaging in its research may be sufficient to bring people from various cultures together.…”

Recent Advances in the Evaluation of Antimicrobial Materials for Resolution of Orthopedic Implant-Associated Infections In Vivo

“…A range of ceramic materials have been used in preclinical orthopedic antimicrobial applications including calcium sulfate, ,− hydroxyapatite, ,− β-tricalcium phosphate, zirconium nitride, borate bioactive glass, and calcium phosphate , (Table ). Several ceramics are intrinsically antimicrobial, such as calcium phosphate and bioactive glass; , however, for orthopedic implant infection applications, ceramics often incorporate antibiotics. For degradable ceramic materials ( e.g.…”