Biocidal activity of metalloacid-coated surfaces against multidrug-resistant microorganisms

Nosocomial infections are a major cause of patient morbidity and mortality. Those associated with contaminated surfaces and the inadequate hand hygiene of healthcare workers (HCWs) are avoidable by cleaning and/or disinfecting environmental surfaces and by appropriate hygiene practices[1‐4]. The transmission of microorganisms responsible for contaminated surface-associated nosocomial infections is well understood. Briefly, patients and HCWs carry and shed microorganisms around them. Surviving microorganisms contaminate near-patient items, which then act as reservoirs for microbial pathogens and become sources of contamination for the hands of HCWs and of colonization for new patients. Microbial colonization of surfaces can be restricted by cleaning procedures that employ effective chemical products. Cleaning reduces the prevalence of microorganisms in the environment and has been demonstrated to be a key-measure in the control of outbreaks associated with methicillin-resistant S. aureus (MRSA), vancomycin-resistant enterococci (VRE) and A. baumannii[1‐7].

In fact, the continuing shedding of microorganisms between consecutive cleansing interventions, and the difficulties of cleaning particular medical devices, frequently result in residual environmental contamination. An approach to reducing the levels of microbial contamination on inanimate surfaces consists of using anti-adhesive coatings to prevent microorganism adhesion[8, 9]. A second approach is to coat the surfaces of materials with disinfectants or with inorganic antimicrobials such as silver or copper ions[10, 11]. Currently being used for catheters and dressings[12], these technologies are now being frequently applied to other types of medical equipment. The rapid release of the adsorbed biocide after implantation[13], cytotoxicity on mammalian cells[14] and the emergence of resistant microorganisms[15] may limit the biomedical device applications of these technologies. Zollfrank el al. recently reported the antimicrobial effects of a newly developed antimicrobial coating of molybdenum trioxide (MoO₃) against two reference bacterial strains, S. aureus ATCC 25923 and Pseudomonas aeruginosa ATCC 10145[16]. The metalloacid material produces oxonium ions (H₃O⁺), which create an acidic pH (pH 5) that is an effective antimicrobial[17]. To assess the value of such coatings in preventing the generation of environmental microbial reservoirs on medical equipment and in diminishing the risk of exposing patients to microorganisms, we studied the biocidal activity of surfaces coated with the MoO₃ metalloacid material. We tested eight multidrug-resistant bacteria strains representative of those that cause nosocomial infections and outbreaks worldwide, a spore-producing, toxin-producing strain of Clostridium difficile, and two fungi.

The data obtained for the contaminated coated and non-coated leadwire sections are reported in Table1. Examples of plate cultures obtained from Mu50 S. aureus-coated leadwires are shown in Figure1. The average colony counts obtained for the non-coated control wires after they were incubated for 2 − 48 hours showed that all the microorganisms tested were able to survive on the surface of the non-coated wires for prolonged periods of time (Table1) (Figure2). Culture of these wires six hours after contamination revealed high bacteria counts in all cases. At 24 hours, very high bacteria counts were detected for Gram-negative organisms. At 48 hours, substantial numbers of bacteria colonies were cultured from wires contaminated with most of the Gram-negative microorganisms (i.e. the ESBL-producing Enterobacteriaceae and P. aeruginosa) and with the two spore-forming microorganisms, A. fumigatus and C. difficile. In contrast, we detected survival of the two staphylococci, E. faecium and C. albicans 24 hours after contamination but never after 48 hours. Overall, these first data confirmed the non-coated leadwire surface as a stand for nosocomial pathogens, capable of harboring viable microorganisms.

In contrast, the coated surfaces exhibited marked biocidal activity, relative to that of the control surface, against all non-spore-forming microorganisms tested (Table1). The biocidal effect of the coated surface differed with respect to the microorganisms in terms of kinetics, intensity and final outcome. Nevertheless, bacterial counts after six to 24 hours of contact with the coated surface were significantly lower than those observed with non-coated surface (p <0.001). The biocidal effect was the fastest against staphylococci and E. faecium, as marked effects were detected after two hours of contact. In contrast, the biocidal effect on Gram-negative microorganisms took longer and was generally evident after four hours. Of note, the spore-forming microorganisms were completely unaffected by the coated surfaces.

The acidic surface reaction is thought to be related to the diffusion of H₃O⁺ ions through the microbial membranes, resulting in altered enzymatic transport systems and inhibition of key metabolic activities[17]. Thus, the acidic-pH-associated biocidal effect produced by the metalloacid-coated surface would be expected to have a universal effect. The variations in susceptibility between Gram-positive and Gram-negative microorganisms, and the resistance exhibited by the spore-forming ones, suggest that the differences among microorganisms with respect to their susceptibility to the biocidal surface may be due to the H₃O⁺ ion permeability of their cell wall and/or cell membrane. In account of a biological cost of antimicrobial resistance, MDR microorganisms may be less fit and have slower growth rates than sensitive strains[19]. Thus, further studies should be done with wild isolates, to guaranty a biocide effect on them. Despite these limitations, our data show that the survival of non-spore-forming multidrug resistant microorganisms was severely diminished on coated surfaces compared to that on control surfaces. Considering the continuous spread of microorganisms in the surroundings of patients, the usual rhythm of cleaning procedures in hospital wards, and the continuous biocidal effect of the metalloacid-coated surface against the multidrug resistant nosocomial pathogens, we suggest that coated device surfaces may provide an permanent means of minimizing microbial contamination between two cleaning procedures.

The mechanism of microorganism killing at pH values <4.0 is non-specific[1]. Thus, in contrast to disinfectants and antibiotics, microbial resistance to this mode of action may not emerge as a result of exposure to MoO₃. In addition, cytotoxicity tests (MTT assays) and thrombogenicity tests of the powder components suggest that they should be safe for human use[16].

In view of these promising in vitro results, we suggest that MoO₃ coating may be an effective means of minimizing microorganism contamination on certain hospital surfaces. Further studies are needed to evaluate the benefits of this coating on medical devices that are frequently touched by HCWs and to determine whether it can be used as a complementary measure for the preventing of the spread of microorganisms to sites near patients in hospital settings.