Development of a high throughput and low cost model for the study of semi-dry biofilms

Transfer of micro-organisms from dry surface biofilms and the influence of long survival under conditions of poor nutrition and moisture on the virulence of Staphylococcusaureus

BACKGROUND

Biofilms on dry hospital surfaces can enhance the persistence of micro-organisms on dry harsh clinical surfaces and can potentially act as reservoirs of infectious agents on contaminated surfaces.

AIM

This study was conducted to quantify the transfer of viable Staphylococcus aureus cells from dry biofilms through touching and to investigate the impact of nutrient and moisture deprivation on virulence levels in S. aureus.

METHODS

Dry biofilms of S. aureus ATCC 25923 and a defective biofilm-forming ability mutant, S. aureus 1132, were formed in 24-well plates under optimized conditions mimicking dry biofilm formation on clinical surfaces. Microbial cell transfer was induced through the touching of the dry biofilms, which were quantified on nutrient agar. To investigate the impact of nutrient and moisture deprivation on virulence levels, dry and standard biofilms as well as planktonic cells of S. aureus ATCC 25923 were inoculated into Galleria mellonella and their kill rates compared.

FINDINGS

Results of this study showed that viable cells from dry biofilms of S. aureus ATCC 25923 were significantly more virulent and readily transferrable from dry biofilms through a touch test, therefore representing a greater risk of infection. The biofilm-forming capability of S. aureus strains had no significant impact on their transferability with more cells transferring when biofilm surfaces were wet.

CONCLUSIONS

These findings indicate that dry biofilms on hospital surfaces may serve as a reservoir for the dissemination of pathogenic micro-organisms in hospitals, thus highlighting the importance of regular cleaning and adequate disinfection of hospital surfaces.

Bacterial viability in dry-surface biofilms in healthcare facilities: a systematic review

BACKGROUND

Bacteria are known to live inside architectural structures called biofilms. Though standard biofilms have been studied extensively for more than 50 years, little is known about dry-surface biofilms (DSBs). Since 2012, DSBs have been described in several scientific papers, but basic knowledge about the viability and culturability of bacteria remains limited.

AIM

To conduct a systematic review to determine whether bacteria inside DSBs are viable, culturable, and enumerable.

METHODS

Eligible articles had to deal with DSBs containing at least one bacterial species involved in healthcare-associated infections, which developed in actual healthcare environments (in-situ) or with the help of any biofilm model (in-vitro).

FINDINGS

Twenty-four articles were included in the review. Whereas most of them isolated viable bacteria (87% in situ; 100% in vitro), no in-situ study quantified culturable bacteria in the biofilm per unit area. Conversely, 100% of in-vitro studies cultured the bacteria from controls and 94.4% supplied an enumeration of them. Culturable bacteria also grew after 78% of the cleaning, disinfection, or sterilization protocols tested. Microscopic observations after staining the samples with live/dead fluorescent probes (Baclight®) showed large amounts of viable cells in culture-negative samples.

CONCLUSION

Our study questions the efficacy of current methods for microbiological monitoring of surfaces, since these methods are only based on bacterial culturability. To improve both surface monitoring and cleaning and disinfection protocols, it is necessary to integrate the concept of DSBs which appears to contain a significant amount of viable but non-culturable bacteria.

Modelling hospital disinfectant against multi-drug-resistant dry surface biofilms grown under artificial human sweat

BACKGROUND

Dry surface biofilms (DSBs) have been found abundantly across hospital surfaces within intensive care units and may explain how nosocomial pathogens can remain virulent and persist on surfaces for extended periods. Testing standards governing the performance of disinfectant products employ planktonic models under routine growth conditions, which are known to be less tolerant than their biofilm counterpart.

AIM

To evaluate biofilm models cultured under artificial human sweat (AHS), a source of nutrient expected on touch surfaces, to assess the antimicrobial performance of common cleaning agents, including a quaternary ammonium, hydrogen peroxide and active chlorine.

METHODS

Five single-species biofilms, using pathogenic bacteria such as Acinetobacter baumannii, Pseudomonas aeruginosa, Staphylococcus aureus and Enterococcus faecalis, were generated on stainless-steel substrates using a sedimentation protocol under both AHS and nutrient-rich conditions for a direct comparison of phenotypic tolerance. The biofilm models were grown over five days followed by desiccation cycles, before being submerged into the disinfectant solutions for up to 25 min. Epifluorescence (EF) microscopy using LIVE/DEAD™ stain was used to visualize microcolony viability.

FINDINGS

The results revealed biofilms cultured under AHS exhibited a greater antimicrobial tolerance and reduced speed of kill for all cleaning agents compared with the routine media; an average reduction of 72.4% vs 96.9%, respectively. EF microscopy revealed traces of viable bacteria across all coupons after disinfection indicating a potential opportunity for regrowth and recontamination.

CONCLUSION

The notable difference in biocidal performance between the two growth conditions highlights potential pitfalls within current antimicrobial test standards, and the importance of accurate representation of the microbial challenge.

An automated contact model for transmission of dry surface biofilms of Acinetobacter baumannii in healthcare

BACKGROUND

Dry surface biofilms (DSBs) have been recognized across environmental and equipment surfaces in hospitals and could explain how microbial contamination can survive for an extended period and may play a key role in the transmission of hospital-acquired infections. Despite little being known on how they form and proliferate in clinical settings, DSB models for disinfectant efficacy testing exist.

AIM

In this study we develop a novel biofilm model to represent formation within hospitals, by emulating patient to surface interactions.

METHODS

The model generates a DSB through the transmission of artificial human sweat (AHS) and clinically relevant pathogens using a synthetic thumb capable of emulating human contact. The DNA, glycoconjugates and protein composition of the model biofilm, along with structural features of the micro-colonies was determined using fluorescent stains visualized by epifluorescence microscopy and compared with published clinical data.

RESULTS

Micrographs revealed the heterogeneity of the biofilm across the surface; and reveal protein as the principal component within the matrix, followed by glycoconjugates and DNA. The model repeatably transferred trace amounts of micro-organisms and AHS, every 5 min for up to 120 h on to stainless-steel coupons to generate a biofilm model averaging 1.16 × 103 cfu/cm2 falling within the reported range for clinical DSB (4.20 × 102 to 1.60 × 107 bacteria/cm2).

CONCLUSION

Our in vitro DSB model exhibits many phenotypical characteristics and traits to those reported in situ. The model highlights key features often overlooked and the potential for downstream applications such as antibiofilm claims using more realistic microbial challenges.

Using visible light to activate antiviral and antimicrobial properties of TiO2 nanoparticles in paints and coatings: focus on new developments for frequent-touch surfaces in hospitals

The COVID-19 pandemic refocused scientists the world over to produce technologies that will be able to prevent the spread of such diseases in the future. One area that deservedly receives much attention is the disinfection of health facilities like hospitals, public areas like bathrooms and train stations, and cleaning areas in the food industry. Microorganisms and viruses can attach to and survive on surfaces for a long time in most cases, increasing the risk for infection. One of the most attractive disinfection methods is paints and coatings containing nanoparticles that act as photocatalysts. Of these, titanium dioxide is appealing due to its low cost and photoreactivity. However, on its own, it can only be activated under high-energy UV light due to the high band gap and fast recombination of photogenerated species. The ideal material or coating should be activated under artificial light conditions to impact indoor areas, especially considering wall paints or frequent-touch areas like door handles and elevator buttons. By introducing dopants to TiO₂ NPs, the bandgap can be lowered to a state of visible-light photocatalysis occurring. Naturally, many researchers are exploring this property now. This review article highlights the most recent advancements and research on visible-light activation of TiO₂-doped NPs in coatings and paints. The progress in fighting air pollution and personal protective equipment is also briefly discussed.

Graphical Abstract

Indoor visible-light photocatalytic activation of reactive oxygen species (ROS) over TiO₂ nanoparticles in paint to kill bacteria and coat frequently touched surfaces in the medical and food industries.

A new spray-based method for the in-vitro development of dry-surface biofilms

The inanimate environment immediately surrounding the patient in healthcare facilities is a reservoir of microorganisms embedded in dry-surface biofilms (DSB). These biofilms, first highlighted in 2012, are increasingly studied, but currently available in-vitro models only allow for the growth of semi-hydrated biofilms. We developed a new in-vitro method under actual dehydration conditions based on the hypothesis that surface contamination is mainly due to splashes of respiratory secretions. The main objective of this study was to show that the operating conditions we have defined allowed the growth of DSB with a methicillin resistant Staphylococcus aureus strain. The second objective was to show that extended-spectrum beta-lactamase-producing Enterobacteriaceae, that is, Klebsiella pneumoniae and Enterobacter cloacae were also able to grow such biofilms under these conditions. Monobacterial suspensions in sterile artificial saliva (SAS) were sprayed onto polyethylene surfaces. Nutrients and hydration were provided daily by spraying SAS enriched with 20% of Brain Heart Infusion broth. The primary outcome was mean surface coverage measured by image analysis after crystal violet staining. The method applied to S. aureus for 12 days resulted in reproducible and repeatable DSB consisting of isolated and confluent microcolonies embedded in extracellular polymeric substances as shown in scanning electron microscopy images. Similar DSB were obtained with both Enterobacteriaceae applying the same method. No interspecies variation was shown between the three strains in terms of surface coverage. These first trials are the starting point for a 3-year study currently in process.

Hydrogen peroxide, sodium dichloro-s-triazinetriones and quaternary alcohols significantly inactivate the dry-surface biofilms of Staphylococcus aureus and Pseudomonas aeruginosa more than quaternary ammoniums

Globally, healthcare-associated infections (HAI) are the most frequent adverse outcome in healthcare delivery. Although bacterial biofilms contribute significantly to the incidence of HAI, few studies have investigated the efficacy of common disinfectants against dry-surface biofilms (DSB). The objective of this study was to evaluate the bactericidal efficacy of seven Environmental Protection Agency (EPA)-registered liquid disinfectants against DSB of Staphylococcus aureus and Pseudomonas aeruginosa. We hypothesized that overall, there will be significant differences among the bactericidal efficacies of tested disinfectants by product type and active ingredient class. We also hypothesized that depending on the species, higher bactericidal efficacies against DSB will be exhibited after 24 h of dehydration compared to 72 h. Wet-surface biofilms of S. aureus and P. aeruginosa were grown following EPA-MLB-SOP-MB-19 and dehydrated for 24 and 72 h to establish DSB. Seven EPA-registered disinfectants were tested against dehydrated DSB following EPA-MLB-SOP-MB-20. Overall, quaternary ammonium plus alcohol, sodium dichloro-s-triazinetrione and hydrogen peroxide products were more efficacious against DSB than quaternary ammoniums for both tested species. While there was no significant difference in the log₁₀ reductions between 24 and 72 h S. aureus biofilms, significantly higher log₁₀ reductions were observed when products were challenged with 24 h P. aeruginosa DSB compared to 72 h P. aeruginosa DSB. Species type, active ingredient class and dry time significantly impact disinfectant efficacy against DSB of S. aureus or P. aeruginosa.

Wet-dry cycles protect surface-colonizing bacteria from major antibiotic classes

Diverse antibiotic compounds are abundant in microbial habitats undergoing recurrent wet-dry cycles, such as soil, root and leaf surfaces, and the built environment. These antibiotics play a central role in microbial warfare and competition, thus affecting population dynamics and the composition of natural microbial communities. Yet, the impact of wet-dry cycles on bacterial response to antibiotics has been scarcely explored. Using the bacterium E. coli as a model organism, we show through a combination of experiments and computational modeling, that wet-dry cycles protect bacteria from beta-lactams. This is due to the combined effect of several mechanisms including tolerance induced by high salt concentrations and slow cell-growth, which are inherently associated with microscopic surface wetness-a hydration state typical to 'dry' periods. Moreover, we find evidence for a cross-protection effect, where lethal doses of antibiotic considerably increase bacterial survival during the dry periods. This work focuses on beta-lactams, yet similar protection was observed for additional major antibiotic classes. Our findings shed new light on how we understand bacterial response to antibiotics, with broad implications for population dynamics, interspecies interactions, and the evolution of antibiotic resistance in vast terrestrial microbial habitats.

Antimicrobial coatings for environmental surfaces in hospitals: a potential new pillar for prevention strategies in hygiene

Recent reports provide evidence that contaminated healthcare environments represent major sources for the acquisition and transmission of pathogens. Antimicrobial coatings (AMC) may permanently and autonomously reduce the contamination of such environmental surfaces complementing standard hygiene procedures. This review provides an overview of the current status of AMC and the demands to enable a rational application of AMC in health care settings. Firstly, a suitable laboratory test norm is required that adequately quantifies the efficacy of AMC. In particular, the frequently used wet testing (e.g. ISO 22196) must be replaced by testing under realistic, dry surface conditions. Secondly, field studies should be mandatory to provide evidence for antimicrobial efficacy under real-life conditions. The antimicrobial efficacy should be correlated to the rate of nosocomial transmission at least. Thirdly, the respective AMC technology should not add additional bacterial resistance development induced by the biocidal agents and co- or cross-resistance with antibiotic substances. Lastly, the biocidal substances used in AMC should be safe for humans and the environment. These measures should help to achieve a broader acceptance for AMC in healthcare settings and beyond. Technologies like the photodynamic approach already fulfil most of these AMC requirements.

Influence of delivery system on the efficacy of low concentrations of hydrogen peroxide in the disinfection of common healthcare-associated infection pathogens

INTRODUCTION

The ability of healthcare-associated infection pathogens to survive on environmental surfaces is well known. Disinfection is employed to reduce or remove these pathogens but disinfection failures still occur. One method with the potential to improve disinfection efficacy is whole-room disinfection with hydrogen peroxide (H₂O₂).

AIM

To determine the influence of delivery system on the efficacy of low-concentration H₂O₂ on common healthcare-associated infection pathogens.

METHODS

SanoStatic (electrostatic spray) was compared with SanoFog (fogging) in terms of performance for delivery of 5% H₂O₂ and trace silver ions for disinfection. The bacterial test challenges were vancomycin-resistant Enterobacterales (VRE), extended-spectrum β-lactamase (ESBL)-producing Klebsiella pneumoniae (ESBLK), carbapenemase-producing Enterobacterales (CPE), meticillin-resistant Staphylococcus aureus (MRSA), Clostridium difficile spores, Bacillus atropheus and Geobacillus stearothermophilus commercial spore strips.

FINDINGS

SanoFog and SanoStatic were effective when tested under the conditions of experimentation reported here. For VRE, ESBLK, CPE and MRSA, SanoFog and SanoStatic were comparable in performance. For C. difficile we concluded the following: SanoFog was most effective for disinfection of C. difficile spores when compared to SanoStatic.

CONCLUSION

Whereas SanoFog and SanoStatic were effective against bacterial cells, the current practice of using SanoFog and SanoStatic together would be effective for disinfection of C. difficile spores based on investigations under the conditions of experimentation reported here. The spore strips results were not comparable to the results either for the vegetation cells (VRE, ESBLK, CPE, and MRSA) or for C. difficile spores.