Evaluation of visible light and natural photosensitizers against Staphylococcus epidermidis and Staphylococcus saprophyticus planktonic cells and biofilm

Antimicrobial photoinactivation (API) has shown some promise in potentially treating different nosocomial bacterial infections, however, its application on staphylococci, especially other than Staphylococcus aureus or methicillin-resistant S. aureus (MRSA) species is still limited. Although S. aureus is a well-known and important nosocomial pathogen, several other species of the genus, particularly coagulase-negative Staphylococcus (CNS) species such as Staphylococcus epidermidis and Staphylococcus saprophyticus, can also cause healthcare-associated infections and foodborne intoxications. CNS are often involved in resilient biofilm formation on medical devices and can cause infections in patients with compromised immune systems or those undergoing invasive procedures. In this study, the effects of chlorophyllin and riboflavin-mediated API on S. epidermidis and S. saprophyticus planktonic cells and biofilm are demonstrated for the first time. Based on the residual growth determination and metabolic reduction ability changes, higher inactivating efficiency of chlorophyllin-mediated API was determined against the planktonic cells of both tested species of bacteria and against S. saprophyticus biofilm. Some insights on whether aqueous solutions of riboflavin and chlorophyllin, when illuminated with optimal exciting wavelength (440 nm and 402 nm, respectively) generate O2-•, are also provided in this work.

Riboflavin- and chlorophyllin-based antimicrobial photoinactivation of Brevundimonas sp. ESA1 biofilms

Some Brevundimonas spp. are globally emerging opportunistic pathogens that can be dangerous to individuals with underlying medical conditions and for those who are immunocompromised. Gram-negative Brevundimonas spp. can form resilient sessile biofilms and are found not only in different confined terrestrial settings (e.g., hospitals) but are also frequently detected in spacecraft which is inhabited by astronauts that can have altered immunity. Therefore, Brevundimonas spp. pose a serious health hazard in different environments, especially in its biofilm form. Conventional antimicrobials applied to disrupt, inactivate, or prevent biofilm formation have limited efficiency and applicability in different closed-loop systems. Therefore, new, effective, and safe biofilm control technologies are in high demand. The present work aimed to investigate antimicrobial photoinactivation (API) of Brevundimonas sp. ESA1 monocultural biofilms mediated by non-toxic, natural photosensitizers such as riboflavin (RF) and chlorophyllin (Chl) with an emphasis of this technology as an example to be safely used in closed-loop systems such as spacecraft. The present study showed that Chl-based API had a bactericidal effect on Brevundimonas sp. ESA1 biofilms at twice the lower irradiation doses than was needed when applying RF-based API. Long-term API based on RF and Chl using 450 nm low irradiance plate has also been studied in this work as a more practically applicable API method. The ability of Brevundimonas sp. ESA1 biofilms to reduce alamarBlue™ and regrowth analysis have revealed that after the applied photoinactivation, bacteria can enter a viable but non-culturable state with no ability to resuscitate in some cases.

Photoinactivation of Salmonella enterica exposed to 5-aminolevulinic acid: Impact of sensitization conditions and irradiation time

The photodynamic inactivation (PDI) represents the potential alternative to traditional antibiotic therapy, and can be applied to treat various bacterial infections, including those caused by Gram-negative bacterial strains. One of the treatment modalities is based on the capacity of bacterial cells to synthesize the excess amounts of porphyrins after exposure to an externally applied 5-aminolevulinic acid (5-ALA), which makes them photosensitive and leads to reduced survival after irradiation with an appropriately selected light source. This study focuses on the sensitization and the photoinduced inactivation of Salmonella enterica cells in PBS containing 0.5 mM 5-ALA, incubated at 37 °C for 4 h or for 20 h and afterwards irradiated with violet LED light (11.1 mW/cm², a peak at 400 nm). It has been found that both amounts and composition of endogenous porphyrins not only depended on the incubation duration, but also were affected by externally induced photo- and chemo-oxidation reactions. The application of different sensitization conditions has revealed that the increasing amounts of endogenously produced porphyrins do not ensure the proportional reduction of bacterial cell survival numbers. The comparative investigations also demonstrated that the presence of endogenously produced porphyrins in the medium results in secondary sensitization of bacterial cells and causes a notably stronger photoinactivation effect in comparison to their externally applied standards.

Inactivation of Gram (-) bacteria Salmonella enterica by chlorophyllin-based photosensitization: Mechanism of action and new strategies to enhance the inactivation efficiency

This study is focused on the enhancement of susceptibility of Gram (-) bacteria S. enterica to chlorophyllin-based (Chl) photosensitization combining it with other antimicrobial tools. In order to find best combinations, the mechanism by which Chl-based photosensitization inactivates bacteria must be identified. Data confirmed that photosensitization (Chl 1.5×10^-5M, for 1-120min, 405nm, 0-46.1J/cm²) reduced S. enterica population, just by 2.05 log (CFU/ml). Fluorimetric measurements indicated that just minor part of Chl was bound to Salmonella in suspension. Addition of sodium azide (NaN₃) (10mM) protected bacteria from killing, what means that ¹O₂ took place in photochemical reactions. Gene expression data confirmed that Chl-based photosensitization induced oxidative stress in bacteria cells, since mostly genes responsible for detoxification of ROS (OxyR, AhpC, GrxA) have been expressed in Salmonella. Moreover, the expression of genes, responsible for the inhibition of oxidative respiration (AtpC), cell division and down-regulation of metabolism (SulA) have been detected. In addition, Chl-based photosensitization induced significant release of intracellular components (absorbing at λ₂₆₀ nm and λ₂₈₀ nm) in bacteria that indicated increased membrane permeability. Thus, the combination of two antimicrobials (Chl-based photosensitization and chitosan (CHS)) with the same target (cellular membrane) in the presence of light drastically reduced viable Salmonella population (by 7.28 log). Combined treatment of photosensitization and high power pulsed UV light (HPPL) was also very effective, since reduced viable Salmonella by 7.5 log. Bacterial regrowth experiments clearly indicated that after both combined treatments Salmonella lost its ability to proliferate, and SEM images confirmed that after both treatments no viable bacteria have been found at all.

Effective photosensitization-based inactivation of Gram (−) food pathogens and molds using the chlorophyllin–chitosan complex: towards photoactive edible coatings to preserve strawberries

This study is focused on the novel approaches to enhance the inactivation of the Gram (−) food pathogen Salmonella enterica and harmful molds in vitro and on the surface of strawberries using the chlorophyllin–chitosan complex.

Inactivation of several strains of Listeria monocytogenes attached to the surface of packaging material by Na-Chlorophyllin-based photosensitization

This study was focused on the possibility to inactivate thermosensitive Listeria monocytogenes ATC(L3)C 7644 and thermoresistant 56 Ly strain by Na-Chlorophyllin (Na-Chl)-based photosensitization in vitro and on the surface of packaging. Comparative analysis of antimicrobial efficiency of photosensitization with conventional surface cleaning was performed. Data indicate that both Listeria strains, after incubation with Na-Chl and following illumination (λ=400nm, 20mWcm(-2)), were inactivated by 7 log in vitro. This treatment cleaned both Listeria strains from packaging surfaces. Comparative analysis indicates that washing with water diminishes pathogens by less than 1 log, 200ppm Na-hypochlorite by 1.7 log, Na-Chl-based photosensitization by 4.5 log. Listeria biofilms were totally removed from the surface by photosensitization at higher photosensitizer concentrations and longer incubation times. In conclusion, both strains of L. monocytogenes can be effectively inactivated by photosensitization in vitro and on the surface of packaging. Listeria biofilms are susceptible to this treatment as well. Comparison of different surface decontamination treatments reveals that photosensitization is much more effective against both Listeria strains than washing with water or 200ppm Na-hypochlorite. Our data support the idea that Na-Chl-based photosensitization is an effective antimicrobial tool which may serve in the future for the development of human and environmentally friendly surface decontamination techniques.