UV doses required for the treatment of biofilms for different microorganisms.
Abstract
A biofilm has been defined as a community of bacteria living in organized structures at a liquid interface. Biofilms can colonize a wide range of domains, including essentially industrial sectors, different natural environments, and also biomedical environments. Bacteria in biofilms are generally well protected against environmental stresses and, as a consequence, are extremely difficult to eradicate. The current study was to investigate the efficacy of different radiations against bacterial biofilms on different surfaces. It was established that the majority of available treatments have proven less effective against pathogenic biofilms, compared to planktonic bacteria. Therefore, new biofilm treatment strategies are needed, including physical treatments such as radiations. UV LEDs offer new solutions to prevent biofilm formation on inaccessible surfaces, such as medical and food equipment and, potentially, sanitary facilities, to limit nosocomial infections, compared to continuous UV irradiation treatment. Moreover, the antimicrobial effectiveness of gamma irradiation is therefore guaranteed in the treatment of bacteria associated with a biofilm, compared to planktonic bacteria. However, limited studies have been conducted to evaluate the inactivation effect of low-energy X-rays on more resistant biofilm pathogens on food-contact surfaces.
Keywords
- biofilm
- bacteria
- UV
- X-rays
- gamma irradiation
- efficacy
1. Introduction
Biofilms consist of structured communities of bacteria, embedded in a self-produced polymeric matrix and adherent to inert or living surfaces [1, 2, 3]. Biofilm mode of growth is an approach in microorganisms to survive harsh growth conditions. Most microorganisms such as
2. Surfaces colonized by biofilms
Biofilms can colonize a wide range of domains, including essentially industrial sectors, different natural environments (soil, sediment, etc.), and biomedical environments [15]. Many bacteria form clumps at the bottom of the containers. Then, they reach the surface of the liquid-type media. However, some bacteria such as
In the medical sector, microbial adhesion resulting in biofilm formation on implanted medical devices is a common occurrence and can lead to serious illness and death [19]. Implanted medical devices like intravascular catheters, urinary catheters, pacemakers, heart valves, stents, and orthopedic implants, normally used for therapeutic purposes, can also be the source of real infectious risks when colonized by bacterial biofilms [20].
3. Biofilms treatment
The majority of available antibacterial treatments have shown their effectiveness against planktonic bacteria. However, these treatments have proven to be ineffective against pathogenic biofilms [21, 22], which are thousands of times more resistant to this type of treatment [23, 24, 25]. It is therefore difficult to eradicate biofilms effectively because of the phenomenon of biofilm recalcitrance [22]. Despite the importance of biofilm treatment either in the medical or environmental sectors, studies into the effectiveness of irradiation on biofilm-associated cells are lacking. Therefore, new biofilm treatment strategies are needed, including physical treatments such as radiations. This review presents an overview of bacterial biofilm development and seeks to highlight the efficacy of radiations against bacterial biofilms.
3.1 Continuous UVC irradiation treatment efficiency on biofilms
Though germicidal UV radiation is widely applied for disinfection of water and food from planktonic bacteria, it may also be used to prevent bacterial growth and colonization on surfaces, as biofilms, within engineered systems [26]. Moreover, the UVC-based method is to be of practical use for disinfection of catheters in the clinic, as they are the major sources of infection [27]. However, higher UV doses would be required to inactivate biofilm-bound bacteria than planktonic bacteria because the biofilm would provide some degree of protection from the effects of UVC irradiation [28].
Torkzadeh et al. [26] have developed an experimental device and method to ensure the growth of biofilms in the presence of UV radiation and to measure the resulting reduction in surface biofilm growth. Under optimal growth conditions and after 48 h of growth, the reduction of the bio-volume of the
In water and wastewater infrastructure, biofilms pose a real problem for disinfection. Until now, the majority of ultraviolet (UV) disinfection studies focus on planktonic bacteria, with limited attention given to UV irradiation of biofilms. Among the few outstanding studies, the study of Myriam et al. [29] focused on the study of UVC dose/biofilm production relationship for five
The UV treatment has evolved a lot since the development of UV light sources from the conventional mercury lamp to the light-emitting diode (LED). It was established that pulsed UV can be more effective than a continuous emitting mode to control biofilms. Moreover, adaptable UV LED is promising to control biofilms in the water distribution system, according to the review of [30]. Luo et al. [30] have, recently, demonstrated that pulsed UV can be more effective than a continuous emitting mode to control biofilms, on one side and that a selective combination of UV LED wavelengths allows targeting damaged biofilm components, on the other hand.
In the medical sector, an application of radiation treatment on catheters looks promising. In this context, the study of Jimmy Bak et al. [31], who proposed a method for disinfecting the inner surface of catheters biofilm, has demonstrated that mean killing rates were 89.6% for 0.5 min exposure, 98% for 2 min exposure, and 99% for 60 min exposure. About 99% of the cells were killed with a UVC dose of 15 kJ m−2. This dose, which is 100 to 1000 times higher than the lethal dose required for planktonic cells, is assumed to be the maximum dose necessary to avoid contamination of newly inserted catheters. The need for high doses to kill mature biofilm and the limited effect of currently available UVC light sources result in a relatively long treatment time of about 60 minutes, hence the need for new UV sources like UV LED.
Recently, Jimmy Bak et al. [31] have tested a newly developed UVC disinfection device, which can be connected to a Luer catheter hub, on polymer tubes contaminated with a wide range of either bacterium, including
On any type of surface contaminated by biofilm, the effectiveness of UVC light in inactivating biofilm-forming microorganisms is mainly due to the ability of DNA molecules to absorb UV photons between 200 and 300 nm, with an absorption peak at 260 nm, at first. Then, this uptake causes damage to the DNA by altering the pairing of nucleotide bases, creating new bonds between adjacent nucleotides on the same DNA strand. This damage occurs particularly between pyrimidine bases [32]. Therefore, to limit UV damages, bacteria generally possess molecular mechanisms to restore DNA lesions [33], which preserve the irradiated biofilm, from damage due to UVC exposure. This repair mechanism has been shown to be effective up to a threshold dose-related to a maximum accumulation of photoproducts and of reactive oxygen species, which can no longer be managed by this mechanism [29]. Our study in 2016, confirmed the oxidative stress through ROS accumulation, following UVC exposure, and has demonstrated that, in the enzymatic ROS-scavenging pathways, catalase and peroxidase enhancement improved the resistance of
We can then conclude that the resistance of bacteria to UVC treatment remains at dose limits. Beyond these doses, there is an exhaustion of the repair system and a sure bacterial death. Hence the need to exceed the dose limits in order to escape bacterial resistance (Figure 1).
3.2 UV LED irradiation treatment efficiency on biofilms
UV LEDs are emerging as competitive light sources because of advantages such as the possible selection of combined-wavelength UV LED [30], adjustable emitting mode, and the designable configuration that facilitate their incorporation into confined spaces. Therefore, UV LEDs offer new solutions to prevent biofilm formation on inaccessible surfaces, such as medical and food equipment and, potentially, sanitary facilities, to limit nosocomial infections. These results imply that surfaces more exposed to bacterial colonization require adequate UVC irradiation to prevent biofilm establishment. Furthermore, continuous surface irradiation may be insufficient as a sole source for biofilm prevention in many circumstances [26]. However, problems with low wall plugs and reliable power supplies still limit the effectiveness of UV LEDs, which further enlightens the prospective of UV in dealing with the biofilm issue in water infrastructure and also in the medical sector.
In this context, the study of Aikaterini et al. [36] on
In parallel, the study of Gora et al. [37] has demonstrated that UV LED irradiation at 265 nm achieved 1.3 log inactivation of biofilm-bound
Moreover, the combination of UV LED and Blue laser was tested on
Concerning the effect of radiations on biofilm matrix, it is well established that bacteria enclosed in a layer of exopolysaccharides are protected by 13% from UVC radiation. It was also confirmed that absorption of UV light by the alginate, an important matrix molecule, translated into a higher survival rate than observed with planktonic cells, for the same UV dose [39]. In effect, alginate water retention seems to be at the origin of the obvious ability to survive severe environments, like UVC exposure. On the other hand, the effect of UV LED on exopolysaccharides (EPS) has not been extensively studied, but it is predicted to be similar to the effect of continuous UVC on EPS. It is then assumed that following the prolonged exposure to UVC radiation, the production of EPS is stimulated [34]. Moreover, in the framework of the development of a profitable strategy to improve the EPS yield, UV irradiation mutagenesis of
Light sources | Microorganisms | UV dose | Inactivation | Reference |
---|---|---|---|---|
UV LED | Biofilm-bound | 8 mJ/cm2 (265 nm) | 1.3 log reduction | Gora et al. [37] |
Less mature | 72–10,000 J∕m2 | 1 log reduction | Aikaterini et al. [36] | |
Mature biofilms (48 and 72 h grown) | 20 000 J∕m2 | 0.8 _ 0.3 log reductions | ||
Continuous low-intensity UVC irradiation | 50.5 μW/cm2 (254 nm) | 95% | Torkzadeh et al. [26] | |
Catheter biofilm | 15 kJ/m2 | 99% | Bak et al. [27, 31] | |
Wastewater | 40 mJ/cm2 |
3.3 Ionizing radiation treatment efficiency on biofilms
Ionizing radiation is a non-thermal destruction technique that inactivates pathogens that may contaminate certain food products, by exposing them to irradiation sources such as high-energy X-rays at about 5 MeV, gamma rays at about 2.5 MeV, or electron beams at about 10 MeV [41]. Compared to these conventional high-energy irradiation techniques, low-energy X-rays have a higher linear energy transfer (LET) value, resulting in a greater relative biological effect (RBE)[42]. Some previous studies have shown that low-energy X-rays is effective in destroying certain planktonic germs such as
Despite of this, we could not simply conclude that low-energy X-rays destroyed EPS in biofilm. Therefore, we could at least postulate that low-energy X-rays irradiation weakened EPS structure in biofilm. Typical EPS mainly comprises homopolymers like cellulose and dextran and heteropolymers of alginate, emulsan, gellan, and xanthan, which maintain the stability of the biofilm matrix [46]. Ionizing irradiation can break down glycosidic bonds and consequently degrade polysaccharides and destabilize the biofilm [47].
Similarly, some in vitro studies also showed that the direct effect of radiation on oral
Concerning gamma irradiation, it is an established technology of well-documented safety and efficacy for the inactivation of pathogenic microorganisms such as
The study of [54] has demonstrated that in bacterial biofilms attached to stainless steel, gamma irradiation at a dose of 10.0 kGy reduced the counts of
Concerning food sterilization,
Light sources | Microorganisms | Dose | Inactivation | Reference |
---|---|---|---|---|
Gamma irradiation | Biofilm-bound | 10.0 kGy | ≥5.1 log CFU/cm2 5.0 log | [54] |
Biofilm-bound | 5.0 log CFU/cm2 | |||
Biofilm-bound | 1.0 kGy | <2 log CFU/cm2 | ||
0.645 kGy | 1 log | [31] | ||
0.531 kGy | 1 log | |||
0.436 kGy | 1 log |
4. Conclusion
This study has demonstrated that ionizing and non-ionizing radiation effectively reduces the populations of both planktonic and biofilm-associated bacteria. However, biofilms are confirmed to be more difficult to eradicate and require enhanced doses for their eradication. It was also confirmed that radiation sensitivity is microorganism specific. Likewise, the influence on radiation sensitivity of the cultured state of the organism, between planktonic and biofilm-associated, is also isolate specific, confirmed for gamma-treated
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