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The grim situation of bacterial infection has undoubtedly become a major threat to human health. In the context of frequent use of antibiotics, a new bactericidal method is urgently needed to fight against drug-resistant bacteria caused by non-standard use of antibiotics. Cold atmospheric plasma (CAP) is composed of a variety of bactericidal species, which has excellent bactericidal effect on microbes. However, the mechanism of interaction between CAP and bacteria is not completely clear. In this paper, we summarize the mechanisms of bacterial killing by CAP in a systematic manner, discuss the responses of bacteria to CAP treatment that are considered to be related to tolerance and their underlying mechanisms, review the recent advances in bactericidal applications of CAP finally. This review indicates that CAP inhibition and tolerance of survival bacteria are a set of closely related mechanisms and suggests that there might be other mechanisms of tolerance to survival bacteria that had not been discovered yet. In conclusion, this review shows that CAP has complex and diverse bactericidal mechanisms, and has excellent bactericidal effect on bacteria at appropriate doses. Key points: • The bactericidal mechanism of CAP is complex and diverse. • There are few resistant bacteria but tolerant bacteria during CAP treatment. • There is excellent germicidal effect when CAP in combination with other disinfectants. [ABSTRACT FROM AUTHOR]

Silver‐based antibacterial nanoparticles have demonstrated indices of possible bacterial resistance after consecutive periods of time, limiting their application on antibacterial surfaces. In this work, we fabricate electrospun composites of polyethersulfone (PES) and silver nanoparticles (AgNPs), displaying highly‐persistent antibacterial behavior after 10 bacterial expositions. The herein‐reported membranes were synthesized by changing the Ag loading (0.05, 0.5, and 5 wt%), adding polyvinylpyrrolidone as a dispersion promoter, and the resulting composites were evaluated after a different number of bacterial expositions against Escherichia coli (E. coli) aiming to determine their bacterial tolerance. SEM micrographs revealed that the PES fiber diameters decreased as a function of the silver content from 798 to 398 nm, attributed to the conductivity and the viscosity improvement by AgNPs introduction in the polymer solution during the electrospun process. Moreover, the surface roughness (determined by atomic force microscopy) and the contact angle changed with the Ag content, increasing the hydrophilic behavior and promoting a better interaction with the bacteria suspended in the solution. Antibacterial activity kinetics of PES/AgNPs fibers showed 75% bacterial inhibition against E. coli in the composite with 0.5 wt% Ag after 3 h of exposition, and 75% after 1.5 h in composites with 5 wt% of Ag. The ca. 25% survival bacteria were used for consecutive bacterial tolerance assays, where the antibacterial behavior was constant after 10 expositions. Interestingly, electrochemical tests confirmed that PES fibers could control the silver‐releasing process. Thus, PES/AgNPs composites avoid the bacterial adaptation process, which makes them suitable as functional antimicrobial nanomaterials. [ABSTRACT FROM AUTHOR]

The present study describes the antibacterial behavior and the bacterial resistance analysis of extremophile Pseudomonas aeruginosa in contact with copper nanoparticles (CuNPs). For this purpose, green synthesis of CuNPs was performed by combined ultrasound-assisted and chemical reduction methods, obtaining semispherical CuNPs ranging from ca. 4-9 nm. Antibacterial activity (AA) of biosynthesized CuNPs demonstrates an antibacterial inhibition of 85 % (LD85) at 400 µg/mL and a minimum bactericidal concentration (MBC) of 800 µg/mL after 3 h of contact. Bacterial adaptation in contact with CuNPs was observed through the consecutive exposition of microorganisms, presenting a significant increase of LD85 values from 400 µg/mL to 6400 µg/mL after 11 expositions. This behavior demonstrates the bacterial growth adaptation with high-dose of CuNPs. The bacterial resistance mechanism was determined through the overproduction of pyocyanin, associated with oxidative stress events, the genomic polymorphism of resistant bacteria obtained by PCR-RAPDs, and the morphological interaction between P. aeruginosa and CuNPs evidenced by transmission electron microscopy (TEM) micrographs. Our results suggest that under controlled CuNPs exposition, extremophile P. aeruginosa can generate bacterial resistance mechanisms, an important issue for the effective design of antimicrobial nanomaterials. [ABSTRACT FROM AUTHOR]

Probiotics are live microorganisms that confer a health benefit to the host when administered in adequate amounts, with the main site of action at the intestinal level. In order to have a maximal effect, probiotic bacteria have to prove a high tolerance for the conditions present in the upper gastro-intestinal tract. The aim of the present study was to assess, from a qualitative point of view, the tolerance of different probiotic bacterial strains and combinations to simulated gastric juice, bile, fructose, lactose and glucose. Also, the influence of food on the bacterial growth after artificial digestion was analyzed. A bacteriologic assessment was realized for three different commercial probiotic products (Bacillus subtilis + Bacillus licheniformis + Pediococcus acidilactici; Enterococcus faecium; Bacillus subtilis + B. licheniformis + B.coagulans + B.indicus + B. clausii), simulating different conditions (artificial digestion method in the absence/presence of food, presence of bile, fructose, glucose, lactose). The obtained results suggested that the tolerance to simulated gastric juice is higher if the food is present in the moment of digestion, regardless of the probiotic bacterial strain or combination. Also, the tolerance to bile was different from one combination to another. The influence of fructose, lactose and glucose are not remarkable from a qualitative point of view. Moreover, the growth of different probiotic bacterial strains is influenced by the simulated gastro-intestinal conditions, sporulated bacteria presenting a slightly higher tolerance compared with non-sporulated strains.

Cross-adaptation phenomena in bacterial populations, induced by sublethal doses of antibacterial solutions, are a major problem in the field of food safety. In this regard, essential oils and their major compounds appear as an effective alternative to common sanitizers in food industry environments. The present study aimed to evaluate the untargeted metabolomics perturbations of Salmonella enterica serovar Enteritidis that has been previously exposed to the sublethal doses of the major components of essential oils: cinnamaldehyde, citral, and linalool (CIN, CIT, and LIN, respectively). Cinnamaldehyde appeared to be the most efficient compound in the assays evaluating the inhibitory effects [0.06% (v/v) as MBC]. Also, preliminary tests exhibited a phenotype of adaptation in planktonic and sessile cells of S. Enteritidis when exposed to sublethal doses of linalool, resulting in tolerance to previously lethal concentrations of citral. A metabolomics approach on S. Enteritidis provided an important insight into the phenomenon of cross-adaptation induced by sublethal doses of major compounds of some essential oils. In addition, according to the results obtained, when single molecules were used, many pathways may be involved in bacterial tolerance, which could be different from the findings revealed in previous studies regarding the use of phytocomplex of essential oils. Orthogonal projection to latent structures (OPLS) proved to be an interesting predictive model to demonstrate the adaptation events in pathogenic bacteria because of the global engagement to prevent and control foodborne outbreaks. [ABSTRACT FROM AUTHOR]

Cross-adaptation phenomena in bacterial populations, induced by sublethal doses of antibacterial solutions, are a major problem in the field of food safety. In this regard, essential oils and their major compounds appear as an effective alternative to common sanitizers in food industry environments. The present study aimed to evaluate the untargeted metabolomics perturbations of Salmonella enterica serovar Enteritidis that has been previously exposed to the sublethal doses of the major components of essential oils: cinnamaldehyde, citral, and linalool (CIN, CIT, and LIN, respectively). Cinnamaldehyde appeared to be the most efficient compound in the assays evaluating the inhibitory effects [0.06% (v/v) as MBC]. Also, preliminary tests exhibited a phenotype of adaptation in planktonic and sessile cells of S. Enteritidis when exposed to sublethal doses of linalool, resulting in tolerance to previously lethal concentrations of citral. A metabolomics approach on S. Enteritidis provided an important insight into the phenomenon of cross-adaptation induced by sublethal doses of major compounds of some essential oils. In addition, according to the results obtained, when single molecules were used, many pathways may be involved in bacterial tolerance, which could be different from the findings revealed in previous studies regarding the use of phytocomplex of essential oils. Orthogonal projection to latent structures (OPLS) proved to be an interesting predictive model to demonstrate the adaptation events in pathogenic bacteria because of the global engagement to prevent and control foodborne outbreaks.

Probiotics are live microorganisms that confer a health benefit to the host when administered in adequate amounts, with the main site of action at the intestinal level. In order to have a maximal effect, probiotic bacteria have to prove a high tolerance for the conditions present in the upper gastro-intestinal tract. The aim of the present study was to assess, from a qualitative point of view, the tolerance of different probiotic bacterial strains and combinations to simulated gastric juice, bile, fructose, lactose and glucose. Also, the influence of food on the bacterial growth after artificial digestion was analyzed. A bacteriologic assessment was realized for three different commercial probiotic products (Bacillus subtilis + Bacillus licheniformis + Pediococcus acidilactici; Enterococcus faecium; Bacillus subtilis + B. licheniformis + B.coagulans + B.indicus + B. clausii), simulating different conditions (artificial digestion method in the absence/presence of food, presence of bile, fructose, glucose, lactose). The obtained results suggested that the tolerance to simulated gastric juice is higher if the food is present in the moment of digestion, regardless of the probiotic bacterial strain or combination. Also, the tolerance to bile was different from one combination to another. The influence of fructose, lactose and glucose are not remarkable from a qualitative point of view. Moreover, the growth of different probiotic bacterial strains is influenced by the simulated gastro-intestinal conditions, sporulated bacteria presenting a slightly higher tolerance compared with non-sporulated strains. [ABSTRACT FROM AUTHOR]

Among clinical isolates of Staphylococcus aureus, borderline oxacillin-resistant S. aureus (BORSA), which is mildly resistant to oxacillin (OXA) without harboring the mecA or mecC gene, is considered a risk factor for further resistance against multiple antibiotics. In this study, BORSA isolates and their derivatives were characterized through antibiotic susceptibility testing and mutation analysis of the genes encoding penicillin-binding proteins (PBPs) and their related proteins, including the promoter region. Eight BORSA isolates were confirmed to harbor the blaZ gene, and hyperproduction of blaZ-encoded penicillinase was predicted based on the minimum inhibitory concentrations (MICs). Of these, four derivative strains that were spontaneously selected based on viability on media containing high concentrations of OXA showed higher MICs than the parent isolates. The minimum bactericidal concentrations, MIC ratios, and TDtest results identified many strains with cefoxitin tolerance. Sequencing of pbp1, pbp2, pbp3, pbp4, gdpP, and yjbH, and the promoter of pbp4 revealed mutations in BORSA isolates and derivatives, despite their absence in parent isolates, suggesting that mutations in PBPs confer OXA/cefoxitin tolerance in BORSA strains. [ABSTRACT FROM AUTHOR]

Background: Currency including banknotes and coins are used in everyday life by everyone. It is, therefore, a universal medium for carrying potentially pathogenic bacteria which can spread and cause infection in the population. Thus, it is of interest to study the bacterial contamination in currency used daily in Chiang Mai province, of which there is scant information. Objectives: To study the quantity and types of bacterial contamination on banknotes and coins circulating in the area surrounding Chiang Mai University Hospital in Chiang Mai province and examine the tolerance of detected bacteria on the currency. Materials and methods: Banknotes and coins, totaling 343 samples, were collected from circulation in the area surrounding Chiang Mai University Hospital in Chiang Mai. The bacterial contamination was studied based on culture methods and identified by morphological and biochemical tests. To examine the bacterial tolerance, the contaminated bacteria recovered from the currency, including Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa and Acinetobacter baumannii, were tested. These bacteria were inoculated on banknotes and coins and the viability of the bacteria at various time points was examined. Results: The amount of bacterial contamination on banknotes, which are linen fibers, was higher than those on coins, which are opaque metals. The gram-positive bacteria were found in a higher amount than gram-negative bacteria, in both banknotes and coins. Human skin flora and pathogenic bacteria, especially S. aureus and pathogenic Enterobacteriaceae, were detected. The bacterial tolerance study revealed that S. aureus, the gram-positive cocci, showed a decrease of viability on banknotes and coins within 6 hours. Meanwhile, gram-negative bacilli presented a decrease of viability in 6 and 1 hours on banknotes and coins, respectively. The bacterial tolerance testing by used and unused currency showed the same results. Conclusion: All banknotes and coins circulating in the area surrounding Chiang Mai University Hospital in Chiang Mai province were demonstrated to be contaminated with bacteria, some of which might be pathogens and dangerous to users. Gram-positive bacteria has higher tolerance than gram-negative bacteria on currency. Bacterial tolerance on banknotes is longer than on coins. The knowledge obtained is advantageous for people in general as an awareness of these harmful bacteria when using the currency.

The aim of this study was to investigate the extent of heavy metal (HM) pollution and its effect on microorganisms from rhizosphere soil in Baia Mare area (Maramureş County, Romania). Two sites with different contamination degrees were included in the study: one with a long history of mining activities and one within a drinking water safeguard zone. Rhizosphere soil samples were characterized with respect to physico-chemical parameters and the Cd, Cu, Pb and Zn contents. Native bacteria were investigated for HM tolerance and biofilm formation under toxic exposure by the microdilution assay. The most resistant strains were identified and the minimum inhibitory concentrations for HMs were determined. Cd, Cu, Pb and Zn exceeded the intervention threshold in Bozânta tailings site, while Pb content exceeded the intervention level within the area of the drinking water treatment plant. Cd showed a very high potential ecological risk in Bozânta area. The long-term exposure to HMs contributed to the selection of HM-tolerant and weakly adherent strains. Biofouling was significantly reduced under the influence of copper ions. Arthrobacter, Rhodococcus and Acidovorax strains with exceptional resistant profiles were isolated from the tailings site, indicating the important role of native microorganisms in rhizosphere ecosystems of contaminated sites. [ABSTRACT FROM AUTHOR]

Objective: To perform a systematic review of the literature on bacterial resistance, tolerance and susceptibility of silver within the context of wound therapy using silver-based dressings. Methods: A literature search was carried out using PubMed, Embase and Cochrane Library databases, the focus was whether results from microbiological experimental in vitro tests with reference strains and clinical wound isolates are reflected in clinical practice with regards to their 'resistance' profiles, comparable with those observed for antibiotics. The search results were allocated to six categories: resistance and resistance mechanism, in vitro tests with standard strains and wound isolates, prevalence and incidence, impact on clinical practice and impact on antibiotic therapy as well as reviews, expert opinions and consensus. Results: Based on all findings of the literature, it cannot be confirmed that a related clinical resistance to silver-ions in silver-based dressings has clinical impact, although endogenous and exogenous genetic resistance patterns have been described and intensively investigated. A translation of these genetic resistance-expression structures to phenotypic appearances, similar to those known for antibiotics, has not been demonstrated for silver in the literature. Conclusion: It can be concluded that there is no definitive evidence available and further studies should be conducted. [ABSTRACT FROM AUTHOR]

Abstract Background Antimicrobial violet-blue light in the region of 405 nm is emerging as an alternative technology for hospital decontamination and clinical treatment. The mechanism of action is the excitation of endogenous porphyrins within exposed microorganisms, resulting in ROS generation, oxidative damage and cell death. Although resistance to 405 nm light is not thought likely, little evidence has been published to support this. This study was designed to establish if there is potential for tolerance development, using the nosocomial pathogen Staphylococcus aureus as the model organism. Methods The first stage of this study investigated the potential for S. aureus to develop tolerance to high-intensity 405 nm light if pre-cultured in low-level stress violet-blue light (≤1 mW/cm2) conditions. Secondly, the potential for tolerance development in bacteria subjected to repeated sub-lethal exposure was compared by carrying out 15 cycles of exposure to high-intensity 405 nm light, using a sub-lethal dose of 108 J/cm2. Inactivation kinetics and antibiotic susceptibility were also compared. Results When cultured in low-level violet-blue light conditions, S. aureus required a greater dose of high-intensity 405 nm light for complete inactivation, however this did not increase with multiple (3) low-stress cultivations. Repeated sub-lethal exposures indicated no evidence of bacterial tolerance to 405 nm light. After 15 sub-lethal exposures 1.2 and 1.4 log10 reductions were achieved for MSSA and MRSA respectively, which were not significantly different to the initial 1.3 log10 reductions achieved (P = 0.242 & 0.116, respectively). Antibiotic susceptibility was unaffected, with the maximum change in zone of inhibition being ± 2 mm. Conclusions Repeated sub-lethal exposure of non-proliferating S. aureus populations did not affect the susceptibility of the organism to 405 nm light, nor to antibiotics. Culture in low-level violet-blue light prior to 405 nm light exposure may increase oxidative stress responses in S. aureus, however, inactivation still occurs and results demonstrate that this is unlikely to be a selective process. These results demonstrate that tolerance from repeated exposure is unlikely to occur, and further supports the potential development of 405 nm light for clinical decontamination and treatment applications.

Due to its properties of remanence in the environment, heavy metal pollution is recognized as a real problem, which affects the sites for long periods of time. Among heavy metals, cadmium is known as one of the most toxic, with negative effects on microorganisms, plants, animals and especially on human health. In this context, the present study was aimed to test the tolerance of different bacterial strains to cadmium, in order to select those that respond best for revealing the starting points of some new research directions in this area. Thus, three bacterial strains were tested at different concentrations of CdCl2, of which only two showed the ability to grow in this medium. It was also observed that Chm1 strain tolerates the highest concentration of cadmium chloride, up to 3,000 mg/l. [ABSTRACT FROM AUTHOR]

Use of silver nanoparticles (SNPs) is increasing in a large number of consumer products. Thus, the possible build-up of the nanoparticles in the environment is becoming a major concern. Aeromonas punctata isolated from sewage showed tolerance to 200 μg/ml SNPs. The growth kinetics data for A. punctata treated with nanoparticles were similar to those in the absence of nanoparticles. There was a reduction in the amount of exopolysaccharides (EPS) in bacterial culture supernatant after nanoparticle-supernatant interaction. EPS capping of the nanoparticles was confirmed by UV-visible, XRD and comparative FTIR analysis. The EPS-capped SNPs showed less toxicity to Escherichia coli, Staphylococcus aureus and Micrococcus luteus compared to the uncapped ones. The study suggests capping of nanoparticles by bacterially produced EPS as a probable physiological defense mechanism. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The presence of antibiotics in soils may increase the selection pressure on soil bacterial communities and cause tolerance to these pollutants. The temporal evolution of bacterial community tolerance to different concentrations of tetracycline (TC), oxytetracycline (OTC) and chlortetracycline (CTC) was evaluated in two soils. The results showed an increase of soil bacterial community tolerance to TC, CTC and OTC only in samples polluted with the highest antibiotic concentrations tested (2000 mg kg−1). The magnitude of those increases was higher in the soil with the lower organic carbon content (1.6%) than in the soil with an organic carbon content reaching 3.4%. In the soil with low organic carbon content, the time-course evolution showed a maximum increase in the tolerance of bacterial communities to tetracycline antibiotics between 45 and 100 incubation days, while for longer incubation times (360 days) the tolerance decreased. In the soil with high organic carbon content, a similar behavior was found for OTC. However, for CTC and TC, slightly increases and decreases (respectively) were found in the bacterial community tolerance at intermediate incubation times, followed by values close to zero for TC after 360 days of incubation, while for CTC they remained higher than in the control. In conclusion, soil pollution due to tetracyclines may cause bacterial community tolerance to these antibiotics when present at high concentrations. In addition, the risk is higher in soils with low organic matter content, and it decreases with time. [Display omitted] • Soil pollution with tetracyclines may increase bacterial community tolerance. • Antibiotic concentrations needed to cause those increases are higher than 500 mg/kg. • The increases were higher in soils with low organic carbon content. • The magnitude of the increases in bacterial community tolerance was time dependent. • Bacterial community tolerance to tetracyclines was maximum after 45–100 days. [ABSTRACT FROM AUTHOR]

Recently, there have been increasing numbers of publications illustrating the potential of light-based antimicrobial therapies to combat antimicrobial resistance. Several modalities, in particular, which have proven antimicrobial efficacy against a wide range of pathogenic microbes include: photodynamic therapy (PDT), ultraviolet light (UVA, UVB and UVC), and antimicrobial blue light (aBL). Using these techniques, microbial cells can be inactivated rapidly, either by inducing reactive oxygen species that are deleterious to the microbial cells (PDT, aBL and UVA) or by causing irreversible DNA damage via direct absorption (UVB and UVC). Given the multi-targeted nature of light-based antimicrobial modalities, it has been hypothesised that resistance development to these approaches is highly unlikely. Furthermore, with the exception of a small number of studies, it has been found that resistance to light based anti-infective agents appears unlikely, irrespective of the modality in question. The concurrent literature however stipulates, that further studies should incorporate standardised microbial tolerance assessments for light-based therapies to better assess the reproducibility of these observations. [ABSTRACT FROM AUTHOR]

Background: Antimicrobial violet-blue light in the region of 405 nm is emerging as an alternative technology for hospital decontamination and clinical treatment. The mechanism of action is the excitation of endogenous porphyrins within exposed microorganisms, resulting in ROS generation, oxidative damage and cell death. Although resistance to 405 nm light is not thought likely, little evidence has been published to support this. This study was designed to establish if there is potential for tolerance development, using the nosocomial pathogen Staphylococcus aureus as the model organism., Methods: The first stage of this study investigated the potential for S. aureus to develop tolerance to high-intensity 405 nm light if pre-cultured in low-level stress violet-blue light (≤1 mW/cm 2 ) conditions. Secondly, the potential for tolerance development in bacteria subjected to repeated sub-lethal exposure was compared by carrying out 15 cycles of exposure to high-intensity 405 nm light, using a sub-lethal dose of 108 J/cm 2 . Inactivation kinetics and antibiotic susceptibility were also compared., Results: When cultured in low-level violet-blue light conditions, S. aureus required a greater dose of high-intensity 405 nm light for complete inactivation, however this did not increase with multiple (3) low-stress cultivations. Repeated sub-lethal exposures indicated no evidence of bacterial tolerance to 405 nm light. After 15 sub-lethal exposures 1.2 and 1.4 log 10 reductions were achieved for MSSA and MRSA respectively, which were not significantly different to the initial 1.3 log 10 reductions achieved ( P =  0.242 & 0.116, respectively). Antibiotic susceptibility was unaffected, with the maximum change in zone of inhibition being ± 2 mm., Conclusions: Repeated sub-lethal exposure of non-proliferating S. aureus populations did not affect the susceptibility of the organism to 405 nm light, nor to antibiotics. Culture in low-level violet-blue light prior to 405 nm light exposure may increase oxidative stress responses in S. aureus , however, inactivation still occurs and results demonstrate that this is unlikely to be a selective process. These results demonstrate that tolerance from repeated exposure is unlikely to occur, and further supports the potential development of 405 nm light for clinical decontamination and treatment applications.