Antibiotic and biocide resistance in methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococcus

Resistance or tolerance? Highlighting the need for precise terminology in the field of disinfection

The terms 'resistance' and 'tolerance' are well defined in the context of antibiotic research. However, in the field of disinfection, these terms are often used synonymously, which creates ambiguity and can lead to misunderstandings and misconceptions. In addition, this inconsistency in terminology makes it difficult to assess the risk of a disinfectant resistance. This general review aims to discuss existing definitions of the terms 'adaptation', 'susceptibility', 'tolerance', 'persistence' and 'resistance' in the light of disinfectants. The most ambiguity is found between tolerance and resistance. Whereas the former describes the not necessarily heritable survival of transient exposure to usually lethal concentrations, resistance is the strictly heritable ability to survive otherwise lethal concentrations of an antimicrobial agent, regardless of exposure time. A simple transfer of experience from antibiotic research is not recommended when assessing the risk of resistance to disinfectants, as there are important differences between antibiotics and disinfectants, although both are antimicrobials: (i) disinfectants are usually applied at concentrations that exceed the minimum inhibitory concentration by orders of magnitude, (ii) the exposure times of disinfectants are in the range of seconds, minutes, or a few hours, (iii) the mode of action of disinfectants is less specific, and (iv) disinfectants often contain more than one active agent with additive or synergistic effects. It is important to recognize that disinfectants, like other antimicrobial agents such as antibiotics, have a dualistic nature and should be used correctly and with caution.

Current insights into the effects of cationic biocides exposure on Enterococcus spp

Cationic biocides (CBs), such as quaternary ammonium compounds and biguanides, are critical for controlling the spread of bacterial pathogens like Enterococcus spp., a leading cause of multidrug-resistant healthcare-associated infections. The widespread use of CBs in recent decades has prompted concerns about the potential emergence of Enterococcus spp. populations exhibiting resistance to both biocides and antibiotics. Such concerns arise from their frequent exposure to subinhibitory concentrations of CBs in clinical, food chain and diverse environmental settings. This comprehensive narrative review aimed to explore the complexity of the Enterococcus' response to CBs and of their possible evolution toward resistance. To that end, CBs' activity against diverse Enterococcus spp. collections, the prevalence and roles of genes associated with decreased susceptibility to CBs, and the potential for co- and cross-resistance between CBs and antibiotics are reviewed. Significant methodological and knowledge gaps are identified, highlighting areas that future studies should address to enhance our comprehension of the impact of exposure to CBs on Enterococcus spp. populations' epidemiology. This knowledge is essential for developing effective One Health strategies that ensure the continued efficacy of these critical agents in safeguarding Public Health.

Biocide resistance in Klebsiella pneumoniae: a narrative review

Klebsiella pneumoniae is among the World Health Organization's list of priority pathogens, notorious for its role in causing healthcare-associated infections and neonatal sepsis globally. Containment of K. pneumoniae transmission depends on the continued effectiveness of antimicrobials and of biocides used for topical antisepsis and surface disinfection. Klebsiella pneumoniae is known to disseminate antimicrobial resistance (AMR) through a large auxiliary genome made up of plasmids, transposons and integrons, enabling it to evade antimicrobial killing through the use of efflux systems and biofilm development. Because AMR mechanisms are also known to impart tolerance to biocides, AMR is frequently linked with biocide resistance (BR). However, despite extensive research on AMR, there is a gap in knowledge about BR and the extent to which AMR and BR mechanisms overlap remains debatable. The aim of this paper is to review and summarise the current knowledge on the determinants of BR in K. pneumoniae and highlight content areas that require further inquiry.

Chlorhexidine Resistance or Cross-Resistance, That Is the Question

Chlorohexidine (CHX) is a widely used biocide in clinical and household settings. Studies over the last few decades have reported CHX resistance in different bacterial species, but at concentrations well below those used in the clinical setting. Synthesis of these findings is hampered by the inconsistent compliance with standard laboratory procedures for biocide susceptibility testing. Meanwhile, studies of in vitro CHX-adapted bacteria have reported cross-resistance between CHX and other antimicrobials. This could be related to common resistance mechanisms of CHX and other antimicrobials and/or the selective pressure driven by the intensive use of CHX. Importantly, CHX resistance and cross-resistance to antimicrobials should be investigated in clinical as well as environmental isolates to further our understanding of the role of CHX in selection of multidrug resistance. Whilst clinical studies to support the hypothesis of CHX cross-resistance with antibiotics are currently lacking, we recommend raising the awareness of healthcare providers in a range of clinical disciplines regarding the potential adverse impact of the unfettered use of CHX on tackling antimicrobial resistance.

Comparing the antibacterial efficacy and functionality of different commercial alcohol-based sanitizers

The use of alcohol-based sanitizers has been recommended as an effective alternative to clean hands, especially in the case when hand washing is not doable. This is especially critical with the COVID-19 pandemic, where personal hygiene is an important factor to deter the spread of the virus. This study assesses and evaluates the differences in antibacterial efficacy and functionalities of five different commercial alcohol-based sanitizers with different formulations. All sanitizers were able to provide instant sanitization functionality, effectively killing 5x105 CFU/mL of inoculated bacteria. However, comparing pure alcohol-based sanitizers against alcohol-based sanitizers with a secondary active ingredient demonstrated that the addition of a secondary active ingredient enhanced the effectiveness and functionalities of the sanitizers. Alcohol-based sanitizers with secondary active ingredients demonstrated a more rapid antimicrobial mode of action, eradicating all 106 CFU/mL of bacteria within 15 seconds of contact, in contrast to the 30 min for purely alcohol-based sanitizers. The secondary active ingredient also provided additional anti-biofilm functionality to prevent opportunistic microbes from attaching and proliferating on the treated surface, leading to serious biofilm formation. On top of that, treatment of surfaces with alcohol-based sanitizers with secondary active ingredients also imparted prolonged antimicrobial protection to the surface lasting up to 24 h. On the other hand, purely alcohol-based sanitizers do not seem to possess such quality with the treated surface being vulnerable to microbial contamination within minutes after application. These results highlighted the benefits of adding a secondary active ingredient in sanitizer formulation. However, care needs to be taken to evaluate the type and concentration of antimicrobial agents chosen as the secondary active ingredient.

Towards a Harmonized Terminology: A Glossary for Biocide Susceptibility Testing

Disinfection is a key strategy to reduce the burden of infections. The contact of bacteria to biocides-the active substances of disinfectants-has been linked to bacterial adaptation and the development of antimicrobial resistance. Currently, there is no scientific consensus on whether the excessive use of biocides contributes to the emergence and spread of multidrug resistant bacteria. The comprehensive analysis of available data remains a challenge because neither uniform test procedures nor standardized interpretive criteria nor harmonized terms are available to describe altered bacterial susceptibility to biocides. In our review, we investigated the variety of criteria and the diversity of terms applied to interpret findings in original studies performing biocide susceptibility testing (BST) of field isolates. An additional analysis of reviews summarizing the knowledge of individual studies on altered biocide susceptibility provided insights into currently available broader concepts for data interpretation. Both approaches pointed out the urgent need for standardization. We, therefore, propose that the well-established and approved concepts for interpretation of antimicrobial susceptibility testing data should serve as a role model to evaluate biocide resistance mechanisms on a single cell level. Furthermore, we emphasize the adaptations necessary to acknowledge the specific needs for the evaluation of BST data. Our approach might help to increase scientific awareness and acceptance.

Combined Biocidal Effect of Gaseous Ozone and Citric Acid on Acinetobacter baumannii Biofilm Formed on Ceramic Tiles and Polystyrene as a Novel Approach for Infection Prevention and Control

Acinetobacter baumannii is a prominent emerging pathogen responsible for a variety of hospital-acquired infections. It can contaminate inanimate surfaces and survive in harsh environmental conditions for prolonged periods of time in the form of biofilm. Biofilm is difficult to remove with only one method of disinfection, so combined disinfection methods and biocidal active substances are needed for biofilm eradication. Additionally, having in mind ecological demands, legislators are more prone using fewer toxic substances for disinfection that produce less solid waste and hazardous disinfection byproducts. Gaseous ozone and citric acid are natural biocidal compounds, and the purpose of this study was to determine their combined biocidal effects on A. baumannii biofilm formed on ceramics and polystyrene. Twenty-four-hour A. baumannii biofilm formed on ceramic tiles and polystyrene was exposed to different combinations of disinfection protocols with 25 ppm of gaseous ozone for 1 h exposure time and 15% citric acid for 10 min exposure. The total number of bacteria was counted afterwards and expressed as CFU/cm2. The determined disinfection protocols of A. baumannii biofilm with combined citric acid and gaseous ozone caused reduction of 2.8 to 5.89 log10 CFU (99.99% inhibition rate) of total viable bacteria for each method, with the citric acid–ozone–citric acid disinfection protocol being most successful in eradication of viable bacteria on both ceramics and polystyrene. In conclusion, gaseous ozone and citric acid showed good combined biocidal effects on A. baumannii biofilm and successfully reduced early A. baumannii biofilm from ceramic and polystyrene surfaces. The given combination of active substances can be a good option for eco-friendly disinfection of hospital inanimate surfaces from A. baumannii biofilm contamination with prior mechanical cleaning.

Efficacy of antifungal agents against fungal spores: An in vitro study using microplate laser nephelometry and an artificially infected 3D skin model

Dermal fungal infections seem to have increased over recent years. There is further a shift from anthropophilic dermatophytes to a growing prevalence of zoophilic species and the emergence of resistant strains. New antifungals are needed to combat these fungi and their resting spores. This study aimed to investigate the sporicidal effects of sertaconazole nitrate using microplate laser nephelometry against the microconidia of Trichophyton, chlamydospores of Epidermophyton, blastospores of Candida, and conidia of the mold Scopulariopsis brevicaulis. The results obtained were compared with those from ciclopirox olamine and terbinafine. The sporicidal activity was further determined using infected three-dimensional full skin models to determine the antifungal effects in the presence of human cells. Sertaconazole nitrate inhibited the growth of dermatophytes, molds, and yeasts. Ciclopirox olamine also had good antifungal activity, although higher concentrations were needed compared to sertaconazole nitrate. Terbinafine was highly effective against most dermatophytes, but higher concentrations were required to kill the resistant strain Trichophyton indotineae. Sertaconazole nitrate, ciclopirox olamine, and terbinafine had no negative effects on full skin models. Sertaconazole nitrate reduced the growth of fungal and yeast spores over 72 h. Ciclopirox olamine and terbinafine also inhibited the growth of dermatophytes and molds but had significantly lower effects on the yeast. Sertaconazole nitrate might have advantages over the commonly used antifungals ciclopirox olamine and terbinafine in combating resting spores, which persist in the tissues, and thus in the therapy of recurring dermatomycoses.

Reduced Susceptibility to Biocides among Enterococci from Clinical and Non-Clinical Sources

BACKGROUND

Wide use of biocidal agents such as benzalkonium chloride (BCC) and chlorhexidine digluconate (CHX) in hospitals and non-hospital environments, has raised concerns over the emergence of non-susceptible strains. Efflux pumps are of known main mechanisms in biocide tolerance which have been rarely addressed in enterococci - members of gut microbiota which can cause serious problems particularly in hospitalized patients. The purpose of this study was to investigate the susceptibility of enterococci from different sources (clinical and fecal isolates) toward BCC and CHX, and its correlation with efflux associated genes. Also, possible link between biocide tolerance and antibiotic resistance was examined.

MATERIALS AND METHODS

One hundred and four enterococcus isolates including clinical (n = 54) and fecal isolates (n = 50) were studied for susceptibility toward BCC, CHX, ciprofloxacin, gentamicin and vancomycin. Twelve efflux associated genes were investigated by polymerase chain reaction assay.

RESULTS

In clinical isolates, reduced susceptibility to CHX and resistance to gentamicin and ciprofloxacin were significantly higher than fecal isolates. Vancomycin resistance was associated with increasing minimum inhibitory concentration of CHX. Among all investigated genes, only three ones, efrA, efrB and emeA were detected which were significantly associated with reduced susceptibility to CHX and were more frequent among clinical isolates. Also, high level resistance to gentamicin was significantly associated with the presence of efrA/B as well as with reduced susceptibility to CHX.

CONCLUSION

As expected, reduced susceptibility to CHX, was significantly higher in clinical isolates. However, the presence of a vancomycin-resistant enterococci among fecal isolates of healthy people which showed resistance/tolerance to studied antimicrobial agents, was unexpected and highlights the need to investigate other non-hospital environments to avoid dissemination of antimicrobial resistance. Correlation between reduced susceptibility to CHX and high level resistance to gentamicin, substantiates monitoring of biocide tolerance particularly in the healthcare settings to control the establishment of antimicrobial resistant strains.

Cetylpyridinium Chloride: Mechanism of Action, Antimicrobial Efficacy in Biofilms, and Potential Risks of Resistance

Antimicrobial resistance is a serious issue for public health care all over the world. While resistance toward antibiotics has attracted strong interest among researchers and the general public over the last 2 decades, the directly related problem of resistance toward antiseptics and biocides has been somewhat left untended. In the field of dentistry, antiseptics are routinely used in professional care, but they are also included in lots of oral care products such as mouthwashes or dentifrices, which are easily available for consumers over-the-counter. Despite this fact, there is little awareness among the dental community about potential risks of the widespread, unreflected, and potentially even needless use of antiseptics in oral care. Cetylpyridinium chloride (CPC), a quaternary ammonium compound, which was first described in 1939, is one of the most commonly used antiseptics in oral care products and included in a wide range of over-the-counter products such as mouthwashes and dentifrices. The aim of the present review is to summarize the current literature on CPC, particularly focusing on its mechanism of action, its antimicrobial efficacy toward biofilms, and on potential risks of resistance toward this antiseptic as well as underlying mechanisms. Furthermore, this work aims to raise awareness among the dental community about the risk of resistance toward antiseptics in general.

Biocide susceptibility testing of bacteria: Development of a broth microdilution method

Biocide susceptibility testing (BST) of bacteria lacks standardised methods. Based on a recently established broth macrodilution BST method, a broth microdilution method for BST was developed. To establish the respective protocol, four reference strains Staphylococcus aureus ATCC® 6538, Enterococcus hirae ATCC® 10541, Escherichia coli ATCC® 10536 and Pseudomonas aeruginosa ATCC® 15442 were investigated for their minimal inhibitory concentrations (MICs) towards quaternary ammonium compounds (benzalkonium chloride), cationic compounds (chlorhexidine), aldehydes (glutardialdehyde) and alcohols (isopropanol) using tryptic soy broth. All combinations of (i) inoculum preparation according to the German Veterinary Medical Society (DVG) or the Clinical and Laboratory Standards Institute (CLSI) with some modifications, (ii) use of 1^st subculture (SC) and 2^nd SC, (iii) direct colony suspension (DCS) with/without glass beads, and (iv) incubation at 37 °C for 24 h, 48 h, and 72 h were compared using seven independent replications. Overall, the reproducibility was high for all abovementioned strain/biocide/parameter combinations. In total, 86.9 % - 100 % of the results were within ± one dilution step of the mode value. The proposed method for a standardised BST protocol comprises (i) two different inoculum densities, (ii) the use of a fresh overnight culture (1^st SC or 2^nd SC), (iii) the preparation of the inoculum suspension by either of the two methods using DCS with or without glass beads, and (iv) the incubation at 37 °C for 24 h. This broth microdilution method will help to harmonize BST of bacterial pathogens in routine diagnostics.

Triclosan has a robust, yet reversible impact on human gut microbial composition in vitro

The recent ban of the antimicrobial compound triclosan from use in consumer soaps followed research that showcased the risk it poses to the environment and to human health. Triclosan has been found in human plasma, urine and milk, demonstrating that it is present in human tissues. Previous work has also demonstrated that consumption of triclosan disrupts the gut microbial community of mice and zebrafish. Due to the widespread use of triclosan and ubiquity in the environment, it is imperative to understand the impact this chemical has on the human body and its symbiotic resident microbes. To that end, this study is the first to explore how triclosan impacts the human gut microbial community in vitro both during and after treatment. Through our in vitro system simulating three regions of the human gut; the ascending colon, transverse colon, and descending colon regions, we found that treatment with triclosan significantly impacted the community structure in terms of reduced population, diversity, and metabolite production, most notably in the ascending colon region. Given a 2 week recovery period, most of the population levels, community structure, and diversity levels were recovered for all colon regions. Our results demonstrate that the human gut microbial community diversity and population size is significantly impacted by triclosan at a high dose in vitro, and that the community is recoverable within this system.

Presence of Qac genes in clinical isolates of methicillin-resistant and methicillin-susceptible Staphylococcus pseudintermedius and their impact on chlorhexidine digluconate susceptibility

OBJECTIVE

To evaluate the presence of quaternary ammonium compound (QAC) (resistance genes, qac A/B, smr, qacG, and qacJ, in clinical isolates of methicillin-susceptible Staphylococcus pseudintermedius (MSSP) and methicillin-resistant S pseudintermedius (MRSP) from dogs and the impact on in vitro chlorhexidine susceptibility.

STUDY DESIGN

Experimental in vitro study.

SAMPLE POPULATION

Seventy isolates from dogs colonized or infected with MRSP (n = 50) or MSSP (n = 20).

METHODS

Agar dilution was used to determine the minimum inhibitory concentration (MIC) of chlorhexidine digluconate. Real-time polymerase chain reaction was used to detect the presence of QAC resistance genes, qacA/B, smr, qacG, and qacJ genes.

RESULTS

One or more qac genes were identified in 52 of 70 (74%) isolates. Overall, there was no association between chlorhexidine MIC and the presence of one or more qac genes (P = .85) or the presence of qacA/B (P = .31), smr (P = .72) or qacJ (P = .93) individually. There was an association between qacG and MIC (P = .012), with a median MIC of 1.5 μg/mL for isolates possessing this gene and 1 μg/mL for those not possessing it.

CONCLUSION

Quaternary ammonium compound resistance genes were present in MRSP and MSSP isolates. With the exception of qacG, the presence of these genes was not associated with increased MIC. All isolates exhibited MIC 5000 to 80 000 times lower than the concentration recommended for use.

CLINICAL SIGNIFICANCE

Despite the presence of QAC genes, chlorhexidine digluconate should be effective against MRSP and MSSP if used correctly.

Adaptive bacterial response to low level chlorhexidine exposure and its implications for hand hygiene

Chlorhexidine digluconate (CHG) is commonly used in healthcare, e.g. in skin antiseptics, antimicrobial soaps, alcohol-based hand rubs and oral or wound antiseptics. Aim of the literature review was to evaluate the potential of bacteria to adapt to low level CHG exposure. A maximum 4fold MIC increase to CHG was found after low level exposure in most of the 71 evaluated bacterial species. A strong adaptive mostly stable MIC change was described in strains or isolates of the healthcare-associated species E. coli, S. marcescens and P. aeruginosa (up to 500fold, 128fold or 32fold, respectively). The highest MIC values after adaptation were 2,048 mg/l (S. marcescens) and 1,024 mg/l (P. aeruginosa). A new resistance to tetracycline, gentamicin, meropeneme or triclosan was found in some adapted isolates. In E. coli horizontal gene transfer was induced (sulfonamide resistance by conjugation), pointing out an additional risk of sublethal CHG. The use of CHG in patient care - but also all other settings such as consumer products and households - should therefore be critically assessed and restricted to indications with a proven health benefit or justifiable public health benefits. Additional CHG has no health benefit when used in alcohol-based hand rubs and is not recommended by the WHO. For routine hand washing of soiled hands the use of plain soap is sufficient, CHG in soaps has no health benefit. In surgical hand antisepsis alcohol-based hand rubs should be preferred to CHG soaps. Implementation of these principles will help to reduce avoidable selection pressure.

Antimicrobial Resistance to Agents Used for Staphylococcus aureus Decolonization: Is There a Reason for Concern?

PURPOSE OF REVIEW

Chlorhexidine gluconate (CHG) and mupirocin are increasingly used for Staphylococcus aureus decolonization to prevent healthcare-associated infections; however, increased use of these agents has led to concerns for growing resistance and reduced efficacy. In this review, we describe current understanding of reduced susceptibility to CHG and mupirocin in S. aureus and their potential clinical implications.

RECENT FINDINGS

While emergence of S. aureus tolerant or resistant to topical antimicrobial agents used for decolonization is well described, the clinical impact of reduced susceptibility is not clear. Important challenges are that standardized methods of resistance testing and interpretation are not established, and the risk for selection for co- or cross-resistance using universal, as opposed to targeted decolonization, is unclear. Evidence continues to support S. aureus decolonization in certain patient groups, although further studies are needed to determine the long-term impact of CHG and mupirocin resistance on efficacy. Strategies to mitigate further development of reduced susceptibility and the consequences of selection pressures through universal decolonization on resistance will benefit from further investigation.

A Descriptive Study of Chlorhexidine as a Disinfectant in Cleft Palate Surgery

OBJECTIVES

Chlorhexidine is seen as the golden standard of disinfectants. It is widely used to clean surgical sites; however, many studies indicate resistance of pathogens to chlorhexidine. One study indicated that pathogenic microorganisms were isolated from the soft palate cleft region in 57% of patients with facial clefts. The objectives of our study were to determine (1) if chlorhexidine application is effective in removing pathogens from the surgical site in these patients, and (2) if any pathogens are isolated, determine if they are resistant to other antimicrobials.

DESIGN

A descriptive observational study.

SETTINGS

A private practice that specializes in facial cleft surgery, with a country-wide patient base. All procedures were executed by one oral and maxillofacial surgeon.

PARTICIPANTS

All patients (N=50) who presented for primary repair of the soft palate cleft were included in the study.

INCLUSION CRITERIA

written consent from parent(s), and patient cleared as systemically healthy by a pediatric physician.

EXCLUSION CRITERIA

patient(s) with systemic infections (eg, flu) and/or any local infections (eg, tonsillitis). There were 25 males and 25 females with an average age of 7 months and 16 days included in the study.

METHODS

Swabs were taken from the surgical site of all 50 patients with cleft soft palate and were sent for culture, identification and antimicrobial sensitivity. The swabs were taken before disinfecting the site as well as after 2 minutes of disinfecting the surgical site with chlorhexidine. Results were compared against each other.

RESULTS

Positive cultures with 28 different pathogenic microorganisms that were identified in 47 patients before cleaning the surgical site with the chlorhexidine. The most dominant pathogens were K. pneumonia (n=22), H. influenza (n=18) and S. aureus (n=10). Of the pathogens found, 13 (46%) were still present on the swabs taken after disinfecting with chlorhexidine. K. pneumonia (n= 13), H. influenza (n=11) and S. aureus (n=9) were still the most prevalent pathogens.

CONCLUSIONS

This study demonstrated that 61 of the total of 113 pathogens isolated (54%), survived after 2 minutes of disinfecting the surgical and surrounding area with chlorhexidine, thus intensifying the chances of post-operative infection.

Growth media and assay plate material can impact on the effectiveness of cationic biocides and antibiotics against different bacterial species

The effectiveness of several cationic disinfectants as well as colistin and polymyxin B were assessed under different growth conditions against Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa strains. These conditions included different media (MH1, MH2, TSB and LB) and plate material (polypropylene and polystyrene). Results showed that Minimum inhibitory and bactericidal concentrations (MIC/MBC) values of colistin and polymyxin B were significantly lower on polypropylene plates when compared to polystyrene plates regardless of media used. There were also differences in MIC/MBC values to certain biocides e.g. chlorhexidine and octenidine particularly for S. aureus and E. coli strains, with polypropylene again showing lower values. Other biocides appear to be mostly unaffected by plate type. Whether biocide efficacy was altered by media composition was organism dependent with S. aureus and E. coli more affected than P. aeruginosa. Lower MIC values were more commonly associated with MH2 media and higher MIC values with TSB media for both polypropylene and polystyrene plates, although there were exceptions. Results obtained for standard strains were, in general, indicative for other S. aureus, E. coli and P. aeruginosa strains tested. This study demonstrates the importance of media composition and plate material on biocide effectiveness and highlights the need for optimized disinfectant testing methods.

SIGNIFICANCE AND IMPACT OF THE STUDY

There are an increasing number of reports of bacterial strains that are multi-drug resistant. The use of biocides as part of infection control is crucial in helping to combat the spread of these particular strains. Unlike for antibiotics, there are few standardized measuring techniques to understand if an isolate has become more resistant to biocides. This study demonstrates the importance of media composition and plate material on variation and reporting of susceptibility of several bacterial species to specific cationic biocides. It is a useful comparison study to highlight the need to standardize biocide susceptibility testing.

Susceptibility of vancomycin-resistant and -sensitive Enterococcus faecium obtained from Danish hospitals to benzalkonium chloride, chlorhexidine and hydrogen peroxide biocides

PURPOSE

In Danish hospitals, the number of infections caused by vancomycin-resistant Enterococcus faecium (VRE faecium) has dramatically increased in recent years. Hospital disinfectants are essential in eliminating pathogenic microorganisms, and reduced susceptibility may contribute to hospital-associated infections. We have addressed whether clinical VRE faecium display decreased biocide susceptibility when compared to vancomycin-sensitive Enterococcus faecium (VSE faecium) isolates.

METHODOLOGY

In total 12 VSE faecium and 37 VRE faecium isolates obtained from Danish hospitals over an extended time period were tested for susceptibility towards three commonly applied biocides, namely benzalkonium chloride, chlorhexidine and hydrogen peroxide.

RESULTS

For benzalkonium chloride, 89 % of VRE faecium strains had a minimal inhibitory concentration (MIC) of 8 mg l^-1, whereas for VSE faecium, only 25 % of the strains had an MIC of 8 mg l^-1. For chlorhexidine, the MIC of 95 % of VRE faecium strains was 4 mg l^-1 or higher, while only 33 % of VSE faecium strains displayed MIC values at the same level. In contrast, both VRE and VSE faecium displayed equal susceptibility to hydrogen peroxide, but a higher minimal bactericidal concentration (MBC) was found for the former. The efflux activity was also assessed, and this was generally higher for the VRE faecium strains compared to VSE faecium.

CONCLUSION

VRE faecium from Danish hospitals demonstrated decreased susceptibility towards benzalkonium chloride and chlorhexidine compared to VSE faecium, where the use of chlorhexidine is particularly heavy in the hospital environment. These findings suggest that biocide tolerance may characterize VRE faecium isolated in Danish hospitals.

Phenotypic chlorhexidine and triclosan susceptibility in clinical Staphylococcus aureus isolates in Australia

Antiseptics such as chlorhexidine gluconate and triclosan are widely used in healthcare settings for both skin antisepsis and decolonisation of Staphylococcus aureus. We determined the minimum inhibitory concentration (MIC) of 198 methicillin susceptible and resistant Staphylococcus aureus clinical isolates to both chlorhexidine and triclosan using an agar dilution method. Of these, 10% (19/198) showed a raised MIC to chlorhexidine and 3% (6/198) showed an elevated MIC to triclosan. The multilocus sequence type (MLST) of each isolate was predicted using a binary method, and although ST93-MRSA-IV was the most common, ST22-MRSA-IV was shown to have statistically higher chlorhexidine MIC values compared with non ST22-MRSA-IV isolates (z = -8.7, p < 0.01). Additionally, isolates from patients known to have failed decolonisation were included and did not demonstrate elevated MIC to the decolonisation antiseptic. Monitoring for non-susceptibility of clinical isolates to biocides is important to determine trends, and may have clinical implications in terms of sub-lethal concentration in residues and concomitant antibiotic resistance.

The Two-Component System ChtRS Contributes to Chlorhexidine Tolerance in Enterococcus faecium

Enterococcus faecium is one of the primary causes of nosocomial infections. Disinfectants are commonly used to prevent infections with multidrug-resistant E. faecium in hospitals. Worryingly, E. faecium strains that exhibit tolerance to disinfectants have already been described. We aimed to identify and characterize E. faecium genes that contribute to tolerance to the disinfectant chlorhexidine (CHX). We used a transposon mutant library, constructed in a multidrug-resistant E. faecium bloodstream isolate, to perform a genome-wide screen to identify genetic determinants involved in tolerance to CHX. We identified a putative two-component system (2CS), composed of a putative sensor histidine kinase (ChtS) and a cognate DNA-binding response regulator (ChtR), which contributed to CHX tolerance in E. faecium Targeted chtR and chtS deletion mutants exhibited compromised growth in the presence of CHX. Growth of the chtR and chtS mutants was also affected in the presence of the antibiotic bacitracin. The CHX- and bacitracin-tolerant phenotype of E. faecium E1162 was linked to a unique, nonsynonymous single nucleotide polymorphism in chtR Transmission electron microscopy showed that upon challenge with CHX, the ΔchtR and ΔchtS mutants failed to divide properly and formed long chains. Normal growth and cell morphology were restored when the mutations were complemented in trans Morphological abnormalities were also observed upon exposure of the ΔchtR and ΔchtS mutants to bacitracin. The tolerance to both chlorhexidine and bacitracin provided by ChtRS in E. faecium highlights the overlap between responses to disinfectants and antibiotics and the potential for the development of cross-tolerance for these classes of antimicrobials.

Triclosan in water, implications for human and environmental health

Triclosan (TCS) is a broad spectrum antibacterial agent present as an active ingredient in some personal care products such as soaps, toothpastes and sterilizers. It is an endocrine disrupting compound and its increasing presence in water resources as well as in biosolid-amended soils used in farming, its potential for bioaccumulation in fatty tissues and toxicity in aquatic organisms are a cause for concern to human and environmental health. TCS has also been detected in blood, breast milk, urine and nails of humans. The significance of this is not precisely understood. Data on its bioaccumulation in humans are also lacking. Cell based studies however showed that TCS is a pro-oxidant and may be cytotoxic via a number of mechanisms. Uncoupling of oxidative phosphorylation appears to be prevailing as a toxicity mechanism though the compound's role in apoptosis has been cited. TCS is not known to be carcinogenic per se in vitro but has been reported to promote tumourigenesis in the presence of a carcinogen, in mice. Recent laboratory reports appear to support the view that TCS oestrogenicity as well as its anti-oestrogenicity play significant role in cancer progression. Results from epidemiological studies on the effect of TCS on human health have implicated the compound as responsible for certain allergies and reproductive defects. Its presence in chlorinated water also raises toxicity concern for humans as carcinogenic metabolites such as chlorophenols may be generated in the presence of the residual chlorine. In this paper, we carried out a detailed overview of TCS pollution and the implications for human and environmental health.

Chlorhexidine Induces VanA-Type Vancomycin Resistance Genes in Enterococci

Chlorhexidine is a bisbiguanide antiseptic used for infection control. Vancomycin-resistantE. faecium(VREfm) is among the leading causes of hospital-acquired infections. VREfm may be exposed to chlorhexidine at supra- and subinhibitory concentrations as a result of chlorhexidine bathing and chlorhexidine-impregnated central venous catheter use. We used RNA sequencing to investigate how VREfm responds to chlorhexidine gluconate exposure. Among the 35 genes upregulated ≥10-fold after 15 min of exposure to the MIC of chlorhexidine gluconate were those encoding VanA-type vancomycin resistance (vanHAX) and those associated with reduced daptomycin susceptibility (liaXYZ). We confirmed thatvanAupregulation was not strain or species specific by querying other VanA-type VRE. VanB-type genes were not induced. ThevanHpromoter was found to be responsive to subinhibitory chlorhexidine gluconate in VREfm, as was production of the VanX protein. UsingvanHreporter experiments withBacillus subtilisand deletion analysis in VREfm, we found that this phenomenon is VanR dependent. Deletion ofvanRdid not result in increased chlorhexidine susceptibility, demonstrating thatvanHAXinduction is not protective against chlorhexidine. As expected, VanA-type VRE is more susceptible to ceftriaxone in the presence of sub-MIC chlorhexidine. Unexpectedly, VREfm is also more susceptible to vancomycin in the presence of subinhibitory chlorhexidine, suggesting that chlorhexidine-induced gene expression changes lead to additional alterations in cell wall synthesis. We conclude that chlorhexidine induces expression of VanA-type vancomycin resistance genes and genes associated with daptomycin nonsusceptibility. Overall, our results indicate that the impacts of subinhibitory chlorhexidine exposure on hospital-associated pathogens should be further investigated in laboratory studies.

Genotypic relatedness and characterization of Staphylococcus pseudintermedius associated with post-operative surgical infections in dogs

Staphylococcus pseudintermedius is a commensal organism of dogs that can also be implicated in surgical site infections (SSIs) in dogs. Particularly with the recent emergence and spread of the ST71-t02-SCCmecII-III multidrug-resistant S. pseudintermedius clonal lineage (MDRSP), it is important to understand the clonal diversity of S. pseudintermedius in SSIs in dogs. The study reported here investigated the genotypic relatedness of 124 S. pseudintermedius isolates from the surgical wounds of 90 dogs admitted to a referral practice in the UK. This study also aimed to understand whether MDRSP is better adapted to survival and persistence in different environments compared with other S. pseudintermedius. Whilst no individual S. pseudintermedius clonal type was primarily responsible for S. pseudintermedius-associated SSIs in dogs, we found that MDRSP was the most represented clonal type among the isolates studied. However, we observed no difference in the level of biofilm production, susceptibility to biocides or carriage of specific virulence determinants between MDRSP and other S. pseudintermedius isolates studied. Interestingly, in the competitive fitness study, MDRSP did not outcompete any member of the other S. pseudintermedius isolates studied in each environment. Our data suggest that the determinants that promote S. pseudintermedius-associated SSIs in dogs are distributed among S. pseudintermedius as a species and are not restricted to a few clonal types. They also provide evidence to support the suggestion that MDRSP is not better adapted to survival or persistence in different environments and is no more virulent than other S. pseudintermedius isolates.

Co-Selection of Resistance to Antibiotics, Biocides and Heavy Metals, and Its Relevance to Foodborne Pathogens

Concerns have been raised in recent years regarding co-selection for antibiotic resistance among bacteria exposed to biocides used as disinfectants, antiseptics and preservatives, and to heavy metals (particularly copper and zinc) used as growth promoters and therapeutic agents for some livestock species. There is indeed experimental and observational evidence that exposure to these non-antibiotic antimicrobial agents can induce or select for bacterial adaptations that result in decreased susceptibility to one or more antibiotics. This may occur via cellular mechanisms that are protective across multiple classes of antimicrobial agents or by selection of genetic determinants for resistance to non-antibiotic agents that are linked to genes for antibiotic resistance. There may also be relevant effects of these antimicrobial agents on bacterial community structure and via non-specific mechanisms such as mobilization of genetic elements or mutagenesis. Notably, some co-selective adaptations have adverse effects on fitness in the absence of a continued selective pressure. The present review examines the evidence for the significance of these phenomena, particularly in respect of bacterial zoonotic agents that commonly occur in livestock and that may be transmitted, directly or via the food chain, to human populations.

Antibiotic and Disinfectant Resistance of Escherichia coli Isolated from Retail Meats in Sichuan, China

To demonstrate the resistance of antibiotics and disinfectants to Escherichia coli isolates, 255 E. coli strains were isolated from 328 retail meat samples in this study. Susceptibility testing results showed that 85.5% isolates were resistant to at least one antibiotic drug. The E. coli isolates showed the highest resistance to sulfamethoxazole (61.6%), followed by tetracycline (61.2%), ampicillin (48.2%), cefalotin (29.8%), and kanamycin (22.4%). The minimum inhibitory concentrations of the disinfectants cetyltrimethylammonium bromide, N,N-didecyl-N,N-dimethylammonium chloride, cetyltrimethylammonium bromide, and cetylpyridinium chloride for E. coli were 16-1,024, 4-1,024, 16-512, and 8-512 mg/L, respectively. The emrE, ydgE/ydgF, mdfA, and sugE(c) genes were commonly present (53.7-83.1%), but the qac and sugE(p) genes were less prevalent (0.0-14.9%). The qac genes were highly associated with antimicrobial resistance. Conjugative transfer experiment indicated that the disinfectant resistance genes, qacF, sugE(p), and qacEΔ1, were located on conjugative plasmids. Pulsed-field gel electrophoresis revealed that the antimicrobial-resistant isolates were associated with the sampling supermarkets or groceries. This study indicated that using quaternary ammonium compounds to decontaminate food processing environments may be ineffective and even provide a selective pressure for strains with acquired resistance to other antimicrobials.

Biocide Susceptibility of Staphylococcus aureus CC398 and CC30 Isolates from Pigs and Identification of the Biocide Resistance Genes, qacG and qacC

OBJECTIVES

Methicillin-resistant Staphylococcus aureus (MRSA), in particular clonal complex (CC) 398, is increasingly found in livestock. Recently, MRSA CC30 was identified in Danish pigs. We determined the susceptibility of porcine S. aureus isolates of CC398 and CC30 to disinfectants used in pig farming (benzalkonium chloride, hydrogen peroxide, formaldehyde, sodium hypochlorite, and caustic soda). Furthermore, efflux pump activity, antimicrobial resistance profiles, hemolysis properties, and the presence of toxic shock syndrome toxin-1 (TSST-1) and Panton-Valentine Leukocidin (PVL)-encoding virulence factors were investigated.

METHODS

Susceptibilities to biocides and antimicrobial agents of 79 porcine S. aureus isolates were determined by the microdilution method. Isolates comprised 21 methicillin-sensitive S. aureus (MSSA) and 40 MRSA isolates belonging to CC398 and 13 MSSA and 5 MRSA isolates belonging to CC30. The presence of quaternary ammonium compound (QAC) resistance efflux pumps was analyzed using an ethidium bromide accumulation assay. The presence of qac resistance genes in active efflux pump positive isolates was determined by whole-genome sequencing data. All isolates were screened for lukPV and tst genes with PCR, and hemolytic activities were determined using an agar plate assay.

RESULTS

S. aureus isolates did not show reduced susceptibility to the biocides tested. However, the QAC resistance gene, qacG, was detected in three MRSA CC30 isolates and the qacC in one MRSA CC30 isolate. CC30 isolates were generally more susceptible to non-beta-lactam antibiotics than CC398. Isolates generally had low hemolytic activity and none encoded PVL or TSST-1.

CONCLUSION

The presence of qac genes in European porcine S. aureus isolates and in livestock-associated MRSA CC30 is for the first time described in this study. This finding is concerning as it ultimately may compromise disinfection with QACs and thereby contribute to the selection and spread of MRSA CC30.

Triclosan: current status, occurrence, environmental risks and bioaccumulation potential

Triclosan (TCS) is a multi-purpose antimicrobial agent used as a common ingredient in everyday household personal care and consumer products. The expanded use of TCS provides a number of pathways for the compound to enter the environment and it has been detected in sewage treatment plant effluents; surface; ground and drinking water. The physico-chemical properties indicate the bioaccumulation and persistence potential of TCS in the environment. Hence, there is an increasing concern about the presence of TCS in the environment and its potential negative effects on human and animal health. Nevertheless, scarce monitoring data could be one reason for not prioritizing TCS as emerging contaminant. Conventional water and wastewater treatment processes are unable to completely remove the TCS and even form toxic intermediates. Considering the worldwide application of personal care products containing TCS and inefficient removal and its toxic effects on aquatic organisms, the compound should be considered on the priority list of emerging contaminants and its utilization in all products should be regulated.

In vitro evaluation of six chemical agents on smooth Brucella melitensis strain

Brucellosis is a zoonosis that disseminated by a variety of ways between animals and humans. The effective disinfection of contaminated environments, soil, feces, and animal bodies plays an irreplaceable role in the prevention and control of brucellosis. To kill Brucella effectively, the bactericidal effects of frequently used disinfectants (including aldehydes, halogens, quaternary ammonium compound, phenolics, and alkalines) and the potential factors that influence disinfection effects were determined in the present study. The results revealed that the minimum bactericidal concentrations (MBCs) of the six disinfectants were all significantly lower than the routinely used concentrations, and all the tested disinfectants were effective against B. melitensis NI. The results of quantitative determination showed that the bactericidal effects of the disinfectants were influenced by their concentration, exposure time, dirty condition and the temperature. Under dirty conditions and a low temperatures, sodium hypochlorite and sodium hydroxide showed better bactericidal effect, while benzalkonium chloride was almost without bactericidal ability. In addition, increasing the disinfectant concentration at low temperatures can improve the bactericidal effect. The present study suggested that Brucella is sensitive to commonly used disinfectants. However, the bactericidal effect is vulnerable to dirty conditions and low temperatures. Thus, it is necessary to test the in vitro sensitivity of disinfectants that are commonly used on farms or the new disinfectant formulations periodically, with the aim of improving the efficacy of animal and human brucellosis prevention programs.

Biocide and copper tolerance in enterococci from different sources

Antimicrobial resistance in enterococci is a matter of concern. A collection of 272 strains (including 107 Enterococcus faecalis and 165 Enterococcus faecium strains) isolated from meat and dairy products, seafood, vegetable foods, wildflowers, animal feces (ewe, goat, horse, mule), and hospitals were tested for sensitivity to biocides of different classes (quaternary ammonium compounds, a bisphenol, and a biguanide) and copper sulfate. Most isolates were inhibited at 25 mg of benzalkonium chloride or cetrimide per liter or at 2.5 mg of hexadecylpyridinium chloride per liter. Few isolates had MICs higher than 25 mg/liter for benzalkonium chloride (2.2%), cetrimide (0.74%), or hexadecylpyridinium chloride (0.37%), although they were all inhibited at 250 mg/liter. The population response to triclosan was very homogeneous, and most isolates (98.16%) were inhibited at 250 mg of triclosan per liter. Chlorhexidine showed the greatest variability, with MICs in a range from 2.5 to 2,500 mg/liter. Remarkably, 74.57% of isolates from clinical samples required 2,500 mg of chlorhexidine per liter for inhibition, compared to much-lower concentrations required for most isolates from other sources. Enterococci were inhibited by copper sulfate in a concentration range from 4 to 16 mM, with no bimodal distribution. However, most isolates required 12 mM (41.91%) or 16 mM (47.43%) for inhibition. The highest percentages of isolates requiring 16 mM CuSO4 were from vegetable foods, seafood, and wildflowers. The results from the present study suggest intermediate levels of copper tolerance and a low incidence of biocide tolerance in the enterococci investigated, except for chlorhexidine in clinical isolates.

Effects of daily bathing with chlorhexidine and acquired infection of methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus: a meta-analysis

OBJECTIVE

Chlorhexidine gluconate (CHG) is a common and safe antimicrobial agent and has been used widely in hand hygiene and skin disinfection; however, whether daily bathing with CHG results in the reduced acquired infection of methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus (VRE) remains inconclusive.

METHODS

We did a meta-analysis searching PubMed, Embase and the Cochrane Central Register database for available studies. Primary outcomes were acquired infection of MRSA, VRE.

RESULTS

In all, twelve articles were available in this review. We found that daily application of chlorhexidine bathing would significantly low the acquired colonization of MRSA [incidence rate ratio (IRR) =0.58, 95% confidence interval (CI): 0.41-0.82] or VRE (IRR =0.51, 95% CI: 0.36-0.73). Remarkably, the using of CHG bathing would significantly reduce the MRSA infection (IRR =0.56, 95% CI: 0.37-0.85), MRSA ventilator associated pneumonia (VAP) (IRR =0.22, 95% CI: 0.07-0.64) and VRE infection (IRR =0.57, 95% CI: 0.33-0.97). No significant publication bias was found in this meta-analysis.

CONCLUSIONS

The application of CHG bathing would significantly decrease acquired infection of MRSA or VRE, which may be an important complementary intervention to barrier precautions.

Staphylococcus aureus but not Listeria monocytogenes adapt to triclosan and adaptation correlates with increased fabI expression and agr deficiency

BACKGROUND

The ability of pathogens to adapt to the widely used biocide, triclosan, varies substantially. The purpose of the study was to examine bacterial adaptation over an extended period of time to low increments of triclosan concentrations. Focus was two human pathogens, S. aureus and L. monocytogenes that previously have displayed inherent high and low adaptability, respectively.

RESULTS

Three strains of L. monocytogenes and two strains of S. aureus including the community-acquired USA300 were exposed to increasing, sub-lethal concentrations of triclosan in triclosan-containing agar gradients. Following 25 days of exposure on agar plates to sub-lethal concentrations of triclosan with a twofold concentration increase every second day, minimum inhibitory concentration (MIC) for S. aureus increased from 0.125 (8325-4) and 0.0625 (USA 300) mg/L to 4 mg/L. The MIC of all three L. monocytogenes strains was initially 4 mg/L and remained unaltered by the exposure. The adapted S. aureus isolates retained normal colony size but displayed increased expression of fabI encoding an essential enzyme in bacterial fatty acid synthesis. Also, they displayed decreased or no expression of the virulence associated agrC of the agr quorum sensing system. While most adapted strains of USA300 carried mutations in fabI, none of the adapted strains of 8325-4 did.

CONCLUSIONS

Adaptability to triclosan varies substantially between Gram positive human pathogens. S. aureus displayed an intrinsically lower MIC for triclosan compared to L. monocytogenes but was easily adapted leading to the same MIC as L. monocytogenes. Even though all adapted S. aureus strains over-expressed fabI and eliminated expression of the agr quorum sensing system, adaptation in USA300 involved fabI mutations whereas this was not the case for 8325-4. Thus, adaptation to triclosan by S. aureus appears to involve multiple genetic pathways.

Multidrug Efflux Pumps in Staphylococcus aureus: an Update

The emergence of infections caused by multi- or pan-resistant bacteria in the hospital or in the community settings is an increasing health concern. Albeit there is no single resistance mechanism behind multiresistance, multidrug efflux pumps, proteins that cells use to detoxify from noxious compounds, seem to play a key role in the emergence of these multidrug resistant (MDR) bacteria. During the last decades, experimental data has established their contribution to low level resistance to antimicrobials in bacteria and their potential role in the appearance of MDR phenotypes, by the extrusion of multiple, unrelated compounds. Recent studies suggest that efflux pumps may be used by the cell as a first-line defense mechanism, avoiding the drug to reach lethal concentrations, until a stable, more efficient alteration occurs, that allows survival in the presence of that agent. In this paper we review the current knowledge on MDR efflux pumps and their intricate regulatory network in Staphylococcus aureus, a major pathogen, responsible from mild to life-threatening infections. Particular emphasis will be given to the potential role that S. aureus MDR efflux pumps, either chromosomal or plasmid-encoded, have on resistance towards different antimicrobial agents and on the selection of drug - resistant strains. We will also discuss the many questions that still remain on the role of each specific efflux pump and the need to establish appropriate methodological approaches to address all these questions.

Biocides--resistance, cross-resistance mechanisms and assessment

IMPORTANCE OF THE FIELD

Antibiotic resistance in bacterial pathogens has increased worldwide leading to treatment failures. Concerns have been raised about the use of biocides as a contributing factor to the risk of antimicrobial resistance (AMR) development. In vitro studies demonstrating increase in resistance have often been cited as evidence for increased risks. It is therefore important to understand the mechanisms of resistance employed by bacteria toward biocides used in consumer products and their potential to impart cross-resistance to therapeutic antibiotics.

AREAS COVERED

In this review, the mechanisms of resistance and cross-resistance reported in the literature toward biocides commonly used in consumer products are summarized. The physiological and molecular techniques used in describing and examining these mechanisms are reviewed and application of these techniques for systematic assessment of biocides for their potential to develop resistance and/or cross-resistance is discussed.

EXPERT OPINION

The guidelines in the usage of biocides in household or industrial purpose should be monitored and regulated to avoid the emergence of any MDR strains. The genetic and molecular methods to monitor the resistance development to biocides should be developed and included in preclinical and clinical studies.

Examination of bacterial resistance to exogenous nitric oxide

While much research has been directed to harnessing the antimicrobial properties of exogenous NO, the possibility of bacteria developing resistance to such therapy has not been thoroughly studied. Herein, we evaluate potential NO resistance using spontaneous and serial passage mutagenesis assays. Specifically, Staphylococcus aureus, Methicillin-resistant S. aureus (MRSA), Staphylococcus epidermidis, Escherichia coli, and Pseudomonas aeruginosa were systematically exposed to NO-releasing 75mol% MPTMS-TEOS nitrosothiol particles at or below minimum inhibitory concentration (MIC) levels. In the spontaneous mutagenesis assay, bacteria that survived exposure to lethal concentrations of NO showed no increase in MIC. Similarly, no increase in MIC was observed in the serial passage mutagenesis assay after exposure of these species to sub-inhibitory concentrations of NO through 20 d.

The efficacy of silver dressings and antibiotics on MRSA and MSSA isolated from burn patients

In this study our objectives were (1) to investigate whether meticillin-resistant Staphylococcus aureus (MRSA) showed an increased tolerance to silver wound dressings compared with meticillin-sensitive S. aureus (MSSA); and (2) to evaluate the effects of bacterial phenotypic states of MRSA and MSSA, and pH, on the activity of silver wound dressings and two antibiotics, ampicillin and clindamycin. Twenty MRSA strains and 10 MSSA strains isolated from burns patients in South Africa were evaluated for their susceptibility to a silver alginate and a silver carboxymethyl cellulose wound dressing, employing a corrected zone of inhibition assay, conducted on Mueller Hinton agar and a poloxamer-based biofilm model. When exposed to the two silver dressings, all 30 S. aureus strains showed susceptibility. Possible enhanced antimicrobial efficacy of the silver dressings occurred when pH was lowered to 5.5, compared with a pH of 7.0. When all S. aureus were grown in the biofilm phenotypic state and exposed to both silver dressings and antibiotics, enhanced tolerance was noted. Susceptibility to silver was overall higher for MRSA when compared with MSSA. This study showed that the effect of pH and bacterial phenotypic state must be considered when the antimicrobial activity of silver wound dressings is being investigated. It is evident from the data generated that both pH and the bacterial phenotypic state are factors that induce changes that affect both antimicrobial performance and bacterial susceptibility.

Dual-resolution molecular dynamics simulation of antimicrobials in biomembranes

Triclocarban and triclosan, two potent antibacterial molecules present in many consumer products, have been subject to growing debate on a number of issues, particularly in relation to their possible role in causing microbial resistance. In this computational study, we present molecular-level insights into the interaction between these antimicrobial agents and hydrated phospholipid bilayers (taken as a simple model for the cell membrane). Simulations are conducted by a novel 'dual-resolution' molecular dynamics approach which combines accuracy with efficiency: the antimicrobials, modelled atomistically, are mixed with simplified (coarse-grain) models of lipids and water. A first set of calculations is run to study the antimicrobials' transfer free energies and orientations as a function of depth inside the membrane. Both molecules are predicted to preferentially accumulate in the lipid headgroup-glycerol region; this finding, which reproduces corresponding experimental data, is also discussed in terms of a general relation between solute partitioning and the intramembrane distribution of pressure. A second set of runs involves membranes incorporated with different molar concentrations of antimicrobial molecules (up to one antimicrobial per two lipids). We study the effects induced on fundamental membrane properties, such as the electron density, lateral pressure and electrical potential profiles. In particular, the analysis of the spontaneous curvature indicates that increasing antimicrobial concentrations promote a 'destabilizing' tendency towards non-bilayer phases, as observed experimentally. The antimicrobials' influence on the self-assembly process is also investigated. The significance of our results in the context of current theories of antimicrobial action is discussed.

Proteomic signature of fatty acid biosynthesis inhibition available for in vivo mechanism-of-action studies

Fatty acid biosynthesis is a promising novel antibiotic target. Two inhibitors of fatty acid biosynthesis, platencin and platensimycin, were recently discovered and their molecular targets identified. Numerous structure-activity relationship studies for both platencin and platensimycin are currently being undertaken. We established a proteomic signature for fatty acid biosynthesis inhibition in Bacillus subtilis using platencin, platensimycin, cerulenin, and triclosan. The induced proteins, FabHA, FabHB, FabF, FabI, PlsX, and PanB, are enzymes involved in fatty acid biosynthesis and thus linked directly to the target pathway. The proteomic signature can now be used to assess the in vivo mechanisms of action of compounds derived from structure-activity relationship programs, as demonstrated for the platensimycin-inspired chromium bioorganometallic PM47. It will further serve as a reference signature for structurally novel natural and synthetic antimicrobial compounds with unknown mechanisms of action. In summary, we described a proteomic signature in B. subtilis consisting of six upregulated proteins that is diagnostic of fatty acid biosynthesis inhibition and thus can be applied to advance antibacterial drug discovery programs.

Resistance and tolerance to tropodithietic acid, an antimicrobial in aquaculture, is hard to select

The antibacterial compound tropodithietic acid (TDA) is produced by bacteria of the marine Roseobacter clade and is thought to explain the fish probiotic properties of some roseobacters. The aim of the present study was to determine the antibacterial spectrum of TDA and the likelihood of development of TDA resistance. A bacterial extract containing 95% TDA was effective against a range of human-pathogenic bacteria, including both Gram-negative and Gram-positive bacteria. TDA was bactericidal against Salmonella enterica serovar Typhimurium SL1344 and Staphylococcus aureus NCTC 12493 and killed both growing and nongrowing cells. Several experimental approaches were used to select mutants resistant to TDA or subpopulations of strains with enhanced tolerance to TDA. No approach (single exposures to TDA extract administered via different methods, screening of a transposon library for resistant mutants, or prolonged exposure to incremental concentrations of TDA) resulted in resistant or tolerant strains. After more than 300 generations exposed to sub-MIC and MIC concentrations of a TDA-containing extract, strains tolerant to 2× the MIC of TDA for wild-type strains were selected, but the tolerance disappeared after one passage in medium without TDA extract. S. Typhimurium mutants with nonfunctional efflux pump and porin genes had the same TDA susceptibility as wild-type strains, suggesting that efflux pumps and porins are not involved in innate tolerance to TDA. TDA is a promising broad-spectrum antimicrobial in part due to the fact that enhanced tolerance is difficult to gain and that the TDA-tolerant phenotype appears to confer only low-level resistance and is very unstable.

Salmonella enterica Enteritidis biofilm formation and viability on regular and triclosan-impregnated bench cover materials

Contamination of food contact surfaces by microbes such as Salmonella is directly associated with substantial industry costs and severe foodborne disease outbreaks. Several approaches have been developed to control microbial attachment; one approach is the development of food contact materials incorporating antimicrobial compounds. In the present study, Salmonella enterica Enteritidis adhesion and biofilm formation on regular and triclosan-impregnated kitchen bench stones (silestones) were assessed, as was cellular viability within biofilms. Enumeration of adhered cells on granite, marble, stainless steel, and silestones revealed that all materials were prone to bacterial colonization (4 to 5 log CFU/cm(2)), and no significant effect of triclosan was found. Conversely, results concerning biofilm formation highlighted a possible bacteriostatic activity of triclosan; smaller amounts of Salmonella Enteritidis biofilms were formed on impregnated silestones, and significantly lower numbers of viable cells (1 × 10(5) to 1 × 10(6) CFU/cm(2)) were found in these biofilms than in those on the other materials (1 × 10(7) CFU/cm(2)). All surfaces tested failed to promote food safety, and careful utilization with appropriate sanitation of these surfaces is critical in food processing environments. Nevertheless, because of its bacteriostatic activity, triclosan incorporated into silestones confers some advantage for controlling microbial contamination.