Antifungal in vitro Activity of Essential Oils against Clinical Isolates of Malassezia pachydermatis from Canine Ears: A Report from a Practice Laboratory

In vitro susceptibility testing of essential oils and their main components against antifungal-resistant Malassezia pachydermatis

Canine Malassezia dermatitis (CMD) and otitis externa are generally treated by antifungal drugs. However, azole-resistant strains have been isolated from canine skin and ear canals worldwide. Phytochemicals isolated from essential oils are effective alternatives for inhibiting Malassezia pachydermatis. To evaluate the usefulness of phytochemicals against azole-resistant isolates, we performed in vitro susceptibility testing using the phytochemicals carvacrol, citral, and thymol. Eight antifungal-resistant isolates were obtained from 7 cases of dermatitis and 1 case of otitis externa during 2022 and 2023 from dogs in Tokyo and Kanagawa, Japan. Fungal susceptibility to carvacrol, citral, and thymol were assessed using the modified broth microdilution method. The minimum inhibitory concentrations (MICs) of the phytochemicals in all isolates were as follows: 0.03 to 0.125% for carvacrol; 0.03 to 0.125% for thymol; and 0.03% to 0.125% for citral. Based on these results, carvacrol, citral, and thymol appear to be effective against azole-resistant strains. The phytochemicals appear to be effective for treating antifungal-resistant cases of CMD and otitis externa.

Palmarosa essential oil inhibits the growth of dandruff-associated microbes by increasing ROS production and modulating the efflux pump

We investigated the antimicrobial efficacy of seven essential oils from four plant families-Lamiaceae, Asteraceae, Zingiberaceae, and Poaceae-against microbes associated with dandruff. The antimicrobial effectiveness of these essential oils was assessed using paper disk agar diffusion and broth micro-dilution techniques. The results demonstrated that two of the essential oils significantly inhibited the growth of dandruff-associated microorganisms, with inhibition zones ranging from 5 ± 1.81 mm to 29.4 ± 2.70 mm in diameter. Among the tested essential oils, Palmarosa (Cymbopogon martinii) exhibited the highest antimicrobial potency, showing a minimum inhibitory concentration (MIC) between 0.27 and 0.55 mg/mL for the fungi Malassezia furfur and Candida albicans, as well as for the bacterium Staphylococcus epidermidis. The essential oil displayed fungicidal activity within the range of 0.55-2.2 mg/mL and bactericidal activity at 0.55 mg/mL. Additionally, post-treatment effects were evaluated by monitoring the re-growth of fungal and bacterial cells after exposure to Palmarosa essential oil. The results revealed significant retardation of the growth of M. furfur for up to 7 h at 2 × MIC and S. epidermidis for up to 5 h at higher concentrations (1 × MIC and 2 × MIC). The study also found that Palmarosa essential oil-induced reactive oxygen species (ROS) production and altered the integrity of microbial membranes, although it did not impact the concentrations of ergosterol and sorbitol. Regarding safety, topical application of Palmarosa essential oil (at concentrations of 1X, 10X, and 20X MIC) on rats and rabbits caused no irritation, and no sub-acute toxic effects were observed either on the skin or systemically. These promising results suggest that Palmarosa essential oil has the potential to be developed into an effective and safe formulation for treating dandruff associated with microbial infections. Further studies could help solidify its clinical applications.

Minimum inhibitory and bactericidal/fungicidal concentration of commercially available products containing essential oils, zinc gluconate, or 4% chlorhexidine for Malassezia pachydermatis, Pseudomonas aeruginosa, and multi-drug resistant Staphylococcus pseudintermedius canine clinical isolates

Skin infections are common complications in both humans and animals. Because of the increased incidence of multi-drug resistant (MDR) skin infections, essential oils have been suggested as potential alternatives to the classic antimicrobials. The goal of this study was to evaluate the minimum inhibitory and bactericidal/fungicidal concentrations (MIC and MBC/MFC) of commercially available products containing essential oils, zinc gluconate, or 4% chlorhexidine. Microbroth dilution technique was performed on clinical isolates of MDR Staphylococcus pseudintermedius (MDR-SP; n = 10), Pseudomonas aeruginosa (PA; n = 10), and Malassezia pachydermatis (MP; n = 10). For MDR-SP, essential oil-containing products showed median MICs of 1:240 and 1:320. The chlorhexidine shampoo had a MIC of 1:128,000 (0.312 µg/mL), whereas zinc gluconate products had median MICs of 1:320 and 1:160. Three essential oil-containing shampoos (MBC 1:40), the zinc gluconate (MBC 1:40), and the chlorhexidine (MBC 1:64,000 [0.625 µg/mL]) reached an MBC. For PA, essential oil-containing products showed median MICs of 1:30 and 1:80. The zinc-gluconate products had a median MIC of 1:160, whereas the chlorhexidine shampoo had a median MIC of 1:4,000 (10 µg/mL). Only the zinc-gluconate products (MBC 1:80) and the chlorhexidine shampoo (MBC 1:2,000 [20 µg/mL]) reached an MBC. For MP, essential oil-containing and zinc-gluconate products showed lower median MICs (1:4,800 and 7,200) for shampoos compared with other formulations (1:160 and 1:320), whereas the chlorhexidine shampoo had a median MIC of 1:80,000 (0.5 µg/mL). These results suggest that natural topical compounds can be an effective alternative to treat skin infections in companion animals. Further in vivo studies are needed to clinically confirm this study's results.

Synergistic effect of essential oils and chlorhexidine against planktonic and biofilm-forming cells of Malassezia pachydermatis

Malassezia (M.) pachydermatis, is often associated with secondary infection of the skin and external auditory canal in dogs and cats. The treatment of Malassezia infections is based on the local application of antifungals often combined with antiseptics. Due to increased resistance of yeast to commonly used antimycotics, especially in biofilm-forming cells, the use of natural substances, e.g. plant essential oils, appears as a new promised option. In this study, the efficacy of selected plant essential oils (EO) - oregano, rosemary, bergamot, clove, cinnamon, and thyme - in combination with chlorhexidine on both planktonic and biofilm-forming cells of M. pachydermatis, was investigated. The checkerboard test was used to determine the effect of chlorhexidine combined with individual EOs. According to the FICI (fractional inhibitory concentration index) in planktonic cells, most combinations showed additive effect, except for thyme and rosemary EO, where a synergistic effect was found (33.3 % and 16.7 % respectively). In the biofilm-forming cells, a synergistic effect was noted in chlorhexidine combined with bergamot EO, recorded in 6 isolates (33.3 %), and with thyme and oregano EO, detected in 3 isolates (16.7 %). A significant decrease (p ˂ 0.05) was found in FIC (fractional inhibitory concentration) compared to MIC (minimum inhibitory concentration), for both planktonic and biofilm-forming cells. Based on the obtained results, we can conclude that the combination of chlorhexidine with EOs achieved better efficiency than when using each agent alone and made it possible to reduce the concentration of both, and a sufficient antifungal and antibiofilm effect was achieved in M. pachydermatis strains.

Extracellular phospholipase production by Malassezia pachydermatis strains and its inhibition by selected antimycotics and plant essential oil components

Extracellular phospholipase (EPL) plays an important role in the pathogenesis of the yeast Malassezia pachydermatis. Currently, the attention of researchers is focused on studying the virulence factors involved in this process and searching solutions to reduce their activity. One of the options is the use of natural remedies as anti-virulence agents. This study is aimed at investigating the production of extracellular phospholipase in M. pachydermatis strains (18 samples) and followed by the time-dependent inhibitory effect of selected azole antifungals (itraconazole, posaconazole and voriconazole) and plant essential oil components (terpinen-4-ol, thymol, carvacrol, eugenol and geraniol), evaluated by Egg Yolk Agar plate method. Almost all strains (17 isolates, (94.4%) were found to be intense EPL producers. A significant, time-dependent inhibition of EPL was noted after 1-, 3- and 6-h exposure of Malassezia cells to itraconazole (26.4%, 47.2% and 50.9%, respectively) compared to exposure to posaconazole (26.4%, 28.3% and 28.3%, respectively) and voriconazole (18.8%, 20.8% and 35.8%, respectively). After one-hour exposure to plant essential oil components, the best inhibitory effect was recorded for eugenol (62.3%), followed by terpinen-4-ol and thymol (56.6%), geraniol (41.5%) and carvacrol (26.4%). A 3-h exposure revealed that thymol retained the best inhibitory effect (88.7%) on EPL production, followed by carvacrol (73.6%), eugenol (56.6%), terpinen-4-ol (52.8%) and geraniol (49.1%). After 6-h exposure, no growth of M. pachydermatis strains exposed to carvacrol was observed, and the inhibitory efficiency for the other tested essential oil (EO) components achieved 88.7%. The obtained results indicate the promising efficacy of plant essential oils components in the inhibition of virulence factors such as EPL production.

In Vitro Antimicrobial Activity of Thymus vulgaris, Origanum vulgare, Satureja montana and Their Mixture against Clinical Isolates Responsible for Canine Otitis Externa

Otitis externa is a frequent inflammation among dogs, mainly caused by bacteria and yeasts that are often resistant to conventional drugs. The aim of the present study was to evaluate the in vitro antibacterial and antifungal activities of commercial essential oils (EOs) from Origanum vulgare, Satureja montana, and Thymus vulgaris, as well as a mixture of these three components, against 47 clinical bacterial strains (Staphylococcus sp., Streptococcus sp., Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumoniae, Serratia marcescens) and 5 Malassezia pachydermatis strains, previously cultured from the ears of dogs affected by otitis externa. The tested Gram-positive bacteria were sensible to the analysed EOs with MICs ranging from 1.25% (v/v) to <0.0195% (v/v); Gram-negative isolates, mainly P. aeruginosa, were less sensitive with MICs from >10% (v/v) to 0.039% (v/v). M. pachydermatis isolates were sensitive to all EOs with MICs from 4.25% (v/v) to 2% (v/v). However, the mixture was active against all bacterial (except one P. aeruginosa strain) and fungal tested isolates. The three EOs and their mixture seem to be an interesting alternative for treating canine otitis externa when conventional antimicrobials are not active.

Yeasts of the Malassezia Genus – Recent Findings

The genus Malassezia is a medically important genus of yeasts that can colonize the skin of humans and other warm-blooded animals. The genus currently comprises 18 species of which four new species were identified recently. The most widely known species, M. pachydermatis, occurs in animals but was detected also in humans, namely at life endangering septicaemias and in prematurely born children. Proliferation of Malassezia occurs most frequently as a result of disturbances in the normal homeostasis of host immunity on the one hand and virulence of these yeasts on the other hand. The successful management of the disease depends on the therapeutic control of overgrowth of the yeasts and any concurrent bacterial infection by local or systemic anti microbial treatment, as well as, on identification and potential correction of the predisposing factors.

Antibiotic resistance profiles and activity of clove essential oil (Syzygium aromaticum) against Pseudomonas aeruginosa isolated of canine otitis

Background and Aim:

Pseudomonas aeruginosa is often isolated from acute and chronic otitis and deep pyoderma in dogs. The increase in bacterial resistance to antibiotics induced the need for alternative therapies to treat infections, with an emphasis on essential oils (EOs). This study aimed to investigate clove oil’s in vitro bactericidal action as a therapeutic alternative against strains of P. aeruginosa isolated from canine otitis.

Materials and Methods:

The antibacterial activity of clove oil was evaluated by determining the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) using the broth microdilution technique in 96-well plates. Serial concentrations of 10–0.31% of the oil were used, equivalent to 104.5–3.26 mg/mL. The susceptibility of isolates to different classes of antibiotics was determined by the disk diffusion technique using 20 antibiotics belonging to eight classes. Isolates resistant to at least one antibiotic of three different classes were considered multidrug-resistant (MDR).

Results:

A high occurrence of resistance was observed for three antibiotics belonging to the cephalosporin classes (cefadroxil, cephalexin, and ceftriaxone), namely, sulfamethoxazole + trimethoprime, doxycycline, and enrofloxacin. The lowest resistance rates were observed for meropenem (4.88%), amikacin (12.20%), and tobramycin (12.2%). All isolates were susceptible to clove oil with an equivalent MIC and MBC from 3.26 to 6.53 mg/mL. Eugenol was the major component of the oil.

Conclusion:

Clove EO was effective against MDR strains of P. aeruginosa, indicating an alternative for developing an efficient and low-cost antimicrobial agent to treat canine otitis.

Antifungal, Antioxidant and Antibiofilm Activities of Essential Oils of Cymbopogon spp

Essential oils (EOs) of Cymbopogon citratus and Cymbopogon proximus are known as sources of monoterpenes and sesquiterpenoids, although their biological activities have not been well investigated. In this study, the compositions of C. citratus and C. proximus EOs of Egyptian origin and their antifungal and antibiofilm properties against Candida spp. and Malassezia furfur were investigated. Antioxidant activities were also evaluated. GC-MS showed the presence of nine and eight constituents in C. citratus and C. proximus EOs, respectively, with geranial and neral as the major compounds of C. citratus EO and piperitone and α-terpinolene as the major compounds of C. proximus EO. Both EOs showed antifungal (MIC values ranging from 1.25 to 20 µL/ mL) and antibiofilm activities (% of reduction ranging from 27.65 ± 11.7 to 96.39 ± 2.8) against all yeast species. The antifungal and antibiofilm activities of C. citratus EO were significantly higher than those observed for C. proximus EO. M. furfur was more susceptible to both EOs than Candida spp. Both EOs exhibited the highest antioxidant activity. This study suggests that C. citratus and C. proximus EOs might be an excellent source of antifungal, antibiofilm and antioxidant drugs and might be useful for preventing Malassezia infections in both medical and veterinary medicine.

In vitro activity of essential oils against microbial isolates from otitis externa cases in dogs

Despite the number of studies focused on the potential use of essential oils (EOs) as an alternative to conventional treatments of canine external otitis, there is controversy about their antimicrobial activity which could be explained by differences in technical or biological aspects. This study focuses on the antimicrobial activity of three single EO compounds (thymol, cinnamaldehyde and carvacrol) and two EOs (clove and oregano) against clinical isolates recovered from canine otitis externa cases (14 bacterial isolates belonging to five different genera and six Malassezia pachydermatis isolates). All compounds showed activity and cinnamaldehyde exhibited the highest bactericidal and fungicidal activity. The susceptibility was lower among bacterial isolates than fungal isolates, being this difference more evident for Gram-positive bacteria. No relationship between antibiotic multi-resistant profile and susceptibility to compounds was observed. To sum up, our results provide appropriate information about appropriate concentrations of promising candidates for the topic treatment of canine otitis.

Anticancer, Enhanced Antibacterial, and Free Radical Scavenging Potential of Fucoidan- (Fucus vesiculosus Source) Mediated Silver Nanoparticles

The present research displays the green synthesis of stable silver nanoparticles (AgNPs). The aqueous solution of Fucoidan from Fucus vesiculosus source (brown marine algae) is used as a reducing and capping agent. UV-Vis spectroscopy, XRD, FT-IR, SEM, EDX, and TEM with selected area electron diffraction are used to characterize the synthesized silver nanoparticles (AgNPs). The synthesized AgNPs exhibit a surface plasmon resonance at 430 nm after 24 h. The characterization results showed that AgNPs are crystalline in nature and exhibit mostly spherical shapes with an average diameter of 4-45 nm. Silver nanoparticles showed effective antibacterial activity against representative pathogens of bacteria. The activities of commercial antibiotics were enhanced by impregnation with the synthesized AgNPs. It also shows good fungicidal and anticancer activity against liver and lung cell lines and shows significant antioxidant efficacy (84%) at 10 μg/ml AgNP concentration against DPPH. The utilization of environmentally synthesized AgNPs offers numerous benefits of ecofriendliness and compatibility for biomedical applications.

Use of Essential Oils in Veterinary Medicine to Combat Bacterial and Fungal Infections

Essential oils (EOs) are secondary metabolites of plants employed in folk medicine for a long time thanks to their multiple properties. In the last years, their use has been introduced in veterinary medicine, too. The study of the antibacterial properties of EOs is of increasing interest, because therapies with alternative drugs are welcome to combat infections caused by antibiotic-resistant strains. Other issues could be resolved by EOs employment, such as the presence of antibiotic residues in food of animal origin and in environment. Although the in vitro antimicrobial activity of EOs has been frequently demonstrated in studies carried out on bacterial and fungal strains of different origins, there is a lack of information about their effectiveness in treating infections in animals. The scientific literature reports some studies about in vitro EOs' activity against animal clinical bacterial and fungal isolates, but in vivo studies are very scanty. The use of EOs in therapy of companion and farm animals should follow careful studies on the toxicity of these natural products in relation to animal species and route of administration. Moreover, considering the different behavior of EOs in relation to both species and strain pathogen, before starting a therapy, an aromatogram should be executed to choose the oil with the best antimicrobial activity.

The Novel Quantitative Assay for Measuring the Antibiofilm Activity of Volatile Compounds (AntiBioVol)

Herein, we present a new test, dubbed AntiBioVol, to be used for the quantitative evaluation of antibiofilm activity of volatile compounds in vitro. AntiBioVol is performed in two 24-well plates using a basic microbiological laboratory equipment. To demonstrate AntiBioVol usability, we have scrutinized the activity of volatilized eucalyptus, tea tree, thyme essential oils, and ethanol (used for method suitability testing) against biofilms of Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans. We have also compared AntiBioVol with the standard disc volatilization method, placing a special stress on evaluating the impact of various technical parameters on the outcomes of the latter method. The obtained results indicate that AntiBioVol allows analyzing the antibiofilm activity of volatile compounds in a high number of repeats and provides semi-quantitative or quantitative results of high repeatability. In comparison to disc volatilization, AntiBioVol is a more space- and cost-effective method that allows analyzing various types of microbial aggregates. Moreover, we have indicated that the possible reasons for the discrepancies in the results obtained by means of the standard disc volatilization method may be related to various parameters of the testing dishes used (height, volume, diameter) and to various volumes of the agar medium applied. In turn, the application of a 24-well plate and a strictly defined AntiBioVol protocol provide a higher control of experimental conditions. Therefore, the application of AntiBioVol may enable an optimization of and introduction of volatile compounds to the fight against infective biofilms.

Malassezia Yeasts in Veterinary Dermatology: An Updated Overview

Lipophilic yeasts of the genus Malassezia are important skin commensals and opportunistic skin pathogens in a variety of animals. The species M. pachydermatis was first isolated from the skin of a captive Indian rhinoceros with an exfoliative dermatitis in 1925, recognized as an important otic pathogen of dogs in the 1950's, and finally accepted, after several years of controversy, as a common cause of canine dermatitis in the 1990's. Since then, there has been considerable research into the biology of Malassezia yeasts and their interaction with their animal hosts. In dogs and cats, M. pachydermatis is associated with ceruminous otitis externa and a "seborrhoeic" dermatitis, wherein pruritic, erythematous skin lesions, often with brown/black greasy, malodourous material matting hairs, preferentially develop in intertriginous areas. Skin disease is favored by folds, underlying hypersensitivity disorders, endocrinopathies, defects of cornification, and in cats, various visceral paraneoplastic syndromes. Diagnosis is based on detecting the yeast in compatible skin lesions, usually by cytology, and observing a clinical and mycological response to therapy. Treatment normally comprises topical or systemic azole therapy, often with miconazole-chlorhexidine shampoos or oral itraconazole or ketoconazole. Management of concurrent diseases is important to minimize relapses. Historically, wild-type Malassezia isolates from dogs and cats were typically susceptible to azoles, with the exception of fluconazole, but emerging azole resistance in field strains has recently been associated with either mutations or quadruplication of the ERG11 gene. These observations have prompted increased interest in alternative topical antifungal drugs, such as chlorhexidine, and various essential oils. Further clinical trials are awaited with interest.

Antimicrobial Activity of Essential Oils against Staphylococcus and Malassezia Strains Isolated from Canine Dermatitis

Staphylococcus spp. bacteria are the most frequently involved agents in canine cutaneous infections. Treatment of these infections is based on antibiotic therapy, that often is not effective because of the antibiotic-resistance of the bacterial strains. Cutaneous staphylococcal infections are often complicated by Malassezia yeasts, that may be resistant to the conventional antifungal drugs. The present investigation was aimed to evaluate the in vitro antimicrobial activity of some essential oils (EOs) in view of a potential cutaneous application. In detail, EOs obtained from lemon verbena (Aloysia triphylla L'Hèr. Britton), cinnamon (Cinnamomum zeylanicum J. Presl), myrrh (Commiphora myrrha (Nees) Engl. var. molmol), lemongrass (Cymbopogon citratus (DC.) Stapf), litsea (Litsea cubeba (Lour.) Pers.), lemon balm (Melissa officinalis L.), oregano (Origanum vulgare L.), savory (Satureja montana L.), and thyme (Thymus vulgaris L.) were assayed against Staphylococcus spp. and Malassezia pachydermatis strains previously isolated from dogs with dermatitis. All EOs were tested by agar disk diffusion and minimum inhibitory concentration methods to verify the antistaphylococcal activity, and by a microdilution method to evaluate the activity against M. pachydermatis. O. vulgare, T. vulgaris, and S. montana showed the best antibacterial activity against all the selected strains, with MICs ranging from 0.29 to 0.58 mg/mL, from 0.58 to 1.16 mg/mL, and from 0.56 to 1.12 mg/mL, respectively, whereas A. triphylla (1.03 mg/mL) and S. montana (1.8 mg/mL) were the most active against M. pachydermatis. After a proper in vivo evaluation, O. vulgare, T. vulgaris, and S. montana EOs could be a promising treatment to combat canine cutaneous mixed infections.