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Antimicrobial resistance is one of the most serious human and animal health issues of our time. For example, Candida albicans strains isolated from pigeons are increasingly showing resistance to clotrimazole, natamycin, flucytosine, amphotericin B, ketoconazole, nystatin, and econazole.

Racing pigeons play a key role in the spread of resistance as they travel hundreds of kilometers in a single race, providing an opportunity for the dissemination of resistant bacteria.

Propolis is a natural substance with antibacterial, antiviral, antioxidant, antiproliferative, antifungal, antiparasitic, anti-inflammatory, antiprotozoal, immunomodulatory, hepatoprotective, antitumoral, cytotoxic, and wound-healing properties. Propolis acts through its numerous biologically active molecules, such as polyphenols and terpenoids.

Ádám Kerek et al. conducted a study to investigate the effectiveness of propolis against Staphylococcus spp., Enterococcus spp., Escherichia coli, Salmonella enterica, Candida albicans, and Trichomonas gallinae isolated from pigeons. For each pathogen, the minimum inhibitory concentration (MIC) and minimum eradication concentration (MEC) were determined for eight isolates using ethanolic extracts of propolis at concentrations of 96%, 90%, 80%, 70%, and 60%.

The study showed that propolis is effective in inhibiting the growth of bacteria, Candida albicans, and Trichomonas gallinae strains. Its effectiveness against Gram-positive bacteria was, in some cases, comparable to that of antibiotics, suggesting propolis as a potential alternative for wound infection treatments. Its remarkable efficacy against Trichomonas gallinae is especially promising as an alternative treatment for this common pigeon infection. It also proved effective against Candida albicans strains:

• Candida albicans: MIC values between 1.56 and 400 μg/mL; MEC values between 3.125 and 800 μg/mL
• Trichomonas gallinae: MEC values between 2.5 and 5 mg/mL
• Staphylococcus spp.: MIC values between 1.56–400 μg/mL; MEC values between 12.5–3260 μg/mL
• Enterococcus spp.: MIC values between 1.56–400 μg/mL; MEC values between 12.5–800 μg/mL

Escherichia coli i Salmonella enterica: MIC values >13,000 μg/mL

Antimicrobial Mechanisms of Action of Propolis:

• Antibacterial activity is attributed to inhibition of nucleic acid synthesis, reduced motility, disruption of cell membrane function, energy metabolism inhibition, biofilm formation prevention, damage to membrane proteins, altered membrane permeability, and reduction of bacterial resistance.
• Against fungi (Candida albicans), propolis disrupts the cell membrane through depolarization and induces apoptosis. It also inhibits the expression of several genes related to pathogenicity, cellular adhesion, biofilm formation, and phenotypic switching. Phenotypic switching, particularly hyphae formation, is one of the key virulence factors in fungi. Candida albicans is the most common pathogenic yeast, with its hyphal form able to penetrate epithelial and endothelial tissues, causing damage.
• Antiparasitic action of propolis involves several mechanisms. It interferes with phospholipid metabolism, lowering levels of phosphatidylglycerol and phosphatidylinositol – both vital components of cell membranes. Additionally, flavonoids such as chalcones significantly inhibit parasite growth. 2,6-dihydroxy-4-methoxy-chalcone enhances sterol content and alters cellular and mitochondrial membrane composition, changing structure and fluidity. Rosmarinic acid and apigenin cause cell lysis, cytoplasmic condensation, and nuclear DNA aggregation. Resveratrol damages the hydrogenosome, a cellular organelle responsible for energy production and redox balance in protozoa. Apigenin and caffeic acid increase the formation of reactive oxygen species (ROS), leading to mitochondrial swelling and apoptosis. Quercetin acts as an iron chelator, while oleanolic acid inhibits surface protease activity essential for host cell entry. Kaempferol affects the expression of the heavy chain of actin and myosin II, impairing parasite adhesion.

T. gallinae is a protozoan that causes yellowish-white deposits in pigeons, typically in the upper respiratory tract and pharyngeal cavity. Mortality is very high in chicks and young birds, while adults are often asymptomatic. It is commonly transmitted through drinking water and bird feed.

Here is an example regarding protozoan viability and propolis activity against T. gallinae: After sample collection, the number of protozoa was counted after 24, 48, and 72 hours. Results clearly show that after 48 hours, the number of protozoa decreased, and they even died in sample 11 (Figure 1).

Figure 1. Initial number of protozoa at 24, 48, and 72 hours/mL for each Trichomonas gallinae strain. A gradual decrease in protozoa count can be clearly observed.

Conclusions

The effectiveness of propolis has been demonstrated through numerous experiments, with results similar to those obtained in the cited study.

• Due to variations in geographic origin and composition, propolis may present different MIC values in different countries. MEC* values are consistently equal to or greater than MIC values. However, the substantial variability observed in propolis may significantly reduce the likelihood of resistance development compared to antibiotics.
• Infection with Trichomonas gallinae is widespread among pigeons, and these findings suggest that propolis shows promise as an alternative to antibiotics for its treatment. It may be used as a topical treatment when applied directly to the oral cavity or mixed with drinking water, potentially replacing ronidazole use.
• Propolis also showed remarkable effectiveness against C. albicans.
• Our research confirmed high efficacy against Gram-positive bacteria. These results suggest that propolis could serve as an effective alternative treatment for external infections caused by Gram-positive bacteria.

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The MEC (Minimum Eradication Concentration) refers to the lowest concentration of a substance required to reduce the number of microorganisms by at least four orders of magnitude, effectively eradicating them. In this study, MEC values are used to assess the effectiveness of propolis in completely eliminating Trichomonas gallinae and other pathogens.

Ádám Kerek, Péter Csanády, Barbara Tuska-Szalay, László Kovács and Ákos Jerzsele, In Vitro Efficacy of Hungarian Propolis against Bacteria, Yeast, and Trichomonas gallinae Isolated from Pigeons—A Possible Antibiotic Alternative?, Resources 2023