Background: Bulgaria is a country with a wide range of medicinal plants, with uses in traditional medicine dating back for centuries. Methods: Disc diffusion assay was used to evaluate the antimicrobial activity of the plant extracts. A cytopathic effect inhibition test was used for the assessment of the antiviral activity of the extracts. The virucidal activity of the extracts, their influence on the stage of viral adsorption, and their protective effect on uninfected cells were reported using the end-point dilution method, and Δlgs was determined as compared to the untreated controls. Results: The results of the study reveal that the antibacterial potential of G. glabra and H. perforatum extracts in Gram-positive bacteria is more effective than in Gram-negative bacteria. When applied during the replication of HSV-1 and HCov-OC-43, only some of the extracts showed weak activity, with SI between 2 to 8.5. Almost all tested extracts inhibited the extracellular virions of the studied enveloped viruses (HSV-1 and HCov-OC-43) to a greater extent than of the non-enveloped viruses (PV-1 and HAdV-5). They inhibited the stage of viral adsorption (HSV-1) in the host cell (MDBK) to varying degrees and showed a protective effect on healthy cells (MDBK) before they were subjected to viral invasion (HSV-1). Conclusion: The antipathogenic potential of extracts of H. perforatum and G. glabra suggests their effectiveness as antimicrobial agents. All 13 extracts of the Bulgarian medicinal plants studied can be used to reduce viral yield in a wide range of viral infections.

Propolis is a natural mixture of resins, wax, and pollen from plant buds and flowers, enriched with enzymes and bee saliva. It also contains various essential oils, vitamins, mineral salts, trace elements, hormones, and ferments. It has been found that propolis possesses antimicrobial, antiviral, and anti-inflammatory properties. We have studied the antiviral activity of six extracts of Bulgarian propolis collected from six districts of Bulgaria. The study was conducted against structurally different viruses: human coronavirus strain OC-43 (HCoV OC-43) and human respiratory syncytial virus type 2 (HRSV-2) (enveloped RNA viruses), human herpes simplex virus type 1 (HSV-1) (enveloped DNA virus), human rhinovirus type 14 (HRV-14) (non-enveloped RNA virus) and human adenovirus type 5 (HadV-5) (non-enveloped DNA virus). The influence of the extracts on the internal replicative cycle of viruses was determined using the cytopathic effect (CPE) inhibition test. The virucidal activity, its impact on the stage of viral adsorption to the host cell, and its protective effect on healthy cells were evaluated using the final dilution method, making them the focal points of interest. The change in viral infectivity under the action of propolis extracts was compared with untreated controls, and Δlgs were determined. Most propolis samples administered during the viral replicative cycle demonstrated the strongest activity against HCoV OC-43 replication. The influence of propolis extracts on the viability of extracellular virions was expressed to a different degree in the various viruses studied, and the effect was significantly stronger in those with an envelope. Almost all extracts significantly inhibited the adsorption step of the herpes virus and, to a less extent, of the coronavirus to the host cell, and some of them applied before viral infection demonstrated a protective effect on healthy cells. Our results enlarge the knowledge about the action of propolis and could open new perspectives for its application in viral infection treatment.

Here we report on the synthesis and characterization of three new N-modified analogues of hemorphin-4 with rhodamine B. Modified with chloroacetyl, chloride cotton fabric has been dyed and color coordinates of the obtained textile materials were determined. Antiviral and virucidal activities of both the peptide-rhodamine B compounds and the dyed textile material were studied. Basic physicochemical properties (acid-base behavior, solvent influence, kinetics) related to the elucidation of structural activity of the new modified peptides based on their steric open/closed ring effect were studied. The obtained results lead to the conclusion that in protic solvent with change in pH of the environment, direct control over the dyeing of textiles can be achieved. Both the new hybrid peptide compounds and the modification of functionalized textile materials with these bioactive hemorphins showed virucidal activity against the human respiratory syncytial virus (HRSV-S2) and human adenovirus serotype 5 (HAdV-5) for different time intervals (30 and 60 min) and the most active compound was Rh-3.

Some new N- and C-modified biomolecular peptide analogues of both VV-hemorphin-5 and VV-hemorphin-7 with varied amino acids (Cys, Glu, His), 1-adamantanecarboxylic acid, and niacin (nicotinic acid) were synthesized by solid-phase peptide synthesis—Fmoc (9-fluorenylmethoxy-carbonyl) chemistry and were characterized in water solutions with different pH using spectroscopic and electrochemical techniques. Basic physicochemical properties related to the elucidation of the peptide structure at physiological pH have been also studied. The results showed that the interaction of peptide compounds with light and electricity preserves the structural and conformational integrity of the compounds in the solutions. Moreover, textile cotton fibers were modified with the new compounds and the binding of the peptides to the surface of the material was proved by FTIR and SEM analysis. Washing the material with an alkaline soap solution did not show a violation of the modified structure of the cotton. Antiviral activity against the human respiratory syncytial virus (HRSV-S2) and human adenovirus serotype 5 (HAdV-5), the antimicrobial activity against B. cereus and P. aeruginosa used as model bacterial strains and cytotoxic effect of the peptide derivatives and modified cotton textile material has been evaluated. Antimicrobial tests showed promising activity of the newly synthesized compounds against the used Gram-positive and Gram-negative bacteria. The compounds C-V, H-V, AC-V, and AH-V were found slightly more active than NH7C and NCH7. The activity has been retained after the deposition of the compounds on cotton fibers.

Propolis, or bee glue, is a complex mixture of resins from plant buds and flowers, wax, pollen and bee saliva. It also contains various vitamins, essential oils, mineral salts, trace elements, hormones and enzymes. It has been found that propolis possesses antimicrobial, antiviral and anti-inflammatory properties. We have studied the antiviral activity of six extracts of Bulgarian propolis collected from six districts of Bulgaria. The study was conducted against structurally different viruses: Human coronavirus strain OC-43 (HCoV OC-43) and Human respiratory syncytial virus type 5 (HRSV-5) (enveloped RNA viruses), Human herpes simplex virus type 1 (HSV-1) (enveloped DNA virus), Human rhinovirus type 14 (HRV-14) (non-enveloped RNA virus) and Human adenovirus type 2 (HadV-2) (non-enveloped DNA virus). The influence of the extracts on the internal replicative cycle of viruses was determined using the cytopathic effect (CPE) inhibition test. The virucidal capacity, their influence on the adsorption stage and their protective effect on uninfected cells were also determined by the end-point dilution method. Residual viral infectivity in propolis samples was compared to untreated controls and Δlgs were calculated. When administered during viral replication, most propolis extracts demonstrated the strongest activity against HCoV OC-43. The influence of propolis extracts on the viability of extracellular virions was expressed to a different degree in the various viruses studied, and the effect was significantly stronger in those with an envelope. Almost all extracts significantly inhibited the step of herpes virus adsorption to the host cell, and some of them showed a protective effect on healthy cells before they were subjected to herpes infection. Our results enlarge the knowledge about the action of propolis and could open new perspectives for its application in viral infections treatment.

A series of novel 1-oxo-2,3,4-trisubstituted tetrahydroisoquinoline (THIQ) derivatives bearing other heterocyclic moieties in their structure were synthesized based on the reaction between homophthalic anhydride and imines. Initial studies were carried out to establish the anti-coronavirus activity of some of the newly obtained THIQ-derivatives against two strains of human coronavirus-229E and OC-43. Their antiviral activity was compared with that of their close analogues, piperidinones and thiomorpholinones, previously synthesized in our group, with aim to expand the range of the tested representative sample and to obtain valuable preliminary information about biological properties of a wider variety of compounds.