There is a growing body of evidence that flavonoids show antibacterial activity against both Gram-positive and Gram-negative bacteria. The mechanisms of action of phenolic compounds on bacterial cell have been partially attributed to damage to the bacterial membrane, inhibition of virulence factors such as enzymes and toxins, and suppression of bacterial biofilm formation. What is more, some natural polyphenols, aside from direct antibacterial activity, exert a synergistic effect when combined with common chemotherapeutics. Many studies have proved that in synergy with antibiotics plant flavonoids pose a promising alternative for therapeutic strategies against drug resistant bacteria. In this review most recent reports on antimicrobial action of polyphenols on Staphylococcus aureus strains are described, highlighting where proven, the mechanisms of action and the structure–activity relationships. Since many reports in this field are, to some extent, conflicting, a unified in vitro and in vivo susceptibility testing algorithms should be introduced to ensure the selection of effective antibacterial polyphenolic compounds with low cytotoxicity and minimal side effects.

Phenolic compounds constitute one of the most promising and ubiquitous groups with many biological activities. Synergistic interactions between natural phenolic compounds and antibiotics could offer a desired alternative approach to the therapies against multidrug-resistant bacteria. The objective of the presented study was to assess the antibacterial potential of caffeic acid (CA) alone and in antibiotic-phytochemical combination againstStaphylococcus aureusreference and clinical strains isolated from infected wounds. The caffeic acid tested in the presented study showed diverse effects onS. aureusstrains with the minimum inhibitory concentration (MIC) varied from 256μg/mL to 1024μg/mL. The supplementation of Mueller-Hinton agar (MHA) with 1/4 MIC of CA resulted in augmented antibacterial effect of erythromycin, clindamycin, and cefoxitin and to the lesser extent of vancomycin. The observed antimicrobial action of CA seemed to be rather strain than antibiotic dependent. Our data support the notion that CA alone exerts antibacterial activity againstS. aureusclinical strains and has capacity to potentiate antimicrobial effect in combination with antibiotics. The synergy between CA and antibiotics demonstrates its potential as a novel antibacterial tool which could improve the treatment of intractable infections caused by multidrug-resistant strains.

Synergistic interactions between commonly used antibiotics and natural bioactive compounds may exhibit therapeutic benefits in a clinical setting. Berberine, an isoquinoline-type alkaloid isolated from many kinds of medicinal plants, has proven efficacy against a broad spectrum of microorganisms. The aim of the presented work was to assess the antibacterial activity of berberine chloride in light of the effect exerted by common antibiotics on fourteen reference strains of Staphylococccus spp., and to evaluate the magnitude of interactions of berberine with these antistaphylococcal antibiotics. In our study minimum inhibitory concentrations (MIC) of berberine chloride against CoNS ranged from 16 to 512 µg/mL. The most noticeable effects were observed for S. haemolyticus ATCC 29970, S. epidermidis ATCC 12228, S. capitis subsp. capitis ATCC 35661, S. galinarium ATCC 700401, S. hominis subsp. hominis ATCC 27844, S. intermedius ATCC 29663 and S. lugdunensis ATCC 49576. The most significant synergistic effect was noticed for berberine in combination with linezolid, cefoxitin and erythromycin. The synergy between berberine and antibiotics demonstrates the potential application of compound combinations as an efficient, novel therapeutic tool for antibiotic-resistant bacterial infections.

The objective of this study was to assess in vitro the antimicrobial activity of ethanolic extract of Polish propolis (EEPP) against methicillin-sensitive Staphylococcus aureus (MSSA) and methicillin-resistant Staphylococcus aureus (MRSA) clinical isolates. The combined effect of EEPP and 10 selected antistaphylococcal drugs on S. aureus clinical cultures was also investigated. EEPP composition was analyzed by a High Performance Liquid Chromatography (HPLC) method. The flavonoid compounds identified in Polish Propolis included flavones, flavonones, flavonolols, flavonols and phenolic acids. EEPP displayed varying effectiveness against twelve S. aureus strains, with minimal inhibitory concentration (MIC) within the range from 0.39 to 0.78 mg/mL, determined by broth microdilution method. The average MIC was 0.54 ± 0.22 mg/mL, while calculated MIC₅₀ and MIC₉₀ were 0.39 mg/mL and 0.78 mg/mL, respectively. The minimum bactericidal concentration (MBC) of the EEPP ranged from 0.78 to 3.13 mg/mL. The in vitro combined effect of EEPP and 10 antibacterial drugs was investigated using disk diffusion method-based assay. Addition of EEPP to cefoxitin (FOX), clindamycin (DA), tetracycline (TE), tobramycin (TOB), linezolid (LIN), trimethoprim+sulfamethoxazole (SXT), penicillin (P), erythromycin (E) regimen, yielded stronger, cumulative antimicrobial effect, against all tested S. aureus strains than EEPP and chemotherapeutics alone. In the case of ciprofloxacin (CIP) and chloramphenicol (C) no synergism with EEPP was observed.

The hospital environment microflora comprise a wide variety of microorganisms which are more or less pathogenic and where staphylococci are one of the most common types. The aim of the presented study was to evaluate the prevalence of the biofilm forming coagulase-negative staphylococci (CoNS) in a hospital environment as a risk factor for nosocomial infections. Among 122 isolated and tested strains of CoNS the most frequent were: S. epidermidis—32 strains, S. haemolyticus—31 strains, S. capitis subsp. capitis— 21 strains, S. hominis—11 strains, S. cohnii subsp. cohnii—nine strains. In case of CoNS, the main molecule responsible for intercellular adhesion is a polysaccharide intercellular adhesin (PIA), encoded on the ica gene operon. The analysis revealed the presence of the icaADBC operon genes in 46.88% of S. epidermidis isolates. IcaA and icaD were present in 34.38% and 28.13% of strains respectively while IcaC gene was present in 37.50% of strains. IcaB gene was found in 21.88% of S. epidermidis strains. In 15 (63%) strains all icaADBC operon genes were observed. The assessment of antibacterial drugs susceptibility demonstrated that analyzed CoNS strains were highly resistant to macrolides and lincosamides and more sensitive to rifampicin and linezolid. Our data indicates that the hospital environment can be colonized by biofilm forming coagulase-negative staphylococci and transmission of these strains can cause an increased risk of serious nosocomial infections.

Synergistic effects between commonly used antibiotics and natural substances may be an alternative to conventional antibacterial therapies. The objective of the presented study was to assess the in vitro antibacterial activity of catechin hydrate (CH) and evaluate the interactions of CH with selected antibiotics using Staphylococcus aureus clinical and reference strains. CH displayed diverse activity towards examined S. aureus strains, with minimal inhibitory concentrations (MICs) ranging from 256 to 2048 µg/mL. The interaction between CH and antibiotics was assessed by an E-test. The most significant synergistic effects were noticed for CH in combination with clindamycin and erythromycin. For cefoxitin and vancomycin a decrease of MIC values in the presence of CH was also observed, but it did not reach statistical significance. The obtained results demonstrate that CH shows antimicrobial activity against Staphylococcus aureus clinical strains. What is more, we proved a synergistic effect of CH with erythromycin and clindamycin.