## 4 Biological Activities of Hop Compounds
### Antimicrobial Activity  
Hops have traditionally been used in beer as a natural preservative because of their high content of bitter acids and polyphenols, which inhibit the growth of a broad spectrum of microorganisms. [PERSON] et al. [54] studied the antimicrobial potential of extracts of two hop varieties. All extracts showed antimicrobial activity against all pathogenic bacteria tested at minimum inhibitory concentrations (MICs) in the range of 8-512 \(\upmu\)g/mL. Gram-positive strains (MICs 8-64 \(\upmu\)g/mL) were more sensitive than Gram-negative strains (MICs \(\geq\) 32 \(\upmu\)g/mL) and yeast (MIC = 512 \(\upmu\)g/mL). The authors identified \(\alpha\)- and \(\beta\)-acids as the main substances involved in the inhibitory effects of hop extracts. Humulinic acid (derived from iso-\(\alpha\)-acids) has also been reported for their antimicrobial activity against _Lactobacillus brevis_[60].  
Today, there is a global concern to find new active agents to combat microbial resistance to antibiotics [61]. In this sense, methicillin-resistant _Staphylococcus aureus_ (MRSA) strains cause different pathologies that are difficult to treat due to their virulence, resistance to almost all useful antibiotics, as well as the formation of persistent biofilms [61]. Plant-derived natural compounds,characterized by the presence of a wide spectrum of active biomolecules, have been studied as potential antimicrobial agents that are useful to treat several diseases caused by multidrug-resistant microorganisms. According to [PERSON] et al. [43], seven flavonoids, among them two natural (\(\alpha\),\(\beta\)-dihydroxanthohumol and 8-prenylnaringenin) flavonoids identified in spent hops extracts, showed remarkable antimicrobial activity against methicillin-sensitive (MSSA) and resistant _Staphylococcus aureus_ (MRSA) strains. Lupulone, xanthohumol, and desmethylxanthohumol also exhibited strong antimicrobial activity against MRSA as well as the ability to inhibit biofilm formation [61]. Similar findings were also found by [PERSON] et al. [62], who reported that humulone, lupulone, and xanthohumol had strong antimicrobial and antibofilm activity against MRSA strains. In this research, the authors indicated that lupulone exhibited the strongest effect, which was followed by xanthohumol. Ethanolic extracts of hops also exhibited an antimycobacterial effect on rifamp-sensitive and resistant strains of _Mycobacterium tuberculosis_[63].  
Some authors have also demonstrated the antifungal activity of hop extracts. [PERSON] et al. [10] tested crude extracts of different parts of hop (leaves, stems, rhizomes, and female cones) and the essential oil of hops against _Zymosetporia tritici_. All extracts and essential oil showed antifungal activity, although only female cones and the essential oil exhibited a visible activity, observing a reduction of 85% and 100% in the growth of _Z. tritici_ at 1.25 g/L, respectively. The authors attributed the antifungal activity of hop cone extracts to the presence of desmethylxanthohumol and co-humulone and concluded that the combination of hops essential oil with synthetic fungicides could be an appropriate strategy to reduce the dose of conventional fungicides in crop protection. Similarly, [PERSON] et al. [2] also reported the antifungal activity of hop seed extract. The results indicated that this extract had a remarkable antifungal effect, which was even better than the positive control, against fungi of the genus Penicillin.