Many plant extracts that can improve poultry growth performance are referred to as antimicrobial products.
Dorman and Deans (2000) and Friedman et al.(2002) found that cinnamaldehyde, thymol, and carvacrol are very active molecules against a wide range of bacterial species in vitro.
In those experiments, eugenol had good antimicrobial activity against some bacterial species such as Listeria monocytogenes and Campylobacter jejuni but had medium to low antimicrobial activity against enter-obacteria.
However, Michiels et al. (2009) tested these extracts on pig intestinal microbiota and found similar effects of eugenol on coliforms compared to cinnamaldehyde, thymol, and carvacrol, and lower effects on lactic acid bacteria.