**7. Synthetic microbiomes and engineered vaccine probiotics**

The World Health Organization (WHO) has defined probiotics as "live microorganisms that, when administered in adequate amounts, confer a health benefit on the host". Dysbiosis of healthy gut microbiota plays a critical role in the dysregulation of microbial ecology that favors colonization of pathogenic bacterial strains and diseases [64, 65].

Interestingly, from healthy specific-pathogen-free chickens, competitive exclusion (CE) products could be designed and administered by crop gavage as feed supplements for broiler chickens [66, 67]. This type of bacteriotherapy effectively protects chickens from bacterial infections, notably salmonellosis [67]. Freeze-dried CE preparations are manufactured and commercialized in many countries [68].

Extensive basic and clinical studies with lactic acid bacteria strains such as *Bifidobacteria* spp., *Bacteroides,* and *Akkermansia* spp. have provided strong evidence of their health benefits. This is achieved through multiple mechanisms and effector molecules [69, 70]. In-feed supplementation with *Bacillus*, *Bifidobacterium,*  *Enterococcus, Lactobacillus, Streptococcus, Lactococcus* spp. And Yeast Saccharomyces have been given as probiotics to inhibit infection of enteric pathogens such as *E. coli* and mitigate antibiotic-associated diarrhea [71, 72].
