## 4 Interactions between Vaccine Administration and Gut Microbiome Status
### Gut Dysbiosis and Its Influence on Vaccine Efficacy in Poultry
[86] observed that various candidate _Salmonella_ vaccines did not affect critical populations in broiler cecal microbiota, including genera such as _Ruminococcus_ (short-chain fatty acid generation), _Fae-calibacterium_ (secretion of anti-inflammatory metabolites [89]), and _Lactobacillus_ (lactic acid secretion). These results resembled those of [PERSON] et al. [87]. Following vaccination of 16-day-old Ross 308 broilers with commercially available live _Salmonella_ Enteritidis and Typhimurium vaccines, there were alterations to the cecal microbiota at 14 days post-oral-gavage vaccination. Whilst no differences in cecal microbiota diversity were observed between broilers of the vaccine treatments and the control group, microbiota composition was influenced by the vaccine treatment. Among broilers vaccinated with _Salmonella_ Enteritidis or Typhimurium, the Firmicutes phylum (including _Lactobacillus_ and _Ruminococcus_ genera) was enriched while Bacteroidetes phylum and _Alistipes_ genus abundance were diminished. Firmicutes are commonly associated with initial establishment in the cecal microbiota of young chicks while the abundance of Bacteroidetes increases as the birds age [90]. Additionally, the _Alistipes_ genus (belonging to the Bacteroidetes phylum) has been identified in broiler chick cecal microbiota [91], but its implication in broiler performance and mucosal immunity is not understood at this time. Similarly, [PERSON] et al. [68] demonstrated that egg-type chickens vaccinated at 1 and 28 days of age with a commercial live _Salmonella_ vaccine did not exhibit negative changes to the gut microbiome; specifically, the only changes in gut microbiome status were associated with vaccine treatments that also supplemented probiotic bacteria in feed during the vaccination period. Moreover, the type of vaccine and its associated \"adjuvant\" may contribute to unique alterations to gut microbiota. For example, [PERSON] et al. [86] determined that three different _Salmonella_ vaccine candidates stimulated unique alterations to cecal microbiota in broiler chicks, where the microbiota composition of cecal pouch content among individuals within a _Salmonella_ vaccine treatment group were more closely related to each other than to samples from different _Salmonella_ vaccine treatment groups. [PERSON] et al. [88] also evaluated the ceca microbiota status in SPF chicks and commercial egg-type pullets following vaccination with a commercial _Salmonella_ vaccine and a live _Salmonella_ challenge. When compared to non-vaccinated control groups, the cecal microbiota composition of 30-day-old vaccinated, non-challenged SPF chicks was dominated by _Faecalibacterium prausnitzii_, _Blautia hominis_, and _Subdoligranulum variabile_ species while the cecal microbiota composition of 27-week-old vaccinated, non-challenged commercial pullets were dominated by _Subdoligranulum variabile_, _Negativbacillus__assiliensis_, and _Mediterraneanbacter glycyrrhizinylticus_[88]. Previously, genera like _Faecalibacterium_ and _Subdoligranulum_ were detected in the cecal pouch contents of broiler chickens [92], and their contribution to the human microbiota is considered beneficial [93, 94]. Alternatively, _Subdoligranulum_ and _Faecalibacterium_ abundance in cecal pouch contents of 42-day-old Ross 708 broiler chicks negatively correlated (\(p=0.03\) and \(p=0.0004\), respectively) with body weight (i.e., the smaller the bird, the greater the abundance of these genera). Furthermore, the implications for the enrichment of these genera in a bird's cecal pouch are not yet understood [95]. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Model** & **Vaccination** & **Coca Prouch Micobital Response** & **Ref.