## 2 Esophageal Cancers
### Microbiota in Gastroesophageal Reflux Disease and Barrett's Esophagus  
Gastroesophageal reflux disease (GERD) is a common, but dangerous condition of the upper gastrointestinal tract. GERD develops as a result of constant irritation of the esophagus by stomach acids. The disease arises from motor abnormalities or anatomical factors such as obesity or hiatal hernia [13]. Without treatment, GERD might result in the development of Barrett's esophagus (BE). BE affects the distal esophagus, that is, normal stratified squamous are replaced by columnar mucosa with intestinal specialized metaplasia. BE is a premalignant condition that might directly result in esophageal adenocarcinoma [14].  
Alterations in the esophageal microbiota are observed in GERD and BE, suggesting that the oral microbiota may play a potential role in disease development. [PERSON] et al., in a 16S rRNA gene survey on a group of 34 patients with normal esophagus, GERD, and BE, have reported that BE and GERD associated microbiota is characterized by a greater ratio of Gram-negative anaerobes/microaerophiles (Bacteroidetes, Proteobacteria, Fusobacteria, and Spirochaetes). The results may suggest that changes in composition of the microbiota in the distal esophagus may lead to inflammation and intestinal metaplasia, as Gram negative bacteria produce lipopolysaccharide (LPS), which activates the innate immune response [15].  
[PERSON] et al. [6] found the microbiota of patients with GERD and BE to be colonized mainly by _Veillonella_, _Prevotella_, _Neisseria_, and _Fusobacterium_ species; species that are not detected in the microbiota of the normal esophagus. Later studies also found _Campylobacter_ spp. and _E. coli_[3] to be present in abnormal human esophagus. [PERSON] et al. highlight differences in microbial composition between patients with normal esophagus, GERD, and BE; _Streptococcus_ was the most abundant genus in three study groups (20% of all bacteria detected), but was lower in the BE study group. The GERD-associated esophagitis microbiota was significantly enriched in _Veillonella_, _Haemophilus_, _Pasteurella_, _Neisseria_, and _Fusobacterium_, which were almost undetectable in healthy subjects. The microbiota related to BE was comparable to that of the reflux esophagitis-associated microbiota; however, it displayed a higher prevalence of _Veillonella_, _Achromobacter_, and _Actinobacillus_ (not recognized in reflux esophagitis microbiota) [10] (Table 1).  
Despite interesting results and well-performed statistical analysis, [PERSON] et al. as well as [PERSON] et al. did not consider other, non-bacterial factors that might influence esophagus inflammation such as smoking, diet, or medical conditions. It would be necessary to extend the analysis by adding the aforementioned variables in order to verify if the increase in the number of Gram-negative bacteria still significantly impacts GERD and BE development. Moreover, as mentioned before, [PERSON] et al.'s study was limited by the culture base techniques that are unable to thoroughly represent the microbiome composition.  
Bacterial population in the distal esophagus was examined by [PERSON] et al. [6] using biopsy samples from 18 subjects (6 subjects with normal esophagus, 6 subjects with reflux esophagitis, and 6 subjects with [PERSON]'s esophagus). From the average of 350 clones in each of subjects, 40 were randomly selected and sequenced. Then, 240 clones (40 clones from each subject) yielded 147 16S rDNA sequences, classified into four phyla (normal esophagus); 139 16S rDNA sequences, classified into six phyla (reflux esophagitis); and 138 16S rDNA sequences, classified into five phyla ([PERSON]'s esophagus). Study indicates that Proteobacteria and Firmicutes presence is dominant in all study groups, however, a significant decrease of Proteobacteria is noticed in [PERSON]'s esophagus patients. Fusobacteria, not present in normal esophagus microbiota, is present in reflux esophagitis and [PERSON]'s esophagus.