The alpha - diversity index can analyze and reflect the richness and evenness of microbial communities in various foods. Chao1, ACE, Simpson, and the Shannon index were calculated to estimate the alpha - diversity of different street foods (Fig. 1). The Ace index, Chao1 index, and Shannon index were positively correlated with alpha - diversity, while the Simpson index was negatively correlated with it. (Predictive functional profiling of microbial communities in fermentative hydrogen) Chao1, ACE, and Shannon indices showed similar trends in the seven foods. The highest richness values of Chao1, ACE, and Shannon were observed in snacks (SN), and the lowest in salad (SA). The ACE index of snacks (SN) was 183, that of salads (SA) was 35, and the ACE index of snacks (SN) was 5.23 times that of SA. The Shannon index of snacks (SN) was 209.04, that of salad (SA) was 38.75, and the Shannon index of snacks (SN) was 5.39 times that of SA. The highest Simpson richness value was found in snacks (SN) (5.29), and the lowest in sulla (SU) (1.74).
As shown in Fig. 2, a total of 9 phyla were detected at the phylum level in seven street food microorganisms, including 4 common phyla, namely Bacteroidota, Cyanobacteria, Firmicutes, and Proteobacteria. The results of hierarchical cluster analysis and community structure analysis based on community composition among different street foods showed that salad (SA) was clustered with snacks (SN), and then clustered with pastries (PA), indicating that the microbial community structures of these three foods were similar. The dominant phylum of these three foods was Proteobacteria. MF and SF were clustered together, and the dominant phylum was Cyanobacteria, with a relative abundance of 47.24% − 50.95%. BE and SU were clustered into one group, and the dominant phylum was Firmicutes, with a relative abundance of 63.72% − 74.22%.
As shown in Fig. 3, a total of 152 genera were detected in the seven street foods at the genus level, of which 9 genera were common, namely Bacillus, Mitochondria _ norank, Streptococcus, Caulobacteraceae _ uncultured, Chloroplast _ norank, Faecalibacterium, Acinetobacter, Lactococcus, and Pseudomonas. Among the common genera, Bacillus had the highest content in SU, with a relative abundance of 66.73%, and its relative abundance in other foods was less than 10.00%. Acinetobacter had the highest content in PA, with a relative abundance of 35.62%. Lactococcus had the highest content in SA, with a relative abundance of 7.77%, followed by BE with a higher content of 4.88%, and the relative abundance of the remaining samples was less than 1.00%. The content of Faecalibacterium in the seven foods was low, and its relative abundance was less than 3.00%. In the clustering and composition of different street food genus - level samples, the closer the samples were, the shorter the branch length was, indicating that the species composition of the two samples was more similar, and the similarity or difference in community structure between samples could be clearly seen. Among them, marinated products (MF) and staple food (SF) were clustered into one group, indicating that the microbial community structures of these two foods were similar. The dominant genus of MF was Pantoea, with a relative abundance of 1.66%. The dominant genus of SF was Weissella, with a relative abundance of 8.61%. PA and SN were clustered into one group. The dominant bacteria of PA and SN were mainly Acinetobacter, with a relative abundance of 35.62% − 10.40%. In addition, SU, BE, and SA were clustered into one group, and the dominant genus of SU was Bacillus, with a relative abundance of 66.73%. The dominant genus of BE was Streptococcus, with a relative abundance of 37.33%. The dominant genus of SA was Pseudomonas, with a relative abundance of 29.89%.
LEfSe analysis results (Fig. 4) showed that when the contribution of different species (LDA Score) was greater than the set value of 4, there were 32 significantly different species at the genus level of 7 street food microorganisms. Among them, there was a significant difference between SF and other foods, with Tumebacilluswei as the main standard genus. There were significant differences in 6 genera between MF and other foods, with Koritha, Escherichia Shigella, Aeromonas Asaia, Phascolarctobacterium, Geobacillus as the main standard genera. There were 4 species of bacteria in SN that were significantly different from other foods, with Acinetobacter, Enhydrobacter, Paracoccus, and Megasphaera as the main standard genera. Four species of bacteria in SA were significantly different from other foods, with Anoxybacillus, Lysinibacillus, Faecalibacterium, and Pelomonas as the main standard genera. Pseudomonas, Cobetia, Massilia, Pseudoalteromonas, Sediminibacterium, and Shewanella were the main standard genera in PA, and 7 genera in SU were significantly different from other foods. Rosenbergiella, Rahnellal, Flavobacterium, Rahnellal, Ruminococcus - torques - group, CHKC1001, Vagococcus norank were the main standard genera. Four bacterial genera in BE were significantly different from other foods, with Pantoea, Pediococcus, Moellerlla, and Psychrobacter as the main standard genera. The results showed that there were many differences in dominant bacteria at the genus level among the seven foods.
Through the difference analysis of KEGG metabolic pathways, the differences and changes in functional genes in different street food microbial communities in metabolic pathways could be observed, which was an effective means to study the metabolic function changes of community samples to adapt to environmental changes (Fig. 5 and Fig. 6). From Fig. 5, these pathways were mainly associated with six major functional categories (KEGG level 1), namely metabolism (48.38%), genetic information processing (18.38%), information processing (13.90%), cellular processes (3.00%), human diseases (1.33%), and organism systems (0.77%). Additionally, level 2 KEGG pathways such as amino acid metabolic pathways, carbohydrate metabolic pathways, replication repair, and energy metabolic pathway were the significant predominant functions in each sample (Fig. 6). In the KEGG first - level pathway, metabolism was the absolute dominant function, indicating that the metabolic activities of the seven food microorganisms were very active. Pathogenic bacteria were the key factors affecting food safety. Human Diseases was the pathway we were concerned about. The level 2 KEGG pathways related to the human disease pathway had a total of 6 pathways, such as infectious diseases, neurodegenerative diseases, cancer, metabolic diseases, immune system diseases, and cardiovascular diseases. SU and SA were the significant predominant functions in each sample. The bacterial genera in MF and SF were mainly involved in the Human Diseases pathway. The relative abundances of SM, FM, and BE in Genetic information Processing were higher than those of other foods, and the relative abundances of each food in the remaining primary pathways were less different (Fig. 5).
As shown in Fig. 6, the relative abundances of SF and MF in the Cell Growth and Death pathway under the cellular processes level pathway were higher than those of other foods; the relative abundances of SF and MF in the translation pathway under the genetic information processing primary pathway were significantly lower than those of the other five foods, and the relative abundances of each food in the other pathways were less different. The relative abundances of the seven foods in the infectious diseases pathway were higher than 40%. The relative abundance of SA in the immune system Diseases pathway was the highest, and the relative abundance of neurodegenerative diseases in PA was higher than 30%. The relative abundances of SU and SA in the cardiovascular disease pathway were low. There was little difference in the relative abundances of each food in the other pathways (Fig. 5 and Fig. 6).