1. Bacterial Flora in Human Oral Cavity Made by Luan Yijun, Zhang Yanhui, Liu Junying, Li Na, Xuxia, Liu Song, Wang Ge, Zhang Tianqi, Ying Wanggui, Li Shengfeng

2. 1.Normal flora in human oral cavity 2.Several dental diseases caused by bacteria 3.What can we do to prevent dental disease

3.  The human mouth is home to more than 500 species of bacteria.  Oral bacteria include streptococci, lactobacilli, staphylococci and corynebacteria( 棒状杆菌 ), with a great number of anaerobes, especially bacteroides( 类杆菌 ).

4. Normal flora changes with age  At birth the oral cavity is sterile  It rapidly becomes colonized from the environment

5.  Streptococcus salivarius is dominant and may make up 98% of the total oral flora until the appearance of the teeth (6 - 9 months in humans)  The eruption of the teeth during the first year leads to colonization by S. mutans （变形链 球菌） and S. sanguis,they will persist as long as teeth remain

6.  Other strains of streptococci adhere strongly to the gums （牙龈） and cheeks but not to the teeth  The creation of the gingival crevice （齿龈裂 缝） area increases the habitat for the variety of anaerobic species found

7.  Bacteroides and spirochetes colonize around puberty

8. Functions of normal flora  occupy available colonization sites  contribute to host nutrition  contribute to immunity  exert microbial antagonism against heterogenous species

9.  Since some of these bacteria are parasites or opportunistic pathogens, if they invade tissues not normally accessible to them, characteristic diseases result back

10.  Dental Plaque （牙菌斑） Dental Plaque （牙菌斑）  Dental Caries （龋齿） Dental Caries （龋齿）  Periodontal Disease （牙周病） Periodontal Disease （牙周病）

11. Dental Plaque  material adhering to the teeth, consists of bacterial cells (60-70% the volume of the plaque), salivary polymers, and bacterial extracellular products  Streptococcus sanguis and Streptococcus mutans

12. a weak attachment of the streptococcal cells to salivary glycoproteins →a pellicle on the surface of the teeth → a stronger attachment → ( glycosyl transferase, an enzyme on the cell surface of Streptococcus mutans, is apparently involved in initial attachment of the bacterial cells to the tooth surface and ) → conversion of Sucrose ( 蔗糖） to dextran （右旋糖酐） and levan （左旋糖酐） polymers → form the extracellular matrix of plaque Mechanism

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15. Dental Caries  the destruction of the enamel （釉质）, dentin （牙质） or cementum （牙骨质） of teeth due to bacterial activities  Streptococcus mutans, Lactobacilli ， Actinomyces （放线菌）, and various proteolytic bacteria

16. Physiological and Biochemical Properties of Streptococcus mutans  a regular component of the normal oral flora of humans  contains the enzyme glycosyl transferase  produces lactic acid, produces more lactic acid and is more acid-tolerant than most other streptococci  stores polysaccharides made from dietary sugars, can be utilized as reserve carbon and energy sources for production of lactic acid

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20. Periodontal Disease  bacterial infections that affect the supporting structures of the teeth (gingiva, cementum （ 牙骨质）, periodontal membrane and alveolar bone)  Actinomyces, streptococci, spirochetes and Bacteroides

21.  The mechanisms of tissue destruction in periodontal disease are not clearly defined but hydrolytic enzymes, endotoxins, and other toxic bacterial metabolites seem to be involved

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23. How to prevent dental diseases  Brush your teeth two times everyday, after arousing and before sleeping  Use toothpastes which contains fluoride  Use dental floss to clean your teeth-gaps everyday