The oral cavity contains hundreds of bacterial species that form complex biofilm communities on teeth and gums. Two key pathogens associated with dental caries are Streptococcus mutans and Lactobacillus casei. These bacteria produce acids by fermenting sugars that demineralize tooth enamel over time, leading to cavities. While everyone harbors caries-causing bacteria like S. mutans, dental caries only develops when there is an imbalance in the microbial community that allows these pathogens to dominate and lower the pH. Studying the oral microbiome provides insights into the pathogenesis of oral diseases and opportunities for prevention and treatment strategies.

1. Normal Oral Microflora
Microflora of Dental Caries
Microflora of the Root Canal ( endo )
Microflora of the Periodontal Pocket (perio)
Oral Microbiology

2. The Importance of Studying Bacterial
Pathogenesis
• A human body has 1 X 1013 eukaryotic
cells and 1 X 1014 bacterial cells
• Microbial infections are the most epidemic
diseases and the leading cause of death
– Diarrhea and enteric bacteria
– Tuberculosis and Mycobacterium
– Ulcer and Helicobacter infection
– Urinary tract infection
– STD
How microbiology is related to dentistry?

4. The first microbes observed: Anton
Van Leeuwenhoek (1632-1723)
Developed the microscope and
was the first to discover oral
bacterial flora: “I didn’t clean
my teeth for three days and
then took the material that had
lodged in small amounts on the
gums above my front teeth…. I
found a few living
animalcules..”

6. What causes dental caries?
• Pre-microbiology era
– Dental caries is the death (decay) of a tissue
• Microbiology period era
– Dental caries is a microbe related disease

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8. Plaque is a complex microbial
community

9. How to identify bacteria within dental
plaque?
• Culture methods
take saliva or plaque, dilute and plate on
appropriate plates, grow to single colonies,
identify by microscopic and biochemical methods
• 16S DNA/RNA based detection
use 2 oligo-nucleotide primers universal to ALL
bacteria 16S rDNA, PCR amplification of the total
saliva or plaque DNA pool, clone the PCR product
and sequence, phylogenetic analysis using
computer database

10. Supragingival Plaque
Predominant cultivable microflora
obtained from occlusal fissures
Bacteria Median percentage Range % isolation
Streptococcus 45 8-86 100
Staphylococcus 9 0-23 80
Acinomyces 18 0-46 80
Propionibacterium 1 0-8 50
Eubacterium 0 0-27 10
Lactobacillus 0 0-29 20
Veilonella 3 0-44 60

11. Supragingival Plaque
Predominant cultivable microflora
obtained from occlusal fissures
Specie Median percentage Range % isolation
S.mutans 25 0-86 70
S. Sanguis 1 0-15 50
S. Oralis 0 0-13 30
S. Anginosus 0 0-3 10
A.Naeslundi 3 0-44 70
L. Casei 0 0-10 10
L. plantarum 0 0-29 10

12. Subgingival plaque
Bacteria Mean percentage Frequency %
Streptococcus 23 100
Actinomyces 42 100
Prevotella 8 93
Veilonella 14 93
S. Sanguis 6 86
A. naeslandii 19 97

13. Current knowledge about bacteria in plaque
• Both culture and DNA/RNA-based techniques
are used for identification and quantification of
oral microorganisms
• Overall, there are ~700 species exist in the oral
cavity
• ~20% of these 700 species have been cultivated
• Both Gram-positive and Gram-negative exist
• Some archaea are found
• Most anaerobic or facultative anaerobic
Who are the bad guys?

14. The first isolation of cariogenic
bacteria by Clark, 1924
Isolation of cariogenic bacteria from
caries lesions
Discovery of Mutans streptococci

15. Keyes and Fitzgerald, 1962’s
Re-isolation of “Mutans streptococci”:
• Streptococcus mutans (human) (same
species Clark isolated in England in 1924)
• Streptococcus sobrinus (human)
• Streptococcus rattus (rats)
• Streptococcus cricetus
• Streptococcus ferus
• Streptococcus macacae
• Streptococcus downeii

17. • The “cariogenic bacteria” – bacteria associated
with dental caries
• Actinomyces – early colonizers and root caries
• A. odontolyticus
• A. naeslundii genospecies 2
• A. isrealii
• A. gerensceriae
• Lactobacilli (L. casei) – caries progression
• Mutans streptococci (S. mutans) – caries
initiation

18. The virulence factors of
cariogenic bacteria
1. Acid production (acidogenicity)
• Lower the pH to below 5.5, the critical pH,
drives the dissolution of calcium
phosphate(hydroxyapatite) of the tooth
enamel
• Inhibit the growth of beneficial bacteria,
promote the growth of aciduric bacteria.
• Further lower the pH, promote progression
of the carious lesion

19. 2. Acid tolerance (aciduricity)
– Allows the cariogenic bacteria to thrive under
acidic conditions while other beneficial bacteria
are inhibited. This results in dominance of the
plaque by cariogenic bacteria

20. • Glucan formation
– Allows the cariogenic bacteria to stick onto the
teeth and form a biofilm
– Glucan mediated biofilms are more resistant
tomechanical removal
– Bacteria in these biofilms are more resistant to
antimicrobial treatments

21. Dental Caries
Carbohydrate (Sucrose)
Cariogenic bacteria such as strep. mutans
Glucans/levans Acids
Plaque formation Demineralization

22. Dental caries is a bacterial infectious
disease
Transmission
• Mother – Child (vertical transmission) -true
for most oral bacteria
• Persons in close contact to the baby
• Horizontal transfer (between spouses) is
rare, only observed in some periodontal
pathogens (i.e. P.gingivalis)
The most common vehicle is saliva

23. New problem: everybody has S.
mutans!
• So…….Why not every body who has S.
mutans develop dental caries?
 S. mutans is not present in high portions
Acid produced is neutralized by urea or ammonia
produced by other bacteria in the plaque
S. mutans is away from the tooth surface so acid
produced is diffused

24. The ecologic plaque hypothesis
 Both pathogenic and commensal (nonharmful)
bacteria exist in a natural plaque. At sound site,
the pathogenic bacteria may exist in low numbers
to cause any clinical effect, or they may exist in
higher numbers, but the acid produced is
neutralized by the action of other bacteria.
 Disease is a result of a shift in the balance of the
residence microflora driven by a change in the
local environment (frequent sugar intake etc).

26. Human genome project
Oral bacteria genome
projects
Metagenomics of the oral
cavity

27. Human genome
Complex genetic
only has
makeup of man
200,000 genes
Each oral bacterium 2000 to 6000 genes.
• With over 1000000 bactrial genes in the oral cavity

28. The dental plaque is a complex
multispecies biofilm

29. Environment
al
Attachment Modification Microbial
Transmission pH, Eh
& Growth of succession
Colonization Pioneer Expose new G+, G-
species receptors (periodontal
Generate pathogens
new
nutrients
Pioneer
colonizers:
S. oralis
Mother to S. mitis
Child S. salivarius Increased
Climax species
S. sanguis community
S. anginosus diversity
S. gordonii

30. That’s all folks! ! ! Have fun ! !