Below is a list of the predominant flora of the oral cavity.  Most of these organisms are bacterial.  You should be familiar with the major organisms and their role in oral disease.

Table of the Microorganisms of the Oral Cavity

Microorganism	Gram Reaction	Oxygen Needs	Physiologic Traits	Association with Disease
Streptococcus salivarius	Positive	Facultative anaerobe	Coccus; a and g hemolysis; found on tongue surface; acid & levan production	Infectious endocarditis
Streptococcus sanguis	Positive	Facultative anaerobe	Coccus; acid production	Plaque flora; pulp infection; infectious endocarditis
Streptococcus mutans	Positive	Facultative anaerobe	Coccus; mostly on teeth; acid, levan, & dextran production	Infectious endocarditis
Lactobacillus acidophilus	Positive	Facultative anaerobe	Bacillus; acid tolerant; acid production	Plaque, caries
Lactobacillus casei	Positive	Facultative anaerobe	Bacillus; acid tolerant; acid production	Plaque, caries
Neisseria species	Negative	Aerobe	Diplococcus; oxidase positive	Normal flora
Actinomyces viscosus	Positive	Anaerobe	Bacillus; proteolytic; acid production	Actinomycosis; gingivitis
Actinomyces naeslundi	Positive	Anaerobe	Bacillus; mineralization; acid production	Periodontitis, caries
Bacteroides melanogenicus	Negative	Anaerobe	Bacillus; found in gingival pockets	Periodontitis; acute necrotizing ulcerative gingivitis (ANUG); subgingival plaque
Bacteroides oralis	Negative	Anaerobe	Bacillus; found in gingival pockets	Periodontitis; ANUG; subgingival plaque
Bacteroides forsythus	Negative	Anaerobe	Bacillus; found in gingival pockets	Periodontitis; ANUG; subgingival plaque
Bacteroides gingivalis	Negative	Anaerobe	Bacillus; found in gingival pockets	Periodontitis; ANUG; subgingival plaque
Fusobacterium nucleatum	Negative	Anaerobe	Acid production; toothpick-shaped rod	Chronic marginal periodontitis
Eikenella corrodens	Negative	Facultative anaerobe	Bacillus; colonies pit the agar	Systemic infection from bites or injury leading to cellulitis and arthritis
Treponema denticola	Not performed	Anaerobe	Spriochete; nutritionally fastidious	ANUG
Treponema vincenti	Not performed	Anaerobe	Spirochete; nutritionally fastidious	ANUG
Corynebacterium species	Positive	Variable	Bacillus; normal flora	Pulp and root canal infection
Porphyromonas gingivalis	Negative	Anaerobe	Bacillus	Periodontitis
Prevotella intermedia	Negative	Anaerobe	Bacillus	Predominant cause of ANUG
Actinobacillus actinomycetemcomitans	Negative	Facultative anaerobe	Bacillus; oxidase positive	Juvenile periodontitis
Candida albicans	Positve	Yeast	Normal flora	Thrush; denture stomatitis; root canal infection

The respiratory system can be divided into the upper and lower respiratory systems.

The upper respiratory system contains

·        the nose and its associated structures, which include ducts from the sinuses and nasolacrimal ducts from the tear-forming apparatus that empty into the nasal cavity

·        the pharynx (throat) and its associated structures, which include the middle ear and the eustachian tubes, and

·        the oral cavity, which contains the tongue.

The lower respiratory system contains

·        the larynx,

·        the trachea,

·        the bronchial tubes, and

·        the alveoli.

The lower respiratory system is essentially sterile, although the trachea may contain a few bacteria.  The upper respiratory system, on the other hand, is home to many normal flora, including some that are potentially pathogenic, given the right circumstances.

To a certain degree, the microorganisms of the upper respiratory system are a reflection of the microbiota of the skin.  For example, both Staphylococcus aureus and Staphylococcus epidermidis can be recovered from nasopharyngeal cultures.  Some organisms are more frequently found among certain age groups than others.  For example, Group A beta hemolytic Streptococcus is often carried by school age children.  Below is a list of some of the bacteria that can be isolated as normal flora from upper respiratory tract cultures:

·        Moraxella species

·        Neisseria species

·        S. epidermidis

The following organisms are only considered normal flora if present in very small numbers.  If present in large numbers, or if found in a pure culture, they are considered pathogenic:

·        S. aureus

·        Haemophilus influenzae

·        Other gram negative bacilli

Finally, below are examples of organisms that are generally always considered pathogenic:

·        Group A, beta hemolytic Streptococcus

·        Streptococcus pneumoniae

In today’s lab, we will compare your oral and respiratory cultures to known cultures of some common flora of the oral and upper respiratory areas.  Use the table on the next page to organize your pattern-recognition abilities and then to compare your isolates with the knowns. 

Remember, ‘colony configuration’ can include types of blood agar hemolysis, color of the colony, texture of the colony—in short, its physical appearance to your discerning eye.  ‘Colony margins’ include descriptive terms such as ‘smooth,’ ‘scalloped,’ ‘irregular,’ and ‘spreading.’  ‘Colony elevation’ is either ‘flat,’ ‘raised,’ or ‘grows below the agar line.’

Bacterial Colony	Colony Configuration	Colony Margins	Colony Elevation
Streptococcus sanguis
Streptococcus pneumoniae
Streptococcus pyogenes
Streptococcus agalactiae
S. epidermidis
S. aureus

Now, examine the blood agar plates of your oral samples from last week. 

1.      Compare the aerobic plate to the anaerobic plate.

a.      Is there any difference in growth patterns?

b.      Did the same organisms grow on both plates?

c.      What do these two plates tell you about the organisms’ oxygen requirements? 

2.      How many different colony types can you distinguish?

3.      How do your isolates compare to the above colony morphologies?

4.      Are your cultures pure or mixed?

5.      Perform a Gram stain of one oral colony of your choice (see next page for instructions).

Next, examine the blood agar plate of your respiratory sample from last week.  You will also want to refer to the Dichotomous Schemes below.

1. Look for patterns of hemolysis among the colonies.
2. Compare your normal flora to the table above.
3. How many colony types do you see?
4. Perform a Gram stain of one respiratory colony of your choice (see next page for instructions).
5. If you have any Gram positive colonies that are beta hemolytic on blood agar, you will need to perform a catalase test to distinguish S. aureus from S. pyogenes.  Both of these organisms are beta hemolytic on blood agar.
6. Any Gram negative diplococci should have an oxidase test performed (why?).
7. Gram positive, catalase negative organisms may need to be further tested:
   1. If beta hemolytic:  Serodiagnostic test for Group A beta hemolytic Streptococcus OR subculture on blood agar and apply a BACITRACIN disk (see below for explanation).
   2. If alpha hemolytic:  subculture on blood agar and apply an OPTOCHIN disk (see below for explanation) to identify Streptococcus pneumoniae.

Gram Negative/Positive Dichotomous Schemes

Gram Stain

Materials: