Lactobacilli
Lactobacillus speciesGeneral Characteristics:
* Lactobacillus species derived from the family Lactobacillaceae Gram positive bacilli.
* Arranged in pairs of short chains.
* Grow in facultative anaerobic conditions.
* Non motile.
* Non spore forming.
* Catalase (-).
* Fermentation of many carbohydrates and forming acids.
*Habitation:
1- Oral cavity.
2- Gastrointestinal tract.
3- Female genital tract.
*Species:-
Lactobacillus acidophilusLactobacillus casei
Lactobacillus fermentum
Lactobacillus brevis
*Taxonomy:-
Lactobacilli are classified into two groups based on glucose fermentation:-1- Homo Fermenters:-
Produce lactic acid from glucose fermentation.
e.g. Lactobacillus casei
Lactobacillus acidophilus
2- Hetero Fermenters:-
Produce 50% lactic acid as well as acetate, ethanol, and carbon dioxide.
e.g. Lactobacillus Fermentum
Lactobacillus brevis
*Cultures:-
Lactobacilli need complex nutritional requirements. They grow under microaerophilic or facultative anaerobic condition and acidic pH.
Lactobacilli can grow on many media:-
1- Blood and chocolate agar (enriched media). Incubate at 37C 2-4 days.
2- Special Selective medium.
Tomato juice agar (pH 5): which contain peptone, casein, tomato juice and agar.
B- Rogosa agar: This medium contains fatty acids, magnesium, purines, pyrimidine, vitamins, and amino acids and the pH is (6-6.5). It also contains sodium acetate and ammonium citrate which is used to inhibition the growth of normal flora and fungi and made this medium selective especially for lactobacilli.
*Oral Ecology:-
At birth oral cavity is sterile and during infant passage through the birth canal, his oral cavity may be contaminated with lactobacillus species from the female vagina, after that and during the first 2 years of his/her life, lactobacilli occur only in a very low numbers. In oral cavity they constitute less than 1% of the total oral flora.
Lactobacillus species and Streptococcus mutans are cariogenic bacteria which cause dental caries.
*Steps of Dental Caries:-
1-Streptococcus mutans is the most important species in the forming of dental caries when a solid surface such as teeth are present and the first step is the colonization on tooth surface.
2-When sugar is ingested, sucrose is broken down into monosaccharides (glucose and fructose), and the presence of sucrose stimulates Streptococcus mutans to synthesis sticky polymers and this allow for adhesions on smooth surface (tooth).
3-Then they ferment sucrose to form a strong acid (lactic acid), which destroys the tooth structure by demineralization of Ca (OH)2 and calcium phosphate mostly hydroxyapatite {Ca5(PO4)3(OH)}.
4-Lactobacillus species will continue to form lactic acid and produce the cavity of dental caries.
*Dental Caries Activity Tests:-
1-Counting of streptococcus mutans and lactobacillus species in the saliva:*High caries activity: If the counting is > 106 CFU/ ml of Streptococcus mutans and >105 CFU/ ml of Lactobacillus species.
* Low caries activity: If the counting is < 105 CFU/ ml of Streptococcus mutans and < 104 CFU/ ml of Lactobacillus species.
2. Snyder test: is used to determine a persons susceptibility to dental caries. The susceptibility is correlated with acid production that is assumed to result from fermentation by cariogenic Lactobacillus species on the teeth or other areas of the mouth.
The Snyder test agar contains 2% glucose and a pH indicator bromocresol green. The pH of the agar is about 4.8, which inhibits the growth of most organisms, but it is ideal for acidophiles such as Lactobacillus species. Saliva samples are inoculated into the tubes and allowed to incubate. If lactobacilli are present in the saliva, it will ferment the glucose and produce lactic acid, causing the pH to drop to about 4.4. This causes the bromocresol green to change from green to yellow. A culture demonstrating a yellow color indicates a person is susceptible to the formation of dental caries.
3. Reductase Test:
Salivary reductase test measures the activity of reductase enzyme present in salivary bacteria. Paraffin-stimulated saliva is collected in a plastic container and an indicator dye "diazoresorcinol" is added to nitrate broth, which colors the saliva blue.The reductase enzyme liberated by the salivary bacteria cause changes in the medium from blue to other colors, which indicate the caries conduciveness of the patients.
ColorTimeScoreConduciveness on cariesBlue15 min1Non-conduciveOrchid15 min2Slightly conduciveRed15 min3Moderately conduciveRed30 sec4Highly conducivePink/White30 sec5Extremely conducive
Lactobacilli under electron microscope
Gram positive lactobacilli