Streptococci - Overview of Detection, Identification, Differentiation and Cultivation Techniques
By: By Jvo Siegrist, Product Manager Microbiology, firstname.lastname@example.org, AnalytiX Volume 7 Article 3
Streptococci are non-motile, microaerophilic, Grampositive spherical bacteria (cocci). They often occur as chains or pairs and are facultative or strict anaerobes. Streptococci give a negative catalase test, while staphylococci are catalase-positive. Because they are unable to synthesize cytochromes, streptococci cannot carry out oxidative phosphorylation. They are able to ferment sugars, but the end product is always lactic acid. Therefore, streptococci are very acid tolerant and count among the lactic acid bacteria order.
There are many natural sources of streptococci, including humans and diverse animals where they often colonize the mucosal surfaces of the mouth, intestinal tract, nasal passages and pharynx. The presence of streptococci in drinking water indicates fecal contamination. Food sources with high risk of contamination include milk and dairy products, eggs, steamed lobster, ground ham, potato salad, custard, rice pudding and shrimp salad. In most streptococcal food poisoning cases, the food was allowed to stand at room temperature for several hours between preparation and consumption. The contamination of the food is most often the result of poor hygiene, handling of the food by infected people, or the use of raw (unpasteurized) milk. Although they can be potent pathogens, some streptococci are commercially important for the production of cheese and yogurt. These include S. lactis, S. cremoris, S. diacelillactis and S. thermophilus, the latter being the most well-known.
For detection, identification, differentiation, enumeration and cultivation of streptococci, Sigma-Aldrich provides a broad range of specific agars and broths (Table 1), Streptococci Diagnostic Tests (Table 2) and a Gram staining kit and component solutions (Table 3).
Streptococci are divided into three groups based on their hemolytic (red blood cell lysing) activity. The hemolytic reaction can be visualized on blood agar plates, such as the non-selective Agars for Differentiation that are listed in Table 1.
|Table 1 .........Media for Streptococci|
|Non-selective Enrichment Broths|
|53286||Fluka||Brain Heart Broth|
|B2551||Sigma||Brewer thioglycollate medium|
|22089||Fluka||Casein peptone Lecithin Polysorbate Broth|
|60865||Fluka||Cooked Meat Broth|
|05121||Fluka||Heart Infusion Broth|
|70122||Fluka||Nutrient Broth No. 1|
|70149||Fluka||Nutrient Broth No. 3|
|40893||Fluka||Peptone Water, phosphate-buffered, Vegitone|
|77187||Fluka||Peptone Water, phosphate-buffered|
|S4681||Sigma||Standard Nutrient Broth No. 1|
|85905||Fluka||Stuart Ringertz Medium|
|90404||Fluka||Thioglycolate Broth with Resazurine|
|T1438||Sigma||Todd Hewitt Broth|
|22092||Fluka||Tryptic Soy Broth|
|51228||Fluka||Tryptic Soy Broth No. 2|
|41298||Fluka||Tryptic Soy Broth, Vegitone|
|T3938||Sigma||Tryptone Soya Broth without Dextrose|
|V5262||Sigma||Veal Infusion Broth|
|Selective Enrichment Broths|
|17157||Fluka||Glucose Azide Broth|
|Non-selective Agars for Cultivation, Enumeration and Isolation|
|70138||Fluka||Brain Heart Infusion Agar|
|D3060||Sigma||Dextrose starch agar|
|D8184||Sigma||Diagnostic sensitivity test agar|
|17197||Fluka||Peptonized Milk Agar|
|70152||Fluka||Plate Count Agar|
|88588||Fluka||Plate Count Agar according to Buchbinder et al.|
|19718||Fluka||Plate Count Agar, Vegitone|
|17257||Sigma||Streptococcus thermophilus Isolation Agar|
|22091||Fluka||Tryptic Soy Agar|
|14432||Fluka||Tryptic Soy Agar, Vegitone|
|51414||Fluka||Tryptic Soya Agar with Polysorbate 80 and Lecithin|
|70159||Fluka||Tryptone Glucose Extract Agar|
|Non-selective Agars for Differentiation|
|70133||Fluka||Blood Agar (Base)|
|B1676||Sigma||Blood Agar Base No. 2|
|70136||Fluka||Deoxyribonuclease Test Agar|
|D2560||Sigma||DNase Test Agar with Toluidine Blue|
|17153||Fluka||LS Differential Agar|
|Selective Agars with Differential System for Differentiation, Detection and Isolation|
|06105||Fluka||Bile Esculin Azide Agar|
|17151||Fluka||Kanamycin Esculin Azide Agar|
|01337||Fluka||Mitis Salivarius Agar|
|Media for Sensitivity Testing|
|97580||Fluka||Mueller-Hinton Agar 2|
In alpha hemolysis, the red blood cells remain intact, but the hemoglobin is converted to biliverdin. This causes a greening of the blood agar plate around the colonies.
S. pneumoniae: Causes bacterial pneumonia, otitis media and meningitis. S. pneumoniae sensitivity to optochin (ethyl hydrocupreine hydrochloride) is the basis of a diagnostic test (Figure 1, Table 2).
Figure 1 ......... Differentiation and identification flow chart of Gram-positive cocci
Table 2 ......... Diagnostic tests for identification and differentiation of streptococci Cat. No. Brand Description 88597 Fluka Catalase Test 08382 Fluka Bacitracin Disks 74042 Fluka Optochin Disks 67886 Fluka PYRase Strips
- Viridans and Others
S. mutans: Associated with dental caries.
S. viridans: Causative agent for endocarditis and dental abscesses.
S. salivarius: Considered to be an opportunistic pathogen.
S. salivarius subsp. thermophilus: Used in the production of cheese and yogurt.
Beta hemolysis is a true hemolysis of erythrocytes by the enzyme hemolysin. Clear zones will appear around the colonies on the blood agar plate. Beta-hemolytic streptococci are further divided into serological groups using specific antibodies that recognize surface carbohydrate antigens (e.g. A, B, etc.).
- Group A
S. pyogenes: Causes infections like strep throat, acute rheumatic fever, scarlet fever, acute glomerulonephritis and necrotizing fasciitis. It is sensitive to bacitracin (Fig. 1), a fact that is employed in a Streptococcus diagnostic test (Table 2). Other Streptococci species may also possess the Group A antigen but are not human pathogens. These include S. dysgalactiae subsp. equisimilis and S. anginosus strains.
- Group B
S. agalactiae: Causes meningitis in neonates and the elderly. Occasionally colonizing the female reproductive tract, they increase the risk for premature rupture of membranes and transmission of the infection to the child.
- Group C
S. equi: Causes strangles infection in horses.
S. zooepidemicus: Causes infections in cattle, horses and other mammals.
- Group D (Enterococci)
Many former Group D streptococci have been reclassified to the genus Enterococcus (e.g. S. faecalis, S. faciem, S. durans, S avium).
S. bovis and S. suis: Still in the Group D streptococci.
Gamma hemolysis is a misnomer as there is actually no hemolysis. Non-hemolytic or gamma-hemolytic streptococci rarely cause disease.
|Table 3 ......... Gram staining kit and component solutions|
|77730||Fluka||Gram Staining Kit|
|94448||Fluka||Gram‘s Crystal violet Solution|
|75482||Fluka||Gram‘s Decolorizer Solution|
|87794||Fluka||Gram’s Fuchsin Solution|
|90107||Fluka||Gram‘s Iodine Solution|
|94635||Fluka||Gram‘s Safranin Solution|