Streptococci: Gram Stain, Introduction, Pathogenecity and Differential Lab Diagnosis
Streptococci in Gram
Streptococci in gram stain showing gram-positive cocci in chains as shown above image. The possible organisms may be Streptococcus pyogenes, Streptococcus agalactiae, Enterococcus species, Streptococcus bovis, viridans streptococci.
Introduction of Streptococci
Streptococci are Gram-positive, non-motile, non-spore-forming, catalase test negative cocci that occur in pairs or chains.
Older cultures may lose their Gram-positive properties. Most of them are facultative anaerobes, and some are strict anaerobes. Most require enriched media i.e. blood agar. They are subdivided into groups by antibodies that recognize surface antigens. These groups may include one or more species. Serologic grouping is based on antigenic differences in cell wall carbohydrates (groups A – V), in cell wall pili-associated protein, and in the polysaccharide capsule in group B streptococci (GBS). Rebecca Lancefield developed the serologic classification scheme in 1933. β-hemolytic strains possess group-specific cell wall antigens, most of which are carbohydrates. These antigens can be detected by immunologic tests and have been useful for the rapid identification of some important streptococcal pathogens. The most important groups of streptococci are A, B, and D. Pharyngitis is caused by group A and they have a hyaluronic acid capsule. S. pneumoniae causes of pneumonia and S. mutans and other so-called viridans streptococci (among the causes of dental caries) do not possess group antigen. S. pneumoniae has a polysaccharide capsule that acts as a virulence factor for the organism. More than 90 different serotypes are known, and all these types differ in virulence.
Pathogenicity of Streptococci
Group A streptococci (S. pyogenes) are responsible for-
Strep throat – a sore, red throat, sometimes with white spots on the tonsils
Scarlet fever – an illness that follows strep throat. It causes a red rash on the body.
Impetigo – a skin infection
Toxic shock syndrome
Cellulitis and necrotizing fasciitis (flesh-eating disease)
Rheumatic fever is a non-suppurative complication of S. pyogenes pharyngitis.
Rheumatic fever is an inflammatory disease affecting primarily the heart and joints. Although severe, it can take an extended period of time to develop.
Group B streptococci (Streptococcus agalactiae)
It can cause blood infections, pneumonia, and meningitis in newborns. Adults can also get group B strep infections, especially if they are elderly or already have health problems. It can cause urinary tract infections, blood infections, skin infections, and pneumonia in adults.
Group D streptococci (Enterococcus) –
Enterococci are distantly related to other streptococci and have been moved into the genus Enterococcus; the most commonly isolated are Enterococcus faecalis and Enterococcus faecium. E. faecalis can cause nosocomial infections, urinary tract infections, bacteremia, endocarditis, meningitis, and can be found in wound infections along with many other bacteria.
It causes pneumonia, acute sinusitis, otitis media, meningitis, bacteremia, sepsis, osteomyelitis, septic arthritis, endocarditis, peritonitis, pericarditis, cellulitis, and brain abscess. It is the most common cause of bacterial meningitis in adults and children and is one of the top two isolates found an ear infection, otitis media.
Differential Laboratory Diagnosis of Streptococci
Streptococci on blood agar
Most species of streptococci are facultative anaerobes. Some grow only in an atmosphere enhanced by carbon dioxide. The classification of species within the genus is complicated because three different schemes are used: Hemolytic patterns:
incomplete (α) hemolysis: Streptococcus pneumoniae, Streptococcus salivarius, viridans are referred to collectively as viridans streptococci, a name derived from Viridis (Latin for “green”), referring to the green pigment formed by the partial, α-hemolysis of blood agar. Encapsulated, virulent strains of S. pneumoniae often forming highly mucoid, glistening colonies (production of capsular polysaccharide) surrounded by a zone of α -hemolysis.
and no hemolysis (γ):Enterococcus faecalis
Bacitracin test (0.04 U) for Streptococcus pyogenes
The bacitracin test is used to determine the effect of a small amount of bacitracin on an organism. S.pyogenes is inhibited by the small amount of bacitracin in the disk (visible zone of inhibition of growth; other beta-hemolytic streptococci usually are not.
CAMP test for Streptococcus agalactiae
CAMP test showing the arrow-shaped zone of enhanced hemolysis (positive) of S. agalactiae and the negative result of S.pyogenes when tested against Staphylococcus aureus.
Biochemical (physiologic) properties
Optiochin test for S. pneumoniae
Optochin i.e. ethylhydrocupreine test is a chemical used in cell culture techniques for S. pneumoniae, which is optochin-sensitive (positive ), from other alpha-hemolytic streptococci such as Streptococcus viridans which are resistant. Optochin Differentiation Disks are recommended for use in the presumptive identification of S. pneumoniae from other alpha-hemolytic streptococci.
Bile solubility test for S. pneumoniae
The bile (sodium deoxycholate) solubility test distinguishes S. pneumoniae from all other alpha-hemolytic (viridans) streptococci. It is bile soluble whereas all other alpha-hemolytic streptococci are bile resistant.