Streptococcus is a genus of coccus gram positive bacteria. They typically grow in chains of pairs as cell division occurs along a single axis in this family of bacteria. Most species in the streptococcus genus are oxidase negative, catalase negative, and most are facultative anaerobes. Many streptococcal species are not intact pathogenic and are part of the commensal human microbiota of the skin, mouth, intestine and upper respiratory tract. However, certain streptococcus species are the pathogenic agent for many cases of pink eye, meningitis, bacterial pneumonia, endocarditis, erysipelas, and necrotizing fasciitis.
Species of Streptococcus are classified based on their hemolytic properties. Alpha-hemolytic species cause oxidation of the iron within the red cells, giving a greenish color to appear on the blood agar. The bacteria produces hydrogen peroxide which oxidizes the hemoglobin to biliverdin. This is also referred to as incomplete hemolysis or partial hemolysis. Beta-hemolytic streptococci cause complete degradation of the red cells, appearing as wide clear areas surrounding the bacterial colonies. Gamma-hemolytic species cause no hemolysis.
The beta-hemolytic species are further classified by the Lancefield grouping. This is a serotype classification that describes the specific carbohydrate present on the bacterial cell wall. There are serotypes for Lancefield groups A-V. For times sake, the only ones that will be discussed are Group A, and Group B.
Strep pneumoniae, often referred to as pneumococcus is the leading cause of bacterial pneumonia. It can also be the etiological agent for otitis media, sinusitis, meningitis and peritonitis. The viridian’s group of alpha-hemolytic streptococci are a large group of commensal organisms. They possess no Lancefield antigens (carbohydrates) and can or can not be hemolytic.
Streptolysin which is an exotoxin is the enzyme produced by the strep species that causes complete hemolysis of the red cells in the media. There are two types of streptolysin; Streptolysin O (SLO) and Streptolysin S (SLS). Streptolysin O is an oxygen-sensitive (labile) cytotoxin which is secreted by most of the Group A Streptococcus (GAS) which interacts with cholesterol in the membrane of the red cells. Streptolysin S is an oxygen-stable cytotoxin also produced by GAS species that affects the innate immune system of the host. It works in preventing the host immune system from clearing the infection.
Group A Strep
Group A strep, otherwise known as S. pyogenes is the causative agent for strep infections, invasive and non-invasive. The most common infection is pharyngitis, otherwise known as strep throat, impetigo, and scarlet fever. All these infections are non-invasive. Invasive GAS infections include necrotizing fasciitis, pneumonia, and bacteremia (bacteria present in the blood). Complications can arise from GAS infections. Rheumatic fever is a disease that affects the joints, kidneys, and the heart valves. It is the consequence of an untreated strep A infection. Antibodies created by the immune system cross-react with proteins in the body which causes an immune-mediated attack on the hosts own cells. Essentially an acquired auto-immune disease. GAS is also implicated in pediatric autoimmune neuropsychiatric disorders associated with streptococcal infections (PANDAS). Autoimmune antibodies affect the basal ganglia causing rapid onset of psychiatric, motor, sleep and other neurological symptoms, primarily in the pediatric population.
GAS is diagnosed by either an rapid strep test or by culture.
Group B Strep
GBS, otherwise known as S. agalactiae, causes pneumonia and more importantly meningitis in neonates and the elderly. The American Congress of Obstetricians and Gynecologists now recommends that all women between 35 and 37 weeks gestation to be tested for group B strep. Those who test positive should be given prophylactic antibiotics during labor. The bacteria can cause premature rupture of the membranes during pregnancy and can colonize and cross the placenta to infect the fetus.
Laboratory diagnosis and Workup
After initial culture and preliminary identification of beta-hemolytic streptococcus is suspected there are further identification tests to make an accurate diagnosis. The first is by Lancefield antigen determination. Commercially available Lancefield antisera is used for the differentiation of beta-hemolytic species. These usually come as small kits and are directed to Lancefield groups A, B, C, F, and G. Antigen detection is demonstrated by agglutination by specific antibodies that are provided.
The PYR test is a rapid colormetric method most often used to distinguish S. pyogenes from other beta-hemolytic species. The PYR tests for the presence of the enzyme pyrrolidonyl aminopeptidase. This enzyme hydrolyzes L-pyrrolidonyl-b-naphthylamide (PYR) to B-naphthylamide, which produces a red color when a specific reagent is added. The test can be performed on paper strips that contain dried chromogenic substrates for the pyrrolidonyl aminopeptidase. S. pyogenes is PYR positive and in the case of an unknown organism displaying S. pyogenes morphology plus being PYR positive, it is acceptable to presumptively identify as S. pyogenes.
Bacitracin susceptibility is another test that can be used to differentiate S. pyogenes from other beta-hemolytic strep species. S. pyogenes has an increased susceptibility to bacitracin. A pure culture must be obtained and streaked on a sheep blood agar plate. A small disk containing 0.04 U of bacitracin is placed on the plate and incubated overnight at 35 degrees celsius in 5% CO2. A zone of inhibition surrounding the disk indicates susceptibility.
With the introduction and advancement of nucleic acid detection and serological methods in the laboratory it is now getting easier and faster to detect strep species without relying on culturing and further confirmation testing. It should be important to note that this does not replace the use of a culture or any of the further testing mentioned above. Serological testing relies on antibodies to anti-streptolysin O and anti-DNase B. The antibody levels against streptolysin O rise within one week of infection and peak around 3-6 weeks. DNase B is a nuclease among many that S. pyogenes uses to escape neutrophil extracellular nets. DNase B is specific for S. pyogenes. The antibody levels to DNase B rise post two weeks infection and reach maximum titers at around 6-8 weeks.
The Optochin test is used to differentiate alpha-hemolytic streptococci. This is either S. pneumoniae or strep viridians. S. pneumoniae species are sensitive to the chemical ethylhydrocupreine hydrochloride, otherwise known as optochin. Optochin disks, sometimes called P disks can be obtained from a commercial vendor. A pure culture is obtain and incubated to allow growth overnight. When growth has occurred, a subculture is plated and a P disk is placed on the agar and incubated overnight at 35 degrees celsius with 5% CO2. The zone of inhibition is then measured and a zone greater than 14 mm indicates sensitivity and allows for the presumptive diagnosis of pneumococci.
The bile solubility test is usually used as confirmatory test for S. pneumoniae that distinguishes it from other alpha-hemolytic species. Sodium deoxycholate (Bile) will lyse the cell wall of pneumococcal species.