Description of Organism
Group A streptococcus, also known as Streptococcus pyogenes is a β-hemolytic streptococcus. Through the use of gram staining we are able to determine that Streptococcus pyogenes is gram positive thus it has a thick peptidoglycan layer containing teichoic acids and stains purple.1,3 Streptococcus pyogenes is capable of growing by utilizing the oxygen in the environment, and when oxygen is absent or not available Streptococcus pyogenes can grow by the means of fermentation or anaerobic respiration.1,3 When bacteria are able to grow with or without the presence of oxygen, they are classified as facultative anaerobes therefore S. pyogenes is considered to be a facultative anaerobe.1,3 The cell shape is spherical and often exists in pairs or chains.1 Some interesting characteristics about S. pyogenes is that it is non-motile and non-spore forming.1 This means that S. pyogenes is not capable of motion and does not produce spores. It also has picky or specific nutritional needs and requires a complex media containing blood or serum in order to be cultured.1 S. pyogenes is transmitted through direct contact from person-to-person. It can also be transmitted by droplet transmission, such as through sneezes, coughs, and even through sharing meals and drinks.1 Some diseases caused by S. pyogenes are Streptococcal pharyngitis and Scarlet Fever and will discussed further below.
Virulence Factors & Mechanism of Pathogenicity
What are virulence factors? Virulence factors are the tools or ways in which a microorganism is able to cause disease, in other words virulence factors are like the weapons and defense that aid the microorganism to cause harm. A microorganism is only pathogenic when it has its virulence factors.3
Capsules help microorganisms to evade from host defenses. S. pyogenes has a hyaluronic capsule which aids in preventing it from being engulfed by the host defenses.1 The host (human body) has polymorphonuclear neutrophils (PMNs) and macrophages which act to protect the host from any foreign microorganisms that enter by means of phagocytosis.1 However, since S. pyogenes has a hyaluronic acid capsule this enables it to pass these defenses and prevents it from being engulfed by both the macrophages and the polymorphonuclear neutrophils.1
The cell wall of the S.pyogenes is composed of a variety of antigenic substances and the components of the cell wall are important virulence factors which aid S. pyogenes to bypass or attack host cells. The thick peptidoglycan layer contributes to the cell wall’s rigidness thus strengthening the cell wall.1 The M proteins are vital virulence factors which allow the S. pyogenes to become resistant to phagocytosis by the PMNs and can bind to a broad selection of host proteins.1 These heat resistant and acid resistant proteins inhibit phagocytosis by permitting the S. pyogenes to penetrate and attach to the epithelial cells thus hindering phagocytes from attaching.3 As a result there is an antiphagocytic effect because of the non-activation of other alternate complement pathways due to the presence of the M protein.1 In order for the S. pyogenes to be virulent they must have M proteins present.1 Surface proteins such as Mac which is secreted and lipoteichoic acid also play a role in preventing phagocytosis. When Mac is secreted it impedes phagocytosis by attaching to CD16 that is on the surface of the PMNs.1 Lipoteichoic acid assists in binding S. pyogenes to the host cell by guiding the bacteria into close proximity with the host cells (by the means of hydrophobic forces) and acts as the first step of adhesin. Allowing other additional adhesins after, encourages further binding (high-affinity binding).1
Erythrogenic toxins released by the S. pyogenes are superantigens which cause the rash we see in Scarlet fever.3 Streptolysins are also virulence factors. For example, Streptolysin O has the ability to damage a variety of cells such as PMNs, lysosomes, and tissue cells because it is toxic to these cells.1
Disease & Disease Symptoms
Have you ever experienced a sore throat along with a general discomfort, and feverishness? Then you may have had streptococcal pharyngitis! Streptococcal pharyngitis is the most common bacterial infection in childhood, and occurs predominantly among 5-15 year old children.1 Bacteria accumulates on the palatine tonsil which is a special lymphoid tissue composed of a variety of cell types that play a role in our innate immune defense such as neutrophils, macrophages and epithelial cells.2 This accumulation leads to a response, in which the cell types of the palatine tonsils recognize the bacterial accumulation and secrete a variety of inflammatory mediators such as cytokines, AMPs, chemokines, and PGs.2 The incubation period of this disease is 2-4 days and the first signs of illness are characterized by a sore throat, fever, general fatigue, and headaches.1 Children typically experience nausea, vomiting and abdominal pain when they have streptococcal pharyngitis. The severity of the disease varies, but majority of cases are mild and self-limiting usually going away within a week.2 Notable findings include: redness, edema (swelling), and lymphoid hyperplasia of the pharynx (posterior region) along with enlarged, inflamed tonsils, discrete non-uniform tonsillopharyngeal exudates, and enlarged submandibular lymph nodes.1 Fever or temperatures of 38.3° C and higher are also symptoms of streptococcal pharyngitis.1
Infection with streptococcal strains which produce a superantigen toxin (erthrogenic/pyrogenic exotoxins) can lead to Scarlet fever.1 Scarlet fever is characterized by a skin rash which appears within the first few days of streptococcal infection and spreads from the trunk to extremities.2 It appears as a red diffuse patch with many deeper red spots that turn white upon pressure.1 The red blush is first on the chest, and then disperses to the rest of the trunk and neck and then to our legs and arms.1 These red rashes are not seen on the palms of our hands or the soles of our feet and on our face.1 These rashes are sandpaper-like and those with scarlet fever have high fevers which vary in severity.3 Because Scarlet fever can be present during Streptococcal pharyngitis, we see puss/fluid filled exudative pharyngitis and tonsillitis and often see red spots on both the hard and soft palate.1 Our tongue is covered by a yellow-white film or coat and when this coating is shed our tongue looks like a red strawberry due to inflammation.1
Disease Complications
Acute rheumatic fever and rheumatic heart disease are serious autoimmune diseases that have the potential to develop following streptococcal pharyngitis.2 Acute rheumatic fever is a non-infectious complication of infection and clinical features related to acute rheumatic fever include arthritis (inflammation of joints), carditis and of course fever.2 Following acute rheumatic fever, a person can develop rheumatic heart disease and this is when there is damage to the valves of the heart as a result of having rheumatic fever.2
Other local infections or complications can occur near the original site of infection (pharynx) such as peritonsillar cellulitis, otitis media, and suppurative cervical lymphadenitis.1 If infection extends up the cribiform plate of the ethmoid it can lead to meningitis, brain abscess and other complications.1
Antimicrobials/Treatment
The drug of choice is penicillin due to its ability to prevent rheumatic fever, narrow spectrum, low cost and it is safe to use.1 Penicillin does this by inhibiting the synthesis of the cell wall of S. pyogenes by obstructing the ability of the bacteria to synthesize peptidoglycan therefore the cell walls of the bacteria weaken and burst leadings to cell death by osmotic pressure.3 You can be given penicillin V which is taken through the oral route for ten days for two or three times daily Amoxicillin can be taken orally for ten days.1 A single injection of penicillin G benzathine may also be given and is only injected once.1 For those allergic to penicillin they could orally take azithromycin for 5 days once daily.1 An important thing to note is that those who have immediate (Type I) hypersensitivity to penicillin should not be given narrow spectrum cephalosporin as they may have increase risk for allergic reactions to cephalosporins.1
Vaccine/Antisera
Currently there are no vaccines available against streptococcal pharyngitis and greater research and development could lead to potential vaccines in the future.2