Group A B-hemolytic streptococcus: The culprit of strep throat

Strep throat vs a sore throat

When researching strep throat, the term pharyngitis comes up quite a bit and some articles even use “strep throat” and “pharyngitis” interchangeably. To start, pharyngitis is simply inflammation of the pharynx, otherwise known as a sore throat. Pharyngitis can be caused by a variety of factors, including both viral and bacterial infections. Strep throat is the result of a bacterial infection, which in turn can cause a sore throat.

Strep throat: Streptococcal bacterial infection

Strep throat is caused by the bacteria Group A b-hemolytic streptococcus (GABHS or GAS). The bacteria are transmitted through respiratory droplets or nasal secretions and its only transmitted from human to human. It is most common in children, with 15-35% of children presenting with sore throat being diagnosed with strep throat. In adults, 5-15% of patients with sore throat have strep throat. Due to the similarities in the clinical manifestations of GABHS infection with other infections, like viral infections, it is important to properly diagnose GABHS infection to prevent unnecessary use of antibiotics.

Immune response to GABHS infection

Figure 1. Immune response against Streptococcus infection.

Interestingly, bacterial that reside in the respiratory tracts also act as a protective layer to GABHS infection. Not only do commensal bacteria compete for nutrients against invading GABHS bacteria, but they also produce bactericidal components to limit GABHS invasion. As for the immune response against GABHS infection, it appears both the innate and adaptive immune responses play a role. The innate immune response will be at the forefront, with epithelial cells in the pharynx recognizing GABHS bacteria as foreign and inducing a response in which cytokines, which are proteins that can promote migration cells, can recruit immune cells to site of infection. Innate immune cells are involved in killing and clearing of the bacteria (Figure 1).

Other pathologies of GABHS infection

Complications to GABHS infection can occur, with 2 different types of manifestation possible being 1) suppurative and 2) non-suppurative complications. Currently, the best method to prevent complications is to treat the infection early, likely at the stage at which pharyngitis is the primary manifestation of the infection.

1)    Suppurative complications is a result to the bacterial infection spreading to other site. GABHS infection manifested as strep throat results in infection of the pharynx. During suppurative complication, patients may experience inflammation of the lymph nodes, abscess formation by the tonsils, or more alarming abscess formation in the throat which may block the airway. Additionally, patients may experience infection spreading to the ear or the bone behind the ear, cause ear pain, often seen as a complication in children. While these complications can be prevented if GABHS is treated promptly, when the infection goes unnoticed it is more likely to cause suppurative complications.

2)    Non-suppurative complications are not a result of the infection spreading, but rather occur after the infection is cleared and are primarily promoted by the immune response resulting from the infection. Rheumatic fever is one of the most reported non-suppurative complication, which manifests itself by inflammation of peripheral tissues causing severe inflammation of the joints. Additionally, inflammation of the kidneys can be observed in patients, as well as Sydenham chorea, which results in involuntary and uncontrollable movements.

Treatment

GABHS falls in the class of gram-positive bacteria, most of which are resistant to penicillin. However, GABHs has not evolved to develop penicillin resistance, and remains sensitive to other b-lactam antibiotics. For patients with penicillin allergy, which then increases the risk of cross reactivity to ither b-lactam antibiotics, alternative antibiotics, such as clindamycin, are used. Alarmingly, GABHS has been showing resistance to alternative antibiotics, creating complications for treatment in patients in with penicillin allergies. Currently, there is exploration in vaccine development as an alternative therapy, however, this has not been approved yet.

References:

1)    Thacharodi, A., Hassan, S., Vithlani, A., Ahmed, T., Kavish, S., Geli Blacknell, N. M., Alqahtani, A., & Pugazhendhi, A. (2024). The burden of group A Streptococcus (GAS) infections: The challenge continues in the twenty-first century. iScience, 28(1), 111677. https://doi.org/10.1016/j.isci.2024.111677

2)    Regoli, M., Chiappini, E., Bonsignori, F., Galli, L., & de Martino, M. (2011). Update on the management of acute pharyngitis in children. Italian journal of pediatrics, 37, 10. https://doi.org/10.1186/1824-7288-37-10

3)    Mustafa, Z., & Ghaffari, M. (2020). Diagnostic Methods, Clinical Guidelines, and Antibiotic Treatment for Group A Streptococcal Pharyngitis: A Narrative Review. Frontiers in cellular and infection microbiology, 10, 563627. https://doi.org/10.3389/fcimb.2020.563627

4)    Mustafa, Z., & Ghaffari, M. (2020). Diagnostic Methods, Clinical Guidelines, and Antibiotic Treatment for Group A Streptococcal Pharyngitis: A Narrative Review. Frontiers in cellular and infection microbiology, 10, 563627. https://doi.org/10.3389/fcimb.2020.563627

5)    Soderholm, A. T., Barnett, T. C., Sweet, M. J., & Walker, M. J. (2018). Group A streptococcal pharyngitis: Immune responses involved in bacterial clearance and GAS-associated immunopathologies. Journal of leukocyte biology, 103(2), 193–213. https://doi.org/10.1189/jlb.4MR0617-227RR

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