Introduction:
Streptococcus agalactiae (group B streptococcus) refers to a gram-positive bacteria that possess the tendency of forming chains. Mostly, it is beta-hemolytic and also exists as a facultative anaerobe. The most prevalent human pathogen is S. agalactiae, and it belongs to group B of the Rebecca Lancefield classification. Group B streptococcus possess bacterial capsules made up of polysaccharides. The species classifications are divided into ten serotypes. However, the division is dependent on the immunologic reactivity contained in the polysaccharide capsule. Generally, GBS is a harmless commensal bacteria and is a significant part of the human micro-biodata (Raabe & Shane, 2019). It mainly colonizes the genitourinary tract of about thirty percent of healthy humans. For newborns, it might result in various diseases like severe invasive infections. In older adults, it might cause malfunctioned immune systems. Besides, it acts as a veterinary pathogen since it may lead to mastitis, especially in dairy cows.
Description of the Microorganism:
S. agalactiae is a bacterium. Its cells are spherical with varied sizes of between 0.6-1.2 micrometers. An electron microscope may be used to reveal more refined details. Not all the species are spherical because others may develop rod-like cells. That depends mainly on the growth condition. The cells arrangement is a characteristic of all Streptococci since the cells are primarily arranged in chains (Bush, 2021). The chains frequently occur in pairs or, in some cases, very extensive chains. The sizes of those chains vary and might be longer if the studied bacteria originate from a fluid culture. During cell division, the cross-walls are oriented in the desired angles of the chain. Immediately after cell division, pairing appearance might remain. The organism is surrounded by the polysaccharide capsule, which is covalently bonded to the cell wall peptidoglycan. It, therefore, creates a mucoid capsule layer that is used in covering the surface of the bacteria. The S. agalactiae cell wall is made up of peptidoglycan, other structures called the teichoic acids and, other proteins (Raabe & Shane, 2019). The cell membrane has a lipid-protein bilayer which is significant in transporting diverse molecules inside and outside the cells by utilizing different transport channels.
Virulence Factors:
The infection of S. agalacticiae occurs depending on cell types, including the epithelial cells, endothelial cells, and macrophages. In helping to overcome such defensive attributes and hide in the host, any organism has to survive many different virulent factors. The virulence factors enable the microorganism to enter the host and protect the microorganism against the host’s immune activities (Bush, 2021). S. agalactie possesses several such mechanisms or factors that permit it to cause different infection types.
First, its capsule serves as the significant virulent factor—the capsular layer composed of polysaccharide act as a medium for serotyping in laboratory referencing. The capsular layer comprises several other repeating units of the monosaccharide galactose. Salinic acid is a vital component of the capsule. It primarily inhibits different alternative pathways by preventing the deposition of the active C3 complement found on the surface of GBS.
Secondly, it contains lipoteichoic acid that contains glycerol phosphate, a vital component present among most Gram-positive Bactria. Diverse functioning in bacteria is categorically associated with the polymer. One of such functions involves facilitating adherence found between the Gram-positive bacteria and eukaryotic cells. The lipoteichoic cells have also been proven to bind in the cell membrane of most epithelial cells and the erythrocytes. However, the occurrence or the binding of LTA to human fetal always occurs in a distinct two-way process (Raabe & Shane, 2019. The first process is hydrophobic interaction, while the other involves the interaction between the embryonic eukaryotic surfaces with glycerolphosphate backbone.
The third virulence factor is hyaluronate lyase, which effectively degrading hyaluronic acid. The importance of hyaluronate lyase is to play the role of a spreading factor to destroy the common connective tissues in the host structure then promote the dissemination of bacteria.
Immunity:
S. agalactiae has protection from innate immunity attacks because they have several oligosaccharides repeating units. The perfect methodology to counteract that is to release specific antibodies specifically designed to target the PS through vaccination. Mostly, the immune response triggered due to PS is mostly nonoptimal and has chemical conjunction to a carrier protein. The carrier protein facilitates the engagement of the T-helper cells to assist in the development of the immune response.
It is a massive challenge for the body to fight against S. agalactiae. The bacteria is highly adaptable to humans because it is a constituent of both the virginal and intestinal flora. It can evade the immune system, and along with the invasive potential, it may cause serious inflammation, sepsis, or even death, especially in the elderly, diabetic patients, and infants. Therefore, the bacterium has the immense capabilities to stay in the healthy host and evolve along with it. While it stays there, it holds its virulence intact. S. agalactiae, as a vital member of GBS, thus produces a diverse range of molecules that slowly permits them to break down the immune system of an individual. One vital molecule in that process is the hemolytic pigment, also known as granadeane. It kills the red cell and several others found in the immune system. This kind of toxin also helps the bacteria aggravate the defensive barrier in the human body, like the blood-brain and the placenta barrier.
Infectious disease information:
Streptococcus agalactiae is known for causing diverse bacterial diseases like meningitis, especially in the neonatal period. It does so by breaching the blood-brain barrier and subsequently invades the Central Nervous System through trans-cellular mechanism approaches. The infectious agents affect different parts of the body, including the throat, sinuses, heart valves, and bloodstream. It causes symptoms including swollen tissues, sore throat, and even rashes. However, the severity of the symptoms depends on the specific area attacked by the bacteria. If such symptoms fail to be treated, they often result in different kinds of complications (Raabe & Shane, 2019). Some of the complications occur due to the infection to the nearby tissues. For example, an ear infection may easily cause sinusitis. Others may develop inflammation of the kidney or even rheumatoid fever. The symptoms could be chronic for people who already have underlying medical conditions. Lastly, it is an opportunistic pathogen that colonizes typically healthy adults asymptomatically. Frequently, it inhabits the vagina, and when transmitted to the newborn, it may even cause severe consequences.
Epidemiology:
Fig 1.0 Show the Schematic representation of sources and transmission routes for the S. agalactiae
The sources are represented in boxes, while the transmission routes are indicated in arrows. The origins and transmission routes include the environment, cattle, and humans. The full arrow representations are sourced from scientific evidence, while any scientific research does not support the dashed arrows. The call-outs have listed the mode of transmission alongside supporting evidence. Intra-species transmission is highlighted in red, while inter-species transmission is represented in blue. The environmental transmission modes are in green.
The known reservoir for the infection of S. agalactiae in the dairy farm is the milk sourced from the infected mammary origins. The bacteria may be prevalent on surfaces with the contaminated milk, including the farmer’s hand and the contaminated milk (Shabayek & Spellerberg, 2018). The transmission type is mainly vertical, involving utero passage or birth canal routes. The bacteria’s portal of entry are the same they used to exist; the mucous membrane, the skin, and the gastrointestinal tracts.
Prevention:
Antibiotics like penicillin can prevent group B bacteria from spreading to the baby, especially during labor or delivery. The doctor can give permit it when the labor begins.
There is an increased use of intravenous intrapartum antibiotic prophylaxis to minimize or prevent the early onset of GBS, especially among infants. The studies were conducted in the 80s, continuing until now (Raabe & Shane, 2019). Penicillin in the initial agent involved intrapartum prophylaxis. Ampicillin could be used as an alternative as well.
Another strategic approach that is still undergoing development to limit the occurrence of GBS disease is vaccination of mothers. That happens in the trimester against S. agalactiae. For pregnant mothers, group B strep screening is necessary. The screening should take place during the 36 or 37th week of pregnancy.
Treatment:
Penicillin G remains the first line of treatment for GBS disease for older people. The duration of the therapy always depends on the clinical representation. For bacteria, ten days of the therapeutic session may be enough.
When exposed to bête-lactam antibiotics, S. agalactiae immediately becomes susceptible. Such antibiotics may include ampicillin and carbapenems. The activity levels vary considerably, depending on the agents to which the bacteria is exposed. For patients with allergies, options are available like erythromycin, fluoroquinolones, and clindamycin. However, a longer treatment duration is needed for meningitis, which may take up to 14 days.
Clinical Relevance:
A multi-drug resistant strain refers to the particular strain that has shown resistance to two or even more antimicrobial drugs. MDR and PEN –NS strains of GBS have since emerged. One of the strains emerged from the patient (H21) and (F49) strains from isolates of the tilapia fish. The effect has since limited the effective treatment of all the patients as the increased resistance has become a threat to public health globally (Li et al., 2021). The principal patients or victims are pregnant mothers as well as neonates. There is a need to revisit the CLSI guidelines to test PEN meningitis and PEN MICs breakpoints using Streptococcus pneumonia as a reference point. The rationale is that the highlighted strains have become increasingly identifiable from meningitis and sepsis patients. However, new antibiotics have been developed to help combat the effects of the MDR strains. Such antibiotics include fourth-generation fluoroquinolones, tetracycline derivatives, and siderophore cephalosporins.
Conclusion:
Streptococcus agilactiae is a Gram-positive organism. It is recognized globally as one of the leading causes of invasive disease in young infants and neonates. Among the infections, it causes diseases such as meningitis and sepsis. The use of penicillin G remains its first line of treatment. Mainly, it is transmitted vertically. Once inside the host cell, it might compromise the cellular infrastructure due to the polysaccharide capsule, making it very hard for the body to fight against it.
References:
Bush, L. (2021). Streptococcal Infections. Retrieved from: https://www.merckmanuals.com/home/infections/bacterial-infections-gram-positive-bacteria/streptococcal-infections
Li, C., et al. (2021). Multidrug-Resistant Streptococcus agalacticiae Strains Found in Human and Fish with High Penicillin and Cefotaxime Non-Susceptibilities. Microorganisms, 8 (7):1055.
Raabe, V. N., & Shane, A.L. (2019). Group B Streptococcus (Streptococcus agalactiae). Microbiology Spectrum, 7, (2), 10.1128/microbiolspec.GPP3=0007-2018
Shabayek, S., & Spellerberg, B. (2018). Group B Streptococcal Colonization, Molecular Characteristics, and Epidemiology. Frontiers in microbiology, 9,437.
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