Streptococcus agalactiae: Introduction, Morphology, Pathogenicity

Introduction

Table of Contents

Streptococcus agalactiae, also known as Group B Streptococcus (GBS), is a Gram-positive bacterium that belongs to the Streptococcaceae family. It is a significant human pathogen and is commonly found in the gastrointestinal and genitourinary tracts of healthy adults. While it may not cause any harm in most healthy individuals, it can be a major concern for certain vulnerable populations, such as newborns and pregnant women.

GBS is a leading cause of infections in neonates, commonly known as early-onset GBS disease, where the bacterium is transmitted from the mother to the baby during childbirth. It can result in serious complications such as sepsis, pneumonia, and meningitis in newborns.

In addition to neonatal infections, GBS can also cause infections in adults, particularly those with compromised immune systems or underlying health conditions. It can lead to urinary tract infections (UTIs), skin and soft tissue infections, and bloodstream infections in these individuals.

The diagnosis of GBS is typically made by isolating the bacterium from clinical specimens like blood, cerebrospinal fluid, or urine. To prevent transmission to newborns, pregnant women are routinely screened for GBS during late pregnancy, and if positive, they are given intrapartum antibiotics to reduce the risk of transmitting the bacterium to the baby.

Antibiotics such as penicillin and ampicillin are commonly used to treat GBS infections. However, the emergence of antibiotic resistance is a growing concern, emphasizing the need for prudent use of antibiotics and continued research into alternative treatments and prevention strategies.

Morphology

Streptococcus agalactiae exhibits a characteristic morphology typical of streptococcal bacteria. Here’s a description of its morphology:

Shape: GBS is a Gram-positive bacterium, meaning it retains the crystal violet stain during Gram staining, appearing purple under the microscope. It is classified as a cocci bacterium, which means it has a spherical or round shape.
Arrangement: Streptococcus agalactiae typically occurs in chains or pairs. The cells are connected in a linear fashion, resembling a string of beads, which is a defining feature of streptococci.
Size: The size of individual S. agalactiae cells ranges from approximately 0.6 to 1.0 micrometers in diameter. These sizes can vary slightly depending on growth conditions and the strain of the bacterium.
Capsule: One significant feature of Streptococcus agalactiae is the presence of a polysaccharide capsule around the bacterial cell. The capsule is an essential virulence factor that helps the bacterium evade the host’s immune system and contributes to its pathogenicity.
Hemolysis: GBS can exhibit different types of hemolysis when grown on blood agar. Most strains of Streptococcus agalactiae show beta-hemolysis, which means they completely lyse red blood cells in the medium, resulting in a clear zone around the colonies. This characteristic helps in its identification in the laboratory.
Growth characteristics: Streptococcus agalactiae is facultatively anaerobic, which means it can grow both in the presence and absence of oxygen. It prefers a temperature range of 35-37°C, which corresponds to human body temperature, making it well-adapted to infect humans.

Pathogenicity

Streptococcus agalactiae is a pathogenic bacterium that can cause a range of infections in different populations. Its pathogenicity is attributed to several virulence factors that allow it to evade the host’s immune system and colonize various body sites. Here are some key aspects of the pathogenicity of S. agalactiae:

Capsule: One of the most important virulence factors of GBS is its polysaccharide capsule. The capsule helps the bacterium resist phagocytosis, which is the process by which immune cells engulf and destroy invading bacteria. By avoiding phagocytosis, GBS can establish infection and evade the host’s immune response.
Adhesion factors: GBS produces surface proteins that enable it to adhere to and colonize various tissues and cells within the host. This adhesion is crucial for establishing infection and forming biofilms, which are communities of bacteria adhered to surfaces and protected by a matrix of extracellular substances.
Toxins: While GBS is not as toxin-producing as some other Streptococcus species, it can produce certain toxins that contribute to tissue damage and inflammation. These toxins may contribute to the severity of infections and the associated symptoms.
Enzymes: Streptococcus agalactiae can produce enzymes that help it invade and damage host tissues. For example, it can produce hyaluronidase, which breaks down hyaluronic acid, a substance that helps hold tissues together. This allows GBS to spread and invade deeper into host tissues.
Immune system evasion: GBS has developed mechanisms to evade the host’s immune system, allowing it to persist and cause infections. The presence of the capsule and other surface proteins can help it escape immune detection and destruction.

Infections caused by Streptococcus agalactiae can range from mild to severe and may include:

Neonatal infections: Early-onset GBS disease can occur in newborns when they are exposed to the bacterium during childbirth. This can lead to serious conditions such as sepsis, pneumonia, and meningitis in neonates.
Infections in pregnant women: GBS can cause urinary tract infections and, in some cases, lead to complications during pregnancy, including preterm labor and chorioamnionitis.
Infections in adults: GBS can cause various infections in adults, including urinary tract infections, skin and soft tissue infections, and bloodstream infections. It is more likely to affect individuals with compromised immune systems or underlying health conditions.

The management of Streptococcus agalactiae infections involves appropriate antimicrobial therapy, and pregnant women are often screened for GBS to prevent transmission to newborns during childbirth. As with other bacterial infections, the rise of antibiotic resistance is a concern, and efforts to promote prudent use of antibiotics and research into alternative treatments and prevention strategies remain crucial.

Lab Diagnosis

The laboratory diagnosis of Streptococcus agalactiae involves a combination of culture-based methods, biochemical tests, and sometimes molecular techniques. Prompt and accurate diagnosis is essential to guide appropriate treatment and prevent the transmission of GBS to vulnerable populations. Here is an overview of the laboratory diagnostic methods:

Specimen collection: The first step in diagnosing GBS is to collect appropriate clinical specimens from the site of infection. Common specimens may include blood, cerebrospinal fluid (in cases of suspected meningitis), urine (in cases of urinary tract infections), or swabs from the affected site (e.g., vaginal, rectal, wound, or throat swabs).
Culture and isolation: The primary method for identifying GBS is through culture on suitable agar media. The most commonly used media for GBS culture is blood agar, which allows for the observation of characteristic colony morphology and hemolysis patterns. GBS colonies typically appear as small, greyish-white, and round colonies with a narrow zone of beta-hemolysis around them.
Gram staining: A Gram stain of the cultured colonies can be performed to observe the characteristic Gram-positive cocci in chains or pairs, which is indicative of GBS.
Catalase test: GBS is catalase-negative, which means it does not produce the enzyme catalase. This can help differentiate it from catalase-positive bacteria, such as Staphylococcus species.
Hippurate hydrolysis test: GBS can hydrolyze hippurate (a compound derived from glycine) using the enzyme hippuricase. The test involves adding a hippurate substrate to a bacterial suspension and detecting the production of hippuric acid, which indicates a positive result for GBS.
CAMP test: The Christie-Atkins-Munch-Petersen (CAMP) test is a confirmatory test used to identify GBS. It involves streaking GBS and Staphylococcus aureus (which produces a hemolysin called CAMP factor) in a line on a blood agar plate. The presence of enhanced hemolysis at the intersection of the streaks indicates a positive CAMP test, confirming the presence of GBS.
Molecular methods: Polymerase chain reaction (PCR) and other nucleic acid-based methods can be used for rapid and specific detection of GBS. These techniques can detect the presence of GBS DNA directly in clinical specimens or from enriched cultures. PCR is especially useful in cases where GBS is challenging to isolate by traditional culture methods.

Treatment

The treatment of Streptococcus agalactiae infections involves the use of antibiotics. The choice of antibiotics and the duration of treatment depend on the type of infection, the severity of the illness, the age and health status of the patient, and antibiotic susceptibility testing results. It’s crucial to treat GBS infections promptly and effectively to prevent complications and transmission to vulnerable populations. Here are some common antibiotics used for treating GBS infections:

Penicillin and Ampicillin: Penicillin and ampicillin are the first-line antibiotics for treating GBS infections due to their effectiveness and low risk of resistance. These antibiotics are commonly used to treat GBS infections in adults, pregnant women, and neonates. Penicillin is often the preferred choice for intrapartum prophylaxis to prevent the transmission of GBS from mother to newborn during childbirth.
Cephalosporins: In cases where patients are allergic to penicillin or ampicillin, certain cephalosporins like cefazolin or ceftriaxone may be used as alternatives. However, individuals with a severe allergic reaction to penicillin should avoid cephalosporins due to a risk of cross-reactivity.
Clindamycin: Clindamycin is an alternative antibiotic used in patients with penicillin allergies or when there is resistance to penicillin. It is effective against many GBS strains; however, resistance to clindamycin has been observed, so susceptibility testing is essential.
Vancomycin: Vancomycin is reserved for cases of severe GBS infections or when other antibiotics are ineffective due to resistance. It is an important drug for treating GBS strains resistant to penicillin and other first-line antibiotics. However, vancomycin use should be judicious to avoid the development of further resistance.
Other antibiotics: In some specific cases, other antibiotics, such as linezolid or daptomycin, may be considered, but they are generally reserved for severe infections and cases with significant antibiotic resistance.

It’s essential to note that the selection of antibiotics should be based on the results of antimicrobial susceptibility testing, which determines the sensitivity of the GBS strain to specific antibiotics.

For pregnant women, intrapartum antibiotic prophylaxis is recommended for those who are at risk of transmitting GBS to their newborns during childbirth. This preventive measure can significantly reduce the incidence of early-onset GBS disease in newborns.

As with any antibiotic treatment, it is crucial to follow the prescribed dosage and complete the full course of treatment to ensure the complete eradication of the bacterium and prevent the development of antibiotic resistance. Patients should always consult with healthcare professionals to determine the most appropriate treatment plan for their specific GBS infection.

Prevention

The prevention of Streptococcus agalactiae infections involves both maternal and neonatal preventive strategies. The goal is to reduce the transmission of GBS from mother to newborn during childbirth and to prevent infections in vulnerable populations. Here are the key prevention measures:

Maternal Screening: Pregnant women are routinely screened for GBS colonization in the late stages of pregnancy, typically between 35 and 37 weeks of gestation. The screening involves obtaining vaginal and rectal swabs to detect the presence of GBS. This screening helps identify women who are colonized with GBS and are at risk of transmitting the bacterium to their newborns during delivery.
Intrapartum Antibiotic Prophylaxis (IAP): Pregnant women who test positive for GBS colonization during screening are offered intrapartum antibiotic prophylaxis (IAP). Antibiotics, usually penicillin or ampicillin, are administered intravenously during labor and delivery to prevent the transmission of GBS to the baby. This preventive measure has been shown to significantly reduce the incidence of early-onset GBS disease in newborns.
Risk-Based Approach: In some cases, maternal GBS status may not be known at the time of labor or delivery. In such situations, a risk-based approach is employed, where IAP is given to women with certain risk factors, such as preterm labor, prolonged rupture of membranes, or fever during labor, as they have an increased risk of GBS transmission to their newborns.
Awareness and Education: Healthcare providers play a crucial role in educating pregnant women about GBS and its potential risks. Increasing awareness among expectant mothers about GBS screening, risk factors, and preventive measures can help improve the uptake of screening and IAP.
Hospital Infection Control Measures: In healthcare settings, infection control measures such as hand hygiene, proper sterilization of equipment, and isolation precautions for infected or colonized individuals help prevent the spread of GBS among vulnerable populations, including neonates and immunocompromised patients.
Antibiotic Stewardship: Appropriate use of antibiotics is essential to prevent the development of antibiotic resistance. Healthcare providers should follow guidelines for GBS management and use antibiotics judiciously to minimize the risk of resistance.

Keynotes

Here are some keynotes on Streptococcus agalactiae (Group B Streptococcus or GBS):

Gram-positive Cocci: Streptococcus agalactiae is a Gram-positive bacterium, appearing purple under the microscope after Gram staining. It is classified as cocci, meaning it has a spherical or round shape.
Chains and Pairs: GBS typically forms chains or pairs of cells, which is a characteristic arrangement of streptococci.
Hemolysis: GBS can exhibit beta-hemolysis on blood agar, causing complete lysis of red blood cells and creating a clear zone around the colonies.
Capsule: GBS possesses a polysaccharide capsule that helps it evade the host’s immune system and contributes to its pathogenicity.
Ubiquitous Colonization: GBS colonizes the gastrointestinal and genitourinary tracts of healthy adults. It is considered a commensal bacterium in these individuals.
Pathogenicity: GBS is a significant human pathogen, causing various infections, particularly in vulnerable populations such as newborns and pregnant women.
Neonatal Infections: Early-onset GBS disease can occur in newborns when the bacterium is transmitted from the mother during childbirth. It can lead to severe conditions such as sepsis, pneumonia, and meningitis in neonates.
Pregnant Women: GBS can cause urinary tract infections in pregnant women and, if transmitted to the baby during childbirth, may result in neonatal infections.
Infections in Adults: GBS can cause infections in adults, especially those with compromised immune systems or underlying health conditions. It can lead to urinary tract infections, skin and soft tissue infections, and bloodstream infections.
Diagnosis: Laboratory diagnosis involves culture, Gram staining, biochemical tests, and molecular methods like PCR for accurate identification.
Antibiotic Treatment: Penicillin and ampicillin are the first-line antibiotics for GBS infections. Alternatives like cephalosporins or clindamycin are used in cases of penicillin allergies or resistance.
Intrapartum Antibiotic Prophylaxis (IAP): Pregnant women colonized with GBS are given IAP to prevent transmission to their newborns during childbirth.
Risk-Based Approach: In the absence of GBS screening results during labor, a risk-based approach may be used to determine whether IAP is necessary based on specific risk factors.
Antibiotic Resistance: The emergence of antibiotic resistance is a concern, emphasizing the need for prudent antibiotic use and continued research into new prevention strategies.
Public Health Significance: GBS infections have important clinical and public health implications, prompting measures to prevent infections in vulnerable populations and manage GBS-related complications.