Biochemical Test of Streptococcus agalactiae: An Overview
Streptococcus agalactiae, commonly known as Group B Streptococcus (GBS) based on the Lancefield classification, is a significant Gram-positive human and animal pathogen. In humans, it is the leading cause of neonatal sepsis and meningitis, while in cattle, it is a primary agent of mastitis. Its accurate and timely identification in clinical and veterinary microbiology relies on a sequence of tests, beginning with cultural characteristics and culminating in specific biochemical reactions and serological confirmation. Unlike the major energy-producing pathways, these biochemical tests—such as the CAMP test and hippurate hydrolysis—are minor metabolic routes that define the organism’s unique enzymatic and fermentation profile, allowing for its accurate distinction from other commensal or pathogenic streptococci and staphylococci. The entire identification process is crucial for effective diagnosis, particularly for the screening of pregnant women, a practice that has dramatically reduced the incidence of early-onset neonatal GBS disease.
Initial Screening: Morphology and Primary Enzymatic Tests
The identification protocol for a suspected GBS isolate begins with general microbiological characterization. A Gram stain is the first step, revealing non-motile, spherical cells (cocci) typically arranged in short chains or pairs, which confirms the isolate’s genus as *Streptococcus* and its Gram-positive nature. Following this, two simple, rapid enzymatic tests are performed to rule out other common bacterial contaminants and close relatives. The Catalase test and the Oxidase test are universally negative for *Streptococcus agalactiae*, a crucial distinction that immediately separates it from the catalase-positive *Staphylococcus* genus, which can also appear as Gram-positive cocci.
The next crucial observation involves the hemolysis pattern on sheep blood agar (BA). *Streptococcus agalactiae* is characterized by Beta-hemolysis, meaning it produces hemolysins that cause complete lysis of the red blood cells in the medium. This results in a clear, transparent zone surrounding the colonies. While this reaction is shared with other serious pathogens like *Streptococcus pyogenes* (Group A Streptococcus), the zone of beta-hemolysis produced by *S. agalactiae* is often narrow, less distinct, or only partially visible, sometimes requiring the specific enhancement provided by the CAMP test for reliable observation. The presence of beta-hemolysis narrows the identification to a few key species, necessitating the use of the definitive biochemical test to achieve species-level identification.
The Definitive Biochemical Test: The CAMP Reaction
The most characteristic and historically significant biochemical test for *Streptococcus agalactiae* is the CAMP test, an acronym derived from the first letters of the researchers who observed the phenomenon: Christie, Atkins, Munch-Petersen. This test is based on a synergistic hemolytic phenomenon observed when *S. agalactiae* is grown in close proximity to a specific, known beta-lysin-producing strain of *Staphylococcus aureus*.
The principle of the CAMP test is enzymatic synergy. *S. agalactiae* produces a diffusible, heat-stable protein known as the CAMP factor. The indicator strain of *Staphylococcus aureus* produces an enzyme called beta-lysin (or sphingomyelinase C), which, by itself, causes only partial damage to red blood cell membranes, a sub-lytic effect. The CAMP factor produced by *S. agalactiae* then binds to these partially damaged membranes and acts synergistically, completing the lysis of the red blood cells. Essentially, the CAMP factor enhances the incomplete hemolysis caused by the staphylococcal beta-lysin.
The test is practically performed by streaking a known beta-lysin-producing *S. aureus* culture down the center of a sheep blood agar plate. The suspected *S. agalactiae* culture is then inoculated in a straight line perpendicular to the *S. aureus* streak, ensuring the two streaks do not physically touch but are separated by 1 to 2 mm. After incubation at 37°C for 18–24 hours, a positive result is identified by an enhanced, distinctive “arrowhead” or “flame-shaped” zone of complete beta-hemolysis that forms only in the intersection area between the two cultures, with the point of the arrow directed toward the *S. aureus* inoculum. A positive CAMP reaction is a highly reliable presumptive identification for Group B Streptococcus, differentiating it clearly from *S. pyogenes* (Group A), which is CAMP negative.
Complementary Enzymatic and Exclusion Tests
*Streptococcus agalactiae* possesses a profile of other specific enzymatic activities that significantly aid in its differentiation from related non-Group B streptococci. One of the most important of these is the Hippurate Hydrolysis test. GBS possesses the enzyme hippuricase (or hippurate hydrolase), which is capable of hydrolyzing sodium hippurate into its products, benzoic acid and glycine. A positive result in this test, typically indicated by a color change following the addition of a chemical reagent, provides strong corroborative evidence for *S. agalactiae* identification, as it is one of the few streptococcal species to possess this enzyme.
Conversely, several key tests are negative, which helps in the exclusion of other species. GBS is consistently negative for Bile Esculin Hydrolysis, meaning it cannot grow in the presence of bile and does not hydrolyze the esculin substrate. This negative result is a key factor in distinguishing *S. agalactiae* from Lancefield Group D streptococci and *Enterococcus* species, which are Bile Esculin positive, turning the media black. GBS is also negative for the PYR (Pyrrolidonyl Arylamidase) test, which is another crucial differentiating factor from *Streptococcus pyogenes* and *Enterococcus* species, both of which are PYR positive. Additionally, it is consistently Urease negative, Coagulase negative, and non-motile, completing a biochemical fingerprint that is unique among the clinically significant streptococci.
Carbohydrate Fermentation Profile and Clinical Relevance
While the CAMP and Hippurate tests are the primary diagnostic tools, the fermentation profile of *S. agalactiae* on various carbohydrates also provides a specific biochemical signature. GBS is a facultative anaerobe that typically ferments D-glucose, maltose, and sucrose, producing acid but usually no gas. However, it is unable to ferment L-arabinose, D-mannitol, and inulin, which is a useful trait in differential identification. Its reaction to lactose and cellobiose is often variable between strains. The fact that it does not ferment mannitol, for instance, is a characteristic that helps in early presumptive identification when a mixture of Gram-positive cocci, including staphylococci, may be present.
The reliable and rapid identification of *Streptococcus agalactiae* through these combined biochemical tests is vital for public health. The clinical relevance is paramount in obstetric settings for the prevention of life-threatening neonatal infections. While traditional biochemical tests form the historical and educational foundation, modern microbiology complements this profile with more advanced confirmatory methods. These include serogrouping using latex agglutination to confirm the presence of the Group B antigen (Lancefield grouping), as well as contemporary molecular techniques like Polymerase Chain Reaction (PCR) and proteomics-based platforms like MALDI-TOF Mass Spectrometry (Matrix-Assisted Laser Desorption/Ionization-Time of Flight Mass Spectrometry). Nonetheless, the classic biochemical profile—Gram-positive cocci, Catalase negative, Beta-hemolytic, CAMP positive, and Hippurate positive—remains the essential metabolic signature for *Streptococcus agalactiae*.