Among the many types, Staphylococci and Streptococci are common bacteria capable of causing a range of infections. While these two groups of bacteria may appear similar under a microscope, distinguishing between them is important for accurate diagnosis and effective treatment. This article will explain the primary laboratory method used to tell these two bacterial groups apart.
Understanding Staphylococci and Streptococci
Staphylococci are typically found on the skin and mucous membranes of humans. These bacteria often appear as grape-like clusters when viewed under a microscope. They can cause various infections, ranging from common skin infections like boils and impetigo to more serious conditions such as food poisoning and severe systemic infections, including those caused by Methicillin-resistant Staphylococcus aureus (MRSA).
Streptococci usually arrange themselves in chains or pairs. These bacteria are frequently associated with infections like strep throat, scarlet fever, and impetigo. More severe infections caused by Streptococci can include pneumonia, sepsis, and meningitis, particularly in newborns. Both Staphylococci and Streptococci are classified as Gram-positive cocci, retaining a purple stain in a laboratory test and having a spherical shape, which makes initial microscopic differentiation challenging.
Distinguishing between these bacterial groups is important because they cause different types of infections and respond to different treatment approaches. Accurate identification guides healthcare providers in selecting the most effective antibiotics, which can significantly improve patient outcomes and help reduce antibiotic resistance. Identifying the specific pathogen involved ensures that patients receive targeted care rather than broad-spectrum treatments.
The Catalase Test Explained
The catalase test is the primary method used to differentiate Staphylococci from Streptococci. This simple biochemical test detects the presence of the enzyme catalase. Catalase functions by breaking down hydrogen peroxide, a common byproduct of bacterial metabolism, into harmless water and oxygen gas.
When a bacterial sample containing catalase is exposed to hydrogen peroxide, the enzyme rapidly converts the hydrogen peroxide, forming oxygen bubbles. Staphylococci possess the catalase enzyme, so they produce a positive result, indicated by bubbling. Conversely, Streptococci lack this enzyme, so no bubbles are produced, resulting in a negative catalase test.
The procedure for the catalase test is straightforward, involving placing bacterial growth on a microscope slide and adding hydrogen peroxide. The observation of bubbles indicates a positive reaction, identifying Staphylococcus, while the absence of bubbles indicates a negative reaction, identifying Streptococcus. This test is quick, easy to perform, and widely used in clinical settings for rapid initial identification of these bacterial groups.
Additional Tests for Specific Identification
While the catalase test effectively differentiates between Staphylococci and Streptococci, further tests are often needed to identify specific species within each group. This detailed identification is important for precise diagnosis and targeted treatment.
For Staphylococci, two tests provide further differentiation. The coagulase test distinguishes pathogenic Staphylococcus aureus from other Staphylococcus species. Staphylococcus aureus produces the coagulase enzyme, which causes blood plasma to clot. Novobiocin sensitivity identifies Staphylococcus saprophyticus, often associated with urinary tract infections. Staphylococcus saprophyticus is resistant to the antibiotic novobiocin, while other coagulase-negative staphylococci are sensitive.
For Streptococci, several tests help differentiate species based on their characteristics. Hemolysis patterns, observed on blood agar plates, categorize streptococci based on their ability to lyse red blood cells. Alpha-hemolysis results in a partial, greenish discoloration around colonies, beta-hemolysis causes a complete clearing, and gamma-hemolysis shows no change. Beta-hemolytic streptococci, such as Streptococcus pyogenes, are further identified using the bacitracin sensitivity test, as Streptococcus pyogenes is sensitive to this antibiotic.
Another important test for Streptococci is optochin sensitivity, which helps identify Streptococcus pneumoniae, a common cause of pneumonia, as it is sensitive to optochin. Additionally, the Lancefield grouping system classifies beta-hemolytic streptococci into different groups (e.g., Group A, B) based on specific carbohydrates in their cell walls. These additional tests collectively provide a comprehensive profile, enabling appropriate treatment plans.