The genera Streptococci and Staphylococci are two of the most commonly encountered bacteria in human health. These distinct groups are responsible for a wide range of illnesses, from mild skin infections to severe, life-threatening systemic diseases. Understanding the fundamental differences between these bacteria is important for recognizing the nature of the infections they cause. Their differing structures, cellular arrangements, and unique toxic mechanisms contribute to the varied health impacts observed in human hosts.
How Structure and Arrangement Set Them Apart
Both Streptococci and Staphylococci are classified as Gram-positive cocci, meaning they are spherical in shape and retain the purple crystal violet stain during a Gram stain procedure. The primary visual difference lies in how their cells divide and arrange themselves. Staphylococci divide in multiple planes, resulting in the formation of irregular, grape-like clusters.
In contrast, Streptococci divide along a single axis, causing the cells to remain attached in chains or pairs. This structure gives them their name, derived from the Greek word for “twisted chain.” This distinct cellular arrangement is the first step in laboratory identification.
A key biochemical test used to differentiate the two is the catalase test. Staphylococci are catalase-positive, possessing the enzyme that breaks down hydrogen peroxide into water and oxygen gas. When hydrogen peroxide is added to a Staphylococcus sample, the resulting production of oxygen bubbles provides a positive result. Conversely, Streptococci lack this enzyme, yielding a negative result with no bubbling.
The typical human habitat also differs between the two groups. Staphylococci, particularly Staphylococcus aureus, frequently colonize the skin and nasal passages. Streptococci, such as Streptococcus pyogenes, are more commonly found in the upper respiratory tract and throat.
Health Impacts Caused by Streptococci
Streptococci are classified using the Lancefield grouping system based on the carbohydrates in their cell walls. Group A Streptococcus (GAS) is one of the most significant human pathogens. The most common infection caused by GAS is streptococcal pharyngitis, or strep throat, which involves inflammation of the tonsils and pharynx. GAS also causes impetigo, a non-invasive skin infection characterized by red sores that form a yellowish crust.
Certain strains of GAS secrete streptococcal pyrogenic exotoxins (SPEs). These toxins act as superantigens, leading to an exaggerated immune response that causes the characteristic sandpaper-like rash of scarlet fever. The rash occurs because the toxins damage the capillaries under the skin. Untreated streptococcal pharyngitis can lead to serious, delayed post-infectious conditions.
Acute rheumatic fever, which affects the joints, skin, and heart valves, occurs weeks after an initial GAS infection, primarily linked to pharyngitis. Another potential complication is post-streptococcal glomerulonephritis, involving inflammation of the kidney filters, which can follow either a throat or skin infection. In rare, severe cases, Streptococci can become invasive, causing conditions like necrotizing fasciitis, or “flesh-eating disease,” which rapidly destroys tissue. Streptococcus pyogenes also secretes cytolytic toxins that contribute to tissue damage during severe invasive infections.
Health Impacts Caused by Staphylococci
The most clinically relevant species is Staphylococcus aureus, which causes a wide array of infections through direct tissue invasion and the production of potent exotoxins. Skin and soft tissue infections are the most frequent manifestation of Staphylococcus infections. These range from minor conditions like pimples and folliculitis to deeper, localized collections of pus known as abscesses or boils.
When the bacteria invade deeper layers, they can cause cellulitis, a spreading infection of the skin and underlying tissues. S. aureus is particularly concerning due to its ability to develop antibiotic resistance. Methicillin-resistant Staphylococcus aureus (MRSA) is a major public health concern because it is resistant to many common antibiotics, making treatment challenging. MRSA infections can be acquired in healthcare settings (HA-MRSA) or in the general community (CA-MRSA), often causing severe necrotizing pneumonia or persistent skin infections.
Beyond direct invasion, Staphylococci produce toxins that mediate specific diseases. Toxic Shock Syndrome (TSS) is a life-threatening condition caused by toxins like Toxic Shock Syndrome Toxin-1 (TSST-1), which triggers a massive, systemic overreaction by the host’s immune system. Symptoms of TSS include high fever, sudden low blood pressure, and multi-organ failure.
Another toxin-mediated illness is staphylococcal food poisoning, caused by the ingestion of pre-formed enterotoxins in contaminated food. These heat-stable toxins cause rapid onset of severe nausea, vomiting, and abdominal cramping. The illness is an intoxication, meaning the symptoms are caused by the toxin itself rather than the multiplication of the bacteria in the host’s body.