The genus Streptococcus is a large and medically significant group of bacteria, ubiquitous in nature and common inhabitants of the human body. These organisms are defined by their spherical shape and tendency to grow in characteristic chain-like formations. While many species exist harmlessly as part of the normal microbial community, others are highly pathogenic, capable of causing a wide spectrum of diseases, from mild infections to life-threatening conditions. Understanding their structure and unique metabolic requirements is necessary to appreciate their diverse ecological roles and impact on human health.
Physical Traits and Classification
Streptococcus are classified as Gram-positive bacteria, meaning their cell wall consists of a thick layer of peptidoglycan that retains a specific stain. These microorganisms are non-motile, lacking the structures required for active movement. The name, derived from Greek, means “twisted chain,” describing their microscopic appearance as round cells (cocci) that divide along a single axis to form chains.
A defining characteristic is the absence of the enzyme catalase, which differentiates them from bacteria like Staphylococcus. Catalase breaks down hydrogen peroxide, a toxic byproduct of oxygen metabolism. Their inability to perform this detoxification is a key laboratory identifier.
Classification relies on two primary systems: the type of hemolysis they cause on blood agar plates and the serological Lancefield grouping. Hemolytic patterns categorize them based on their ability to lyse red blood cells. Beta-hemolysis is the complete destruction of red blood cells, resulting in a clear zone. Alpha-hemolysis is a partial breakdown that leaves a greenish discoloration. Gamma-hemolysis indicates no lysis.
The Lancefield system uses specific carbohydrate antigens in the cell wall to assign a letter-based group, such as Group A (S. pyogenes) or Group B (S. agalactiae).
Energy Needs and Metabolism
The metabolism of Streptococcus species is fundamentally tied to fermentation, classifying them as aerotolerant anaerobes or obligate fermenters. These bacteria rely almost exclusively on glycolysis to break down sugars like glucose, rather than using oxygen for cellular respiration. Glycolysis converts glucose into pyruvate, which is then channeled into lactic acid fermentation.
In this pathway, pyruvate is reduced to lactate, regenerating coenzymes necessary for glycolysis to continue, resulting in a net gain of two ATP molecules per glucose. This reliance on fermentation produces significant amounts of lactic acid as a byproduct, which rapidly acidifies the environment.
Although they lack catalase, Streptococcus species are considered aerotolerant because they possess alternative enzymes, such as NADH oxidase and superoxide dismutase, that offer protection against oxygen toxicity. This allows them to survive in oxygenated environments despite their fermentative nature. They are also fastidious, requiring complex nutritional sources, often including blood or serum components, to grow in a laboratory setting. Their small genomes reflect an adaptation to the nutritionally rich environments of their hosts, leading to the loss of pathways needed to synthesize their own complex nutrients.
Role in Normal Human Flora
Many Streptococcus species maintain a permanent, commensal residence within the human body, coexisting with the host without causing harm. They are prominent members of the microbial communities found primarily in the oral cavity and upper respiratory tract. The Viridans group streptococci, a diverse collection of alpha-hemolytic species, are abundant in the mouth and throat.
These organisms contribute to dental plaque formation. For instance, Streptococcus mutans converts dietary sucrose into lactic acid, a fundamental step in the development of dental caries.
As commensals, these organisms provide colonization resistance by competing with harmful bacteria for space and nutrients. They also produce bacteriocins, which are antimicrobial peptides that inhibit the growth of other bacteria, including potential pathogens like Streptococcus pneumoniae. The presence of these species helps maintain the balance of the upper respiratory tract microbiome. Disruptions, such as those caused by antibiotic use, can allow opportunistic pathogens to colonize and cause infection.
How Streptococcus Causes Disease
Disease occurs when Streptococcus strains express specific virulence factors that allow them to overcome host defenses, leading to tissue invasion and damage. These factors enable the bacteria to adhere to host cells, evade the immune system, and spread.
A significant mechanism of immune evasion is the formation of a polysaccharide capsule, such as the hyaluronic acid capsule produced by Streptococcus pyogenes. This capsule mimics host connective tissue, preventing immune cells from recognizing and engulfing the bacteria.
The M protein is another potent virulence factor, facilitating attachment to host tissues. It also aids immune evasion by binding to regulatory proteins of the host’s complement system, preventing the immune cascade that targets bacteria for destruction.
Pathogenic species also secrete powerful enzymes and toxins that damage host cells and aid spread. Hemolysins, such as Streptolysin O and S, destroy red blood cells and other host cells by creating pores in their membranes. Pyrogenic exotoxins can trigger the characteristic rash seen in scarlet fever. These mechanisms lead to a range of illnesses, from common strep throat (pharyngitis) caused by Group A Streptococcus to severe, invasive infections like cellulitis, sepsis, and necrotizing fasciitis. Infections can also lead to non-suppurative, immune-mediated complications such as rheumatic fever and glomerulonephritis.