Strep Colonization: When Does It Become an Infection?

Streptococcus are common bacteria found on and in the human body. Many are harmless residents, while others can cause disease. The simple presence of these bacteria is known as colonization, where the microorganisms live on a host without causing any signs of illness. This harmless presence can shift to an active infection under certain circumstances.

Understanding Strep Colonization

Strep colonization refers to the presence of Streptococcus bacteria on or within the body without eliciting any symptoms or an immune response from the host. In this state, the bacteria grow and multiply but do not cause harm, existing as part of the body’s commensal flora. This is different from an infection, where the bacteria actively invade tissues and cause illness, triggering a defensive reaction from the immune system.

Several groups of Streptococcus are commonly involved in colonization. Group A Streptococcus (GAS), or Streptococcus pyogenes, is a known colonizer, as is Group B Streptococcus (GBS), or Streptococcus agalactiae. Another species is Streptococcus pneumoniae, which is frequently found in the respiratory tract. The Viridans group of streptococci are also common colonizers, particularly in the mouth.

Common Sites and Prevalence of Strep Colonization

Different Streptococcus species colonize specific anatomical locations. The throat and nasopharynx are the primary reservoirs for Group A Strep and S. pneumoniae. Group B Strep predominantly colonizes the gastrointestinal and genitourinary tracts. The bacteria can spread from the digestive system to the female genital tract. Viridans streptococci are a major component of the normal oral microbiota, residing on the surfaces of teeth and mucosal tissues in the mouth.

The prevalence of strep colonization varies by bacterial species and population group. For instance, GBS colonization is common in pregnant women, with the gastrointestinal tract serving as the main reservoir. GAS colonization of the pharynx is frequently seen in school-aged children, who can carry the bacteria without any signs of strep throat. The presence of these bacteria is a normal aspect of human microbiology.

The Transition from Colonization to Infection

Strep colonization can shift to an active infection, a transition influenced by host and environmental factors. A primary trigger is a weakened immune system, which can occur in newborns, the elderly, or people with health conditions like diabetes or cancer. In these individuals, the body’s defenses are less capable of keeping the colonizing bacteria in check.

Disruption of the body’s microbial balance, for example through antibiotic use, can allow pathogenic strep to proliferate. Bacteria gaining access to normally sterile sites is another factor. A break in the skin can allow GAS to cause a skin infection, while aspiration can lead S. pneumoniae into the lungs, causing pneumonia.

The specific traits of the bacterial strain, such as its ability to produce toxins, also play a part in its potential to cause disease. These transitions can lead to illnesses like strep throat from GAS, neonatal disease from GBS, or pneumonia from S. pneumoniae.

Detection and Management of Strep Colonization

Strep colonization is identified through laboratory tests, such as culturing a swab taken from the colonized site, like the throat or vagina. However, routine screening for all types of strep colonization in the general population is not a standard practice. Screening is reserved for specific situations where the risk of transmission and subsequent infection is a concern.

A primary example of targeted screening is for Group B Strep in pregnant women. Healthcare providers screen for GBS late in pregnancy because a colonized mother can transmit the bacteria to her baby during childbirth, leading to serious neonatal infections like sepsis and meningitis. To prevent this, women who test positive for GBS colonization are given antibiotics during labor.

This prevention strategy has significantly reduced the incidence of early-onset neonatal GBS disease. In other cases, such as asymptomatic GAS throat colonization, treatment is not recommended unless there are specific circumstances, like during an outbreak.