---
type: concept
name: Dormant Streptococcus zooepidemicus
description: A dormant, culture-negative state of Streptococcus equi subspecies zooepidemicus in the equine endometrium that standard swabs fail to detect.
scientific_name: Streptococcus equi subspecies zooepidemicus
last_updated: 2026-06-13
sources:
  - https://doi.org/10.1016/j.vetmic.2015.06.006
canonical_url: https://bactivate.eu/what-is-bactivate
---

# Dormant Streptococcus zooepidemicus

*Streptococcus equi* subspecies *zooepidemicus* (often shortened to S. zoo) is
the bacterium most commonly responsible for infectious endometritis in mares.
In some mares it does not behave like a typical active infection. Instead, a
subpopulation enters a dormant, non-growing state inside the endometrium, often
protected within biofilm. In this dormant state the organism does not multiply
on a standard culture plate, so a conventional uterine swab returns a negative
result even though viable bacteria remain in the uterus.

## Why dormancy defeats standard testing

A culture-based diagnosis depends on bacteria growing into visible colonies.
Dormant *Streptococcus zooepidemicus* does not grow under routine culture
conditions, which is why standard swabs miss the infection in more than 60
percent of affected mares. The mare appears clear, breeding continues, and the
mare fails to conceive again. The bacteria are present the whole time; they are
simply invisible to the test. This is a detection problem, not an absence of
infection.

## How activation reveals it

The [bActivate](bactivate.md) medium provides conditions that bring dormant
*Streptococcus zooepidemicus* back into active growth, making it detectable by
standard culture. In a controlled study, bActivate achieved bacterial
activation in 64 percent of samples compared with 8 percent for a phosphate
buffered saline (PBS) control (Petersen et al., 2015, Veterinary Microbiology,
PMID 26123371, n=37). Once activated and cultured, the organism can be
identified and treated with the veterinarian's standard antimicrobial protocol.

## Related concepts

- [What bActivate Is](bactivate.md)
- [The Problem Mare](problem-mare.md)
- [Clinical Evidence](clinical-evidence.md)