Table 4.

Fecal Inflammatory Biomarkers to Be Assessed in the Enterics for Global Health Study

Biomarker	Target	Description	Rationale
Myeloperoxidase	Neutrophils	A peroxidase enzyme belonging to the heme-containing proteins, produced largely in neutrophils and in smaller quantities in monocytes [80, 81]. MPO is considered a biomarker for neutrophils, inflammatory activity in the gastrointestinal tract, and neutrophil damage [81]. Fecal MPO is a biomarker for IBD [82].	Shigella detections in MAL-ED were associated with increases in MPO, and the association depended on Shigella quantity, such that MPO levels were more highly elevated as the quantity of Shigella increased [10]. Diarrheagenic Escherichia coli, specifically EAEC, EPEC, and ETEC, was also associated with elevated MPO, though to a lesser extent [83].
Calprotectin	Neutrophils	A protein biomarker of leukocytes and neutrophil damage during intestinal inflammation [23–25]. Fecal calprotectin helps to distinguish between IBD and noninflammatory bowel conditions and monitor IBD activity [84].	Increased levels were observed among those with bacterial compared to viral infections during diarrhea [24]. Calprotectin was elevated in shigellosis at levels higher than other bacterial diarrheas (Clostridioides difficile, Salmonella, Campylobacter, and EIEC) [85].
Lipocalin-2 (NGAL)	Neutrophils; enterocyte damage	A circulatory protein commonly referred to as neutrophil gelatinase-associated lipocalin (NGAL). NGAL is responsible for the delivery of molecules including steroids, free fatty acids, and hormones to body organs [86]. It is an indicator of innate immunity [87], found in a variety of cells including neutrophils, and possesses antibacterial and anti-inflammatory functions, in addition to providing protection against cell and tissue stress [86]. NGAL is an indicator of enterocyte damage and acute and chronic renal injury [86, 88]. It is also a biomarker for intestinal inflammation and is associated with IBD [87].	Studies in a Shigella murine model demonstrate that sensitivity for Shigella may be higher compared to MPO [89]. Lipocalin-2 decreases rapidly following inflammation [90].
Hemoglobin	Fecal occult blood	The iron-containing protein present in RBCs responsible for transporting oxygen to organs and tissues [91]. Hemoglobin is a marker of RBCs and its presence in stool indicates the presence of blood. Fecal hemoglobin helps to identify IBD patients with active inflammation [92].	Shigella is the main cause of dysentery among children globally [7]. Presence of RBCs was predictive of shigellosis and other bacterial diarrhea in the systematic review.

Abbreviations: EAEC, enteroaggregative Escherichia coli; EIEC, enteroinvasive Escherichia coli; EPEC, enteropathogenic Escherichia coli; ETEC, enterotoxigenic Escherichia coli; IBD, inflammatory bowel disease; MAL-ED, Etiology, Risk Factors, and Interactions of Enteric Infections and Malnutrition and the Consequences for Child Health and Development; MPO, myeloperoxidase; NGAL, neutrophil gelatinase-associated lipocalin; RBCs, red blood cells.