Table 3. Examples of different types of heterologous protein expression systems for engineered probiotics.

Expression Type	Examples of engineered strains	Purposes	Reference
Intracellular (pCYT)	Lc. lactis, L. plantarum, L. casei	Production of antigens and enzymes	[30,43,99,160,174–177]
Secretion (pSEC)	Lc. lactis, L. plantarum, L. helveticus, L. salivarius, L. casei, L. plantarum, L. acidophilus, L. gasseri, L. paracasei	Secretion of antigens, immunomodulatory proteins, enzymes	[26,33,42,178,179,180]
Cell wall anchoring (pCWA)	Lc. lactis, L. delbrueckii, L. brevis, L. helveticus, L. johnsonii, L. crispatus, and L. salivarius, L. plantarum, Lactobacillus zeae, L. sakei, and L. casei, L. gasseri	Display of antibodies, antigens, and enzymes	[17,181–183,184–187]