Bacteriophage infection of Streptococcus thermophilus is a growing concern in the mozzarella cheese industry. One method to control this problem may be to replace S. thermophilus with a starter coccus from a different genus of lactic acid bacteria. This work evaluated the possibility of using genetically modified Pediococcus spp. for this approach. Electroporation was used to introduce genes for lactose utilization from Lactococcus lactis into strains of P. acidilactici and P. pentosaceus. The resulting lactose-positive transformants, P. acidilactici SAL and P. pentosaceus SPL-2, rapidly reduced the pH of lactose broth, accumulated [14C]lactose at a rate higher than a lactococcal control, and showed relatively high phospho-β-galactosidase activity. When paired with Lactobacillus helveticus LH100 in 9% reconstituted skim milk, P. acidilactici SAL and P. pentosaceus SPL-2 demonstrated synergistic growth with LH100. Milk fermented with Pediococcus-LH100 starter pairs also contained significantly less free galactose than milk fermented with a control starter blend of LH100 and S. thermophilus TA061. Mozzarella cheese made with lactose-positive Pediococcus-LH100 blends was compositionally similar to cheese made with the control starter blend, but production required 60-90 additional minutes. In an attempt to decrease the time required to produce mozzarella, Pediococcus spp. were transformed with lactococcal genes from an extracellular serine proteinase ora n oligopetpide transport system. Constructs which expressed each system were obtained, but these strains did not display improvement in the ability to clot 9% reconstituted skim milk.Studies to screen P. acidilactici and P. pentosaceus for lysogeny detected temperate bacteriophage in three strains of P. acidilactici. Morphological characterization of these new phages demonstrated that they had small isometric heads with non-contractile tails and thus belonged to the B I group of the family Siphovirdae. Further characterization based on DNA-DNA homology and protein profiles suggested that the P. acidilactici phages can be separated into at least two different species. As a whole, the results reported here suggest that due to their slower growth in milk, P. acidilactici SAL and P. pentosaceus SPL-2 cannot be used as direct replacements for S. thermophilus but may be suited for use as adjuncts to the traditional S. thermophilus/Lactobacillus sp. starter blend.