Lactobacillus acidophilus is a Gram-positive, rod-shaped, non-spore forming, homofermentative bacterium that is a normal inhabitant of the gastrointestinal and genitourinary tracts. Since its original isolation by Moro (1900) from infant feces, the “acid loving” organism has been found in the intestinal tract of humans, breast-fed infants, and persons consuming high milk, lactose, or dextrin diets. Historically, Lactobacillus acidophilus is the Lactobacillus species most often implicated as an intestinal probiotic capable of eliciting beneficial effects on the microflora of the gastrointestinal tract (Klaenhammer and Russell (2000) “Species of the Lactobacillus acidophilus complex,” Encyclopedia of Food Microbiology, 2:1151-1157. Robinson et al., eds. (Academic Press, San Diego, Calif.). Lactobacillus acidophilus can ferment hexoses, including lactose and more complex oligosaccharides, to produce lactic acid and lower the pH of the environment where the organism is cultured. Acidified environments (e.g., food, vagina, and regions within the gastrointestinal tract) can interfere with the growth of undesirable bacteria, pathogens, and yeasts. The organism is well known for its acid tolerance, survival in cultured dairy products, and viability during passage through the stomach and gastrointestinal tract. Lactobacilli and other commensal bacteria, some of which are considered probiotic bacteria that “favor life,” have been studied extensively for their effects on human health, particularly in the prevention or treatment of enteric infections, diarrheal disease, prevention of cancer, and stimulation of the immune system. Lactobacilli have also been studied for their influence on dairy product flavor, and functional and textural characteristics. Genetic characterization of other Lactobacillus species (e.g., L. johnsonii and L. rhamnosus) has been described (see e.g., U.S. Pat. No. 6,476,209; U.S. Pat. No. 6,544,772; U.S. Patent Publication Nos. 20020159976, 2003013882 & 20040009490; PCT Publication No. WO 2004/031389; PCT Publication No. 2003/084989; PCT Publication No. WO 2004/020467).
Bacterial growth requires specific transport systems to import nutrients from the external environment. Lactic acid bacteria transport molecules into and out of the cell via three systems: primary transport, secondary transport, and group translocation. In primary transport, chemical (primarily ATP), electrical, or solar energy is used to drive transport. ATP-binding cassette (ABC) transporters are the most abundant class of primary transport systems in lactic acid bacteria. In this system, ATP hydrolysis is linked with substrate translocation across the membrane for both the import of sugars and compatible solutes and the export of products such as drugs or toxins that are undesirable to the cell, or cellular components that function outside of the cell, such as cell wall polysaccharides. In general, ABC transporters are relatively specific for their substrates, but some are multispecific, such as the multidrug transporters.
Secondary transport systems use electrochemical gradients to provide the energy for sugar translocation. They comprise symporters, which cotransport two or more solutes, uniporters, which transport one molecule, and antiporters, which countertransport two or more solutes. Symporters generally couple the uphill movement of the substrate to the downhill movement of a proton (or ion), antiporters use the ion gradient for excretion of a product, and uniporters do not use a coupling ion (Poolman (2002) Antonie van Leeuwenhoek 82:147-164).
Group translocation involves the phosphoenolpyruvate (PEP)-dependent phosphotransferase system (PTS), which couples the uptake of a carbohydrate or alditol with its phosphorylation (Poolman (2002), supra). The phosphate group originates from the conversion of PEP into pyruvate, and the subsequent phosphorylation involves the energy coupling proteins, Enzyme I and HPr, as well as substrate-specific phosphoryl transfer proteins IIA, IIB and IIC.
Multidrug transporters may be separated into two major classes, secondary multidrug transporters and ABC transporters. Secondary multidrug transporters may be further divided into distinct families, including the major facilitator superfamily (MFS), the small multidrug resistance family (SMR), the resistance-nodulation-cell division family (RND), and the multidrug and toxic compound extrusion family (MATE) (Putman et al. (2000) Microbiol. Mol. Biol. Reviews 64:672-693). Secondary multidrug transporters use the electrochemical gradients, as described herein, to extrude drugs from the cell. ABC-type multidrug transporters use energy from ATP hydrolysis to pump drugs out of the cell (Putman et al. (2000), supra).
Bacteria are able to metabolize various carbohydrates by utilizing transport proteins and enzymes with different carbohydrate specificities, in addition to employing diverse regulatory mechanisms, such as catabolite repression. The isolation and characterization of these proteins allows for the development of essential probiotic products with numerous applications, including those that benefit human and/or animal health, and those concerned with food production and safety. The proteins can also be used in developing transgenic plants with altered growth or survival capabilities.