**1. Introduction**

Coronary ischemia, known as the coronary disease, is one of the modern civilization diseases, whose cause is coronary atherosclerosis (so-called atherosclerotic coronary plaque) in over 90% of cases, leading to their stenosis. One of the risk factors for the formation of atherosclerotic coronary plaques is hypercholesterolemia, in particular elevated concentration of LDL cholesterol. Scientific data exist indicating that consumption of fermented milk products reduces the level of cholesterol in humans. Some of the studies (on animals and human volunteers) indicate that the reduction of the cholesterol level in blood serum is caused by lactic acid bacteria present in fermented milk drinks. Numerous in vitro studies demonstrate that the capacity to reduce the cholesterol level may be exhibited not only by the strains with

documented probiotic traits but also some "traditional" lactic acid bacteria used in the production of cheese, cream, or fermented milk products. The role of *Lactococcus* in dairy fermentation is mostly down to the production of lactic acid; however these bacteria utilize less than 0.5% of lactose from milk. Only *Lactococcus lactis* is applicable in the dairy industry, with its two subspecies: *Lactococcus lactis* subsp. *lactis* and *Lactococcus lactis* subsp. *cremoris*. These subspecies comprise the basic component of dairy mesophilic starter cultures, used for the production of cream, buttermilk, cottage cheeses, cheeses, and fermented milk [1–3]. In the process and functional terms, *Lactococcus* possess all the traits required for starter cultures: the capacity to ferment lactose, resistance to low pH, low temperature, and high concentrations of cooking salt. They are characterized by stability and suitable survival time during lyophilization and freezing and in the storage process of starter cultures [1, 2, 4].

Furthermore, lactic acid bacteria have the capacity to reduce the level of cholesterol in simulated conditions, i.e., culture media. It is known that lactic acid bacteria are not capable of metabolizing cholesterol, meaning its transformation into other compounds. It has been noted that bacterial cells are capable of binding cholesterol, consisting in adhesion of substances by the cell wall or its assimilation into cell wall. It has also been suspected that lactic acid bacteria are capable of deconjugating bile salts being the component of bile, followed by coprecipitation of cholesterol with deconjugated bile acids. Furthermore, tests on gnotobiotic animals demonstrated that hydrolysis of bile enhances its secretion and thus may contribute to reduction of the cholesterol level in blood serum. Moreover, the cholesterol level in the human organism may be also influenced by exopolysaccharides (EPS) produced by numerous lactic acid bacteria species. It is believed that these bacteria, similar to fiber, can bind cholesterol and bile acid molecules present in intestines and remove them from the human organism.

In many cases, results of in vitro studies are not unambiguous, or lack of their reproducibility has been determined. It turns out that also in vivo tests conducted on human volunteers or experimental animals do not produce unambiguous results or that their results are divergent. Considering that it is difficult to explain as to why this happens, such studies are frequently criticized for methodological and technical errors and lack of reproducibility.
