**5.4 Influence of** *Lactococcus* **dead cell biomass concentration on cholesterol removal**

The sparse literature data on cholesterol removal by inactivated cells prove that lactic acid bacteria monocultures are capable of removing cholesterol from culture media even after their thermal death [21, 24, 25, 43, 95]. The amount of cholesterol

**85**

*Cholesterol Uptake and Survival of* Lactococcus lactis *Strains in Fluids Simulating the Human…*

removed by inactivated cells is considerably lower than by biologically active cells, which likely stems from the fact that in the case of dead cells cholesterol may not be built into the cell wall or cytoplasmic membrane, but it only undergoes adhesion by the cells. Furthermore, Ziarno [4] demonstrated that biomass of dead (thermally inactivated) cells of *Lactococcus*, originating from industrial monocultures and multi-species cultures, influences cholesterol uptake from the M17 culture broth. The highest amount of cholesterol was removed in the cultures containing 10x con-

from the M17 broth. Bacterial biomasses with 1x concentration removed 1.4–1.9 fold less cholesterol. Tenfold diluted biomass of dead bacterial cells bound from

biomasses. The fact that cholesterol removal occurs even when the bacterial cells are dead confirms that the physical binding of cholesterol molecules by the cell wall (adhesion) is one of the mechanisms of cholesterol removal by *Lactococcus* cells.

**6. Survival of** *Lactococcus* **cells in the human gastrointestinal tract**

Literature contains studies confirming the capacity of lactic bacilli to survive under in vivo conditions in the human alimentary tract [28, 30, 35, 52, 99, 100]. The factors with a significant impact on lactic acid bacteria survivability in the alimentary tract include low gastric pH value, intestine peristalsis, presence of bile acids in the pancreatic fluid and various digestive enzymes present in the individual sections of the alimentary tract, presence of nutrients, as well as bacteria passage time through the alimentary tract and their initial count [35, 50, 101–103]. The mentioned factors result in a decrease of lactic acid bacteria survival rate, but at the same time they may constitute a criterion for the selection of probiotic strains [28, 104].

In order to determine the survival rate of lactic acid bacteria, scientists first determine their resistance to low pH present in certain sections of the gastrointestinal tract. Gastric fluid comprises of the secretion of foveolar cells secreting mucus, chief cells secreting digestive enzymes (pepsin), and parietal cells secreting hydrochloric acid. The pH of gastric fluid is between 1.5 and 3.0. Secretion of gastric fluid is inhibited when the pH drops below 2.0. The temperature inside the stomach is over 37°C, and the alimentary content, depending on the individual physiological and emotional circumstances, remains in the stomach for average 1–3 h [4]. Results of in vitro tests concern survival rate of different lactic acid bacteria strains under conditions imitating low pH of the gastric fluid [18, 35, 41, 105–109]. Strains traditionally used to manufacture dairy products have also been commonly found to survive the conditions of gastric fluid [101]. Also the study of Lankaputhra and Shah [107] indicated that numerous lactic bacilli strains survived perfectly the conditions simulating the pH of gastric fluid. Another subject of the study is the capacity of lactic acid bacteria to survive during transport through subsequent sections of the alimentary canal. Here, a particular significance is exhibited by the section of the small intestine [102, 110, 111]. Literature data show that bile salts comprise a serious obstacle for lactic acid bacteria, as they contain toxic bile acids [19, 106, 107, 112]. Ziarno and Bartosz [113] provided evidence for the influence of cell biomass on the survival of lactic bacilli in model intestinal fluid. Cholesterol influences the composition and functioning of the bacterial cell wall and membrane, thus producing change in the relationship with the surrounding environment, such as resistance to bile acids, pH, or temperature [46, 48, 50]. Cholesterol uptake by bacterial cells is not neutral to them and results in a change of, among others, the profile of fatty acids of the cell membrane [23, 25, 38, 50]. Cell responds to stress conditions of the environment with a change of the composition of the cell membrane, and it may result in an increase of the

of cholesterol, thus 1.7–2.3-fold less than 1× concentrated

of cholesterol

centrated biomass of dead bacterial cells, from 0.074 to 0.083 g/dm3

*DOI: http://dx.doi.org/10.5772/intechopen.88462*

0.021 to 0.029 g/dm3

#### *Cholesterol Uptake and Survival of* Lactococcus lactis *Strains in Fluids Simulating the Human… DOI: http://dx.doi.org/10.5772/intechopen.88462*

removed by inactivated cells is considerably lower than by biologically active cells, which likely stems from the fact that in the case of dead cells cholesterol may not be built into the cell wall or cytoplasmic membrane, but it only undergoes adhesion by the cells. Furthermore, Ziarno [4] demonstrated that biomass of dead (thermally inactivated) cells of *Lactococcus*, originating from industrial monocultures and multi-species cultures, influences cholesterol uptake from the M17 culture broth. The highest amount of cholesterol was removed in the cultures containing 10x concentrated biomass of dead bacterial cells, from 0.074 to 0.083 g/dm3 of cholesterol from the M17 broth. Bacterial biomasses with 1x concentration removed 1.4–1.9 fold less cholesterol. Tenfold diluted biomass of dead bacterial cells bound from 0.021 to 0.029 g/dm3 of cholesterol, thus 1.7–2.3-fold less than 1× concentrated biomasses. The fact that cholesterol removal occurs even when the bacterial cells are dead confirms that the physical binding of cholesterol molecules by the cell wall (adhesion) is one of the mechanisms of cholesterol removal by *Lactococcus* cells.
