## 1 Introduction  
_Lactococcus lactis_ strains are widely used as starter bacteria in the production of cottage cheese and fermented milk. They not only produce lactic acid, but also contribute to the sensory features of cheese. They belong to the order _Eubacteriales_ and the family _Streptococcaeae_. _L. _lactis_ are Gram-positive cocci forming single pairs or short chains (0.5-1.5 \(\upmu\)m) in their growth medium. These facultative anaerobic and catalase-negative bacteria do not form spores. The optimal temperature for their growth is 30 \({}^{\circ}\)C, but they can also grow between 10 \({}^{\circ}\)C and 37 \({}^{\circ}\)C at higher salt concentrations (>4% sodium chloride) [1]. The production of acid from different sugars by these microorganisms varies depending on the species. They are not able to grow in a medium containing 6.5% NaCl or at pH 9.6. They play important roles in the protection and safety of dairy products through the secretion of antimicrobial agents, e.g., lactic acid, diacetyl, hydrogen peroxide, and bacterioxins. In addition to the morphological and biochemical properties of _Lactococcus_ strains, it is important to determine the technological and probiotic potential of strains isolated from fermented milk [2, 3]. Lactic acid bacteria (LAB) present in raw milk mainly come from teats and udders, milking and storage equipment, and the animal's environment [4]. Therefore, wildtype lactococci isolated from milk can be new strains with desirable properties [5]. For easier and more effective isolation of strains, raw milk should be subjected to spontaneous lactic fermentation, which contributes to the multiplication of bacteria; interruption of the process in a timely manner helps to isolate lactococci. Their ability to ferment lactose is critically important, particularly for their use as starter cultures in milk processing [6; 7]. In cheese production, lactococci not only ferment lactose, lower the pH and cause milk to coagulate, but they also take part in the formation of specific sensory characteristics based on the enzymatic breakdown of milk proteins and fats [8]. Scientists and practitioners believe that the selection of autochthonous strains adapted to production conditions, is a guaranteed method of reproducibility and standardisation of the product without losing its typical features [9]. Despite the availability of various types of starter cultures containing _L. lactis_, there is still a need to look for new strains combining the desired technological features (resistance to various NaCl concentrations or temperatures and acidifying abilities) and primarily probiotic functional features, taking into account safety aspects. Probiotic microorganisms should survive in the human digestive tract and help in the maintenance of intestinal microbiota. They should also be resistant to antibiotics and exert health-promoting effects such as immunomodulation, anti-inflammatory activity, antagonistic activity against pathogens, inhibition of toxin activity, enhancement of nutrient bioavailability, and reduction in lactose intolerance [10; 11]. The safety of probiotic strains for the consumer is also very important, and this includes e.g., non-pathogenicicity and non-toxicity, appropriate origin, and lack of haemolytic potential and gelatinase activity. Among the features related to non-toxicity, the lack of amino acid decarboxylases is important, because biogenic amines cause a number of adverse effects in consumers, e.g., headaches or skin irritation [12]. These properties should be confirmed by the GRAS (generally regarded as safe) status [13]. Strains intended for starter cultures should also be viable during the technological processes and storage of products. Therefore, the use of a good starter culture containing defined strains can guarantee the safety, quality, and acceptability of fermented dairy products obtained in a traditional and innovative way [14].