AIM: Orthodontic treatment can be done by mobile and fixed orthodontic appliances. Fixed orthodontic appliance is constructed by brackets which are fixed on the patient's teeth by the composite material and archwires which are tied in the brackets with ligatures or with clips integrated in the brackets. Brackets are usually made by stainless steal, titanium, ceramics or plastic, and archwires are made of different alloys of iron, nickel, titanium and molybdenum. NiTi wires have been modified for aesthetic and functional reasons, rhodium coating of the wire improves aesthetic properties of the wire while nitrided surface of the wire shows better mechanical properties. The length of the orthdontic treatment and difficulty maintaining oral hygiene during orthodontic treatment can lead to the development and action of cariogenic bacteria such as Streptococcus and Lactobacillus. Acidic media and bacterial activity in presence of carbohydrates can lead not only to damage of tooth enamel and caries but also to corrosive damage of dental alloys, release of metal ions and development of hypersensitivity. For the sake of maintenance of oral health, biological methods such as probiotics are introduced nowadays, which are based on restoring microbiological ecological balance. One of the probiotic bacteria used to maintain oral health is Lactobacillus reuteri. Favorable effect of L. reuteri is attributed to the ability of these bacteria to produce antimicrobial substances such as reuterin, reutericin and reutericyclin. Corrosion of metallic materials can be induced by the presence of microorganisms such as different strains of aerobic and anaerobic bacteria and fungi. Probiotic bacteria show some corrosive potential, however their impact on orthodontic materials has not yet been sufficiently investigated. The aim was to determine if the probiotics have additional effect on the surface of two principle alloys used in dentistry above the influence of saliva and whether this influence is modified by the alloy's coating. MATERIALS AND METHODS: Research was conducted on four types of archwires with dimensions 0.508×0.508 mm – three nickel-titanium (NiTi) alloys with uncoated surfaces, rhodium coated and titanium nitride coated, as well as one archwire made of stainless steel (SS). The archwires were exposed to pure artificial saliva (pH=4.8), saliva with the addition of probiotic bacteria Lactobacillus reuteri Prodentis, and saliva containing BioGaia Prodentis probiotic supplement for a period of 28 days at a temperature of 37±2°C. Unexposed archwires were used as control. Surfaces of the exposed and unexposed wires were qualitative examined by the scanning electron microscope (SEM) and quantitatively by atomic force microscope (AFM). Type and speed of archwire corrosion were examined by using the electrochemical methods of cyclic polarization and electrochemical impedance spectroscopy. Mechanical properties of the wires such as friction, microhardness and surface roughness were also analysed. A universal material testing machine was used to measure the friction of exposed and exposed wires. Microhardness was analysed according to Vickers, surface roughness was analysed by means of a scanning electron microscope and an atomic force microscope. Raman spectroscopy was used to identify metal oxides on the alloy surface. RESULTS: Tested probiotic supplement and probiotic bacteria Lactobacillus reuteri influences corrosive and mechanical properties of the examined archwires. The influence depends on the alloy type and the coating. Probiotic supplement has greater influence than the bacteria themselves. The bacteria increase surface roughness of uncoated NiTi in addition to the influence of saliva (p < 0,001), but they decrease corrosive influence of saliva when the alloy is coated with rhodium or nitride. In stainless steel the bacteria decrease the corrosive influence of saliva. BioGaia probiotic supplement induces a higher surface roughness in all NiTi archwires but has no significant influence on friction or microhardness. Additional substances in the probiotic supplement influence corrosion above the influence of probiotic bacteria alone.