The objectives of this study were to establish the prevalence of intestinal Spirochetes of the genus Brachyspira in Czech dogs and to determine the susceptibility of obtained B. pilosicoli isolates to selected antibacterial substances. Spirochetes were diagnosed microscopically in 23 out of 1139 samples of dogs' excrements, primarily intended for a parasitological testing. The cultivation of positive samples provided 10 brachyspira isolates, which were, on the basis of their biochemical activity and the results of the species-specific PCR, identified as B. pilosicoli (9 isolates) and B. hyodysenteriae (1 isolate). These dogs came from households. All the 7 tested isolates B. pilosicoli were sensitive to metronidazole and doxycycline, uniformly resistant to erythromycin, partly sensitive to cefazoline, lincomicine and ampicilline except for one isolate of B. pilosicoli, which was resistant to ampicilline.The second part of study was focused on dogs with diarrhoea that came from animal shelters, where a high prevalence of 58% (10/17) of B. pilosicoli was found. Spirochetal colitis, susceptibility testingIn the past, spirochetes have been observed and isolated from colonic excrements and mucosa of both healthy dogs and dogs with diarrhoea. However, the pathogenic importance of spirochetes and their role in the onset of canine diarrhoea have not been sufficiently explained until now (Pindak et al. 1965; Turek and Meyer 1978; Meyer et al. 1982;Lee and Hampson 1994;Duhamel et al. 1995, 1997, Fellstrom et al. 2001Oxberry and Hampson 2003). Weakly haemolytic and indole-negative spirochetes colonising colon and rectum of healthy dogs were characterised by PCR method specific for 16S rRNA and MLEE (multilocus enzyme electrophoresis) and classified as a new species designated as "Brachyspira canis". These spirochetes probably represent common canine intestinal commensals (Duhamel et al. 1998;Oxberry and Hampson 2003). Another species that has been isolated from canine and human faeces and the faeces of various animals in connection with diarrhoea and colonic infections is Brachyspira pilosicoli (Trott et al. 1996). This species tends to be detected in puppies with diarrhoea affected by a parasitic coinfection of Giardia spp. In adult dogs, it represents frequent finding also in the individuals without clinical symptoms (Turek and Meyer 1978;Duhamel et. al 1996). The cultures isolated from dogs were able to colonise chicken enterocytes via a mechanism similar to the one described in isolates of B. pilosicoli obtained from humans and pigs (Muniappa et al. 1996), which implies their pathogenic potential. Brachyspira pilosicoli is nowadays considered to be the causative agent of canine intestinal spirochetosis. Data on Brachyspira isolation from dogs came from studies on smaller groups of dogs or from sporadic cases of enteric infections documented more in detail (Oxberry and Hampson 2003; Skrzypcak et al. 2007). Reports on susceptibility of B. pilosicoli originating from dogs have appeared only rarely until now (Adac...