[Identification of Citrobacter species and their occurrence in raw products and foods]

Fifty-nine strains of bacteria were tested in study that were isolated from raw materials and foods (raw milk, farm milking parlors, sausage meat, raw potatoes, cheese, frozen oven-ready foods, confectionery products, cold dishes), and they were included in the genus Citrobacter using a commercial diagnostic kit ENTEROtest 16 (Lachema a.s., Brno), numeric identification program TNW (Czech Collection of Microorganisms, Faculty of Science of Masaryk University at Brno) and identification key (O'Hara et al., 1995). The results of the ENTEROtest itself, including OXI and ONPtests, did not provide satisfactory discrimination of detected strains to the level of species since 64.4% were identified by the genus by TNW program or designated as intermediary strains. Correct species identification was obtained in 33.9% only (Tab. I). Five next conventional tests (dulcitol, alpha-methyl-glucoside, raffinose, melibiose, arginine dihydrolase) were used and their results successfully specified 94.9% tested strains to the species level (Tab. I). A dichotomic identification key (O'Hara et al., 1995) enabled to classify the strains on the basis of the results of ENTEROtest 16 and two conventional tests (dulcitol, melibiosis) relatively easily, and it can be recommended for routine identification of typical Citrobacters from foods. Only ten strains were classified in a wrong way (16.9%) due to false results in ENTEROtest (Tab. II). Tab. III shows the strains classified wrongly by both identifications systems. The tested set contained Citrobacter braakii in a majority of cases (30 strains), followed by C. freundii (17 strains), C. youngae (6 strains), 2 strains of genomospecies 10 and one strain of C. koseri, C. amalonaticus and C farmeri. Tab. IV shows the presence of Citrobacter species in the particular raw materials and foods. The most frequently present species in raw materials: C. braakii (26 strains, 68.4%), C. freundii (3 strains, 7.9%) and C. youngae (3 strains, 7.9%). Foods contained these three species only: C. freundii (14 strains, 66.7%), C. braakii (4 strains, 19.0%) and C. youngae (3 strains, 14.3%). The percentage of the three most frequently present species in raw materials is substantially different from their percentage in foods (Fig. 1). The higher percentage of C. freundii presence in foods can be ascribed to secondary contamination.