Vergleichende Untersuchungen zum Nachweis von Salmonellen in äthiopischem Hüttenkäse (Ayib) mit verschiedenen kulturellen Verfahren

Investigations were carried out on survival and growth of Salmonella during traditional making of ayib (Ethiopian cottage cheese), the most effective method for isolation of Salmonella from ayib and the incidence of Salmonella in ayib, some other foods and drinking water in Ethiopia. In the first part of experiments ayib was produced in the laboratory with similar process to that of traditional making of ayib in Ethiopia. Raw milk was artifically contaminated with three different Salmonella serovars (S. Typhimurium, S. Enteritidis und S. Infantis), and after addition of starter culture incubated at 27 °C for 24 h. During the process of fermentation the pH of soured milk dropped from 6,79 to 4,53 within 24 h and growth of S. Typhimurium, S. Enteritidis and S. Infantis reached a cell density of 4.8 x 10 5 , 3.8 x 10 8 and 3.1 x 10 7 /ml, respectively. None of the Salmonella serovars could survive the heating process (curd formation) combined with the low pH. The average pH of fresh ayib was 4.37, dropped to 4.17 in 72 h and remained relatively constant thereafter. In ayib contaminated with Salmonella after heating process, all tested serovars could survive at least for 24 h and the product can be considered free from Salmonella only after 4 days at which time, however, ayib is no more palatable due to sensoric properties. A total of 44 different culture procedures were compared for their efficacy in isolation of Salmonella from ayib. In general a higher recovery of Salmonella has been observed in all methods involving a preenrichment procedure. The best result in terms of sensitivity (100 %) and specificity (100 %) could be obtained with combinations of Rappaport-Vassiliadis Soya Pepton Broth (RVS)/Mannitol Lysine Cristal Violet Brilliant Green Agar (MLCB) and RVS/Xylose Lysine Desoxycholate Medium (XLD). The lowest sensitivity rate (77.8 %) was recorded in the combination of Muller-Kaufmann Tetrathionate Medium (MKT)/Rambach Agar. Comparison between Oxoid Salmonella Rapid Test (OSRT), Modified Semisolid Rappaport-Vassiliadis (MSRV) and the reference method (preenrichment/selectiv enrichment/isolation) revealed that these three methods can be used with similar effectiveness. When the alternative procedures employed in the reference method were compared, their effectiveness in yielding higher rate of sensitivity decreased in the following order: Selenit Cystin Broth (SC)/Brilliant Green Agar (BGA) (97.4 %), SC/MLCB (94.7 %), Rappaport-Vassiliadis enrichment broth (RV)/BGA (92.3 %) and RV/MLCB (89.5 %). The MSRV medium could be used with higher effectiveness when inoculated from preenrichment medium than from selective enrichment medium. All methods involving preenrichment exhibited a higher efficacy in isolation of S. Typhimurium than in other Salmonella serovars. From the total of 639 food and water samples tested for Salmonella in Ethiopia (Awassa) only 7 (1.1 %) were found to contain Salmonella. The samples contaminated with Salmonella were ayib (1), raw milk (1), kitfo (1), cream cakes (1) and water (3). The three different Salmonella serovars isolated for the first time in Ethiopia were S. Braenderup, S. Hall and a rough form.