Enumeration of Enterobacteriaceae in various foods with a new automated most-probable-number method compared with petrifilm and international organization for standardization procedures.
An automated most-probable-number (MPN) system (TEMPO, bioMérieux, Marcy l'Etoile, France) for enumeration of Enterobacteriaceae (EB) was compared with methods involving violet red bile glucose agar (VRBG) (International Organization for Standardization [ISO] method 21528-2) (ISO-VRBG) and Petrifilm (PF-EB). The MPN partitioning (three different volumes with 16 replicates of each) is done automatically in a disposable card. Bacterial growth is indicated by acid production from sugars, lowering the pH of the medium, and quenching the fluorescence of 4-methylumbelliferone. After incubation, the number of nonfluorescent wells is read in a separate device, and the MPN is calculated automatically. A total of 411 naturally contaminated foods were tested, and 190 were in the detection range for all methods. For these results, the mean (+/- standard deviation) counts were 2.540 +/- 1.026, 2.547 +/- 0.995, and 2.456 +/- 1.014 log CFU/g for the ISO-VRBG, PF-EB, and automated MPN methods, respectively. Mean differences were -0.084 +/- 0.460 log units for the automated MPN results compared with the ISO-VRBG and 0.007 +/- 0.450 for the PF-EB results compared with the ISO-VRBG results. The automated MPN method tends to yield lower numbers and the PF-EB method tends to yield higher numbers than does the ISO-VRBG method (difference not significant; Kruskal-Wallis test, P = 0.102). Thus, the average difference was highest between the automated MPN method and the PF-EB method (-0.091 +/- 0.512 log units). Differences between the automated MPN and ISO-VRBG results of > 1 log unit were detected in 3.4% of all samples. For 3.9% of the samples, one comparison yielded differences of < 1 log CFU/g and the other yielded > 1 but < 2 log CFU/g, which means that the differences are possibly > 1 log CFU/g. For the ISO-VRBG method, confirmation of isolates was necessary to avoid a bias due to the presence of oxidase-positive glucose-fermenting colonies. The automated MPN system yielded results comparable to those of the confirmed Enterobacteriaceae ISO-VRBG method but required only 24 h of analysis time.
Paulsen
P
P
Institute of Meat Hygiene, Meat Technology and Food Science, Department of Veterinary Public Health and Food Science, University of Veterinary Medicine Vienna, Veterinaerplatz 1, A-1210 Vienna, Austria. peter.paulsen@vu-wien.ac.at
Borgetti
C
C
Schopf
E
E
Smulders
F J M
FJ
D000362:Agar; D001331:Automation; D015169:Colony Count, Microbial / Q000379:methods* / Q000592:standards*; D004755:Enterobacteriaceae / Q000302:isolation & purification*; D005506:Food Contamination / Q000032:analysis*; D005516:Food Microbiology; D006801:Humans; D012016:Reference Values; D012680:Sensitivity and Specificity; D013997:Time Factors
D000362:Agar