PubMed · 26759
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R F Cosgrove, G T Jones. 1978. A modified agar diffusion assay for amphotericin B.. https://doi.org/10.1111/j.2042-7158.1978.tb13249.x
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We report that colonial Escherichia coli cells on various solid media can develop modest genetic competence. Using an on-filter culture system, we found that E. coli colonies on CaCl2-containing agar were transformed in the presence of plasmid DNA. Interestingly, transformation also occurred on LB-agar, various moist foods and even on H2O-agar. These results suggest that some populations of colonial E. coli in various environments could become transformable regardless of the surrounding Ca2+ concentration.
Five countries representative of laboratories 1-5 evaluated 11 different selective media, designed to suppress mould and bacterial growth and support yeasts growth, for the recovery of yeast populations from blue veined cheeses. In addition, qualitative results were also incorporated. The yeast enumeration values were subjected to statistical analysis using analysis of variance (ANOVA) and the Tukey-Kramer multiple comparison test. With the exception of Laboratory 3, none of the other laboratories was successful in recovering yeasts on all the media. Six of the media proved inadequate for the enumeration of yeasts in the mould invested environment and were therefore omitted from statistical analysis. No significant differences in quantitative data obtained on Rose-Bengal Chloramphenicol Agar (RBCA), Dichloran Rose-Bengal Chloramphenicol Agar (DRBC), Dichloran 18% Glycerol Agar (DG18), and Malt extract agar supplemented with NaCl and oxytetracycline (MES) were detected by four of the collaborating laboratories whereas one laboratory found RBCA to be superior for yeast enumeration. DG18 and Malt Extract Agar with Biphenyl (MEB), however, were ranked superior based on qualitative results compared to the other media, attributed to distinctive individual yeast colonies and mould inhibition. RBCA, DRBC, DG18, and MES on the other hand, all proved to be adequate in supporting yeast colony development for quantitative analysis in samples obtained from blue veined cheeses.
A completely selective enrichment procedure was compared with two partially nonselective ones for the detection of Listeria monocytogenes in cheeses. After enrichment for approximately 48 h, the enrichment media were streaked on selective agars and presumptive Listeria colonies were confirmed using PCR. In some cases, PCR was also performed directly on the enrichment broth. The conventional, completely selective enrichment procedure was not always the best choice for the detection of stressed L. monocytogenes in cheeses. Especially in the case of semi-hard cheeses from pasteurized milk and soft cheeses of the blue veined and the red smear types, the methods that incorporated a nonselective enrichment step gave better results than the completely selective method. For mold ripened, soft cheeses, the results were highly dependent on the brand of cheese and time of sampling, but the best results were obtained using the completely selective enrichment procedure.