The microbial flora of 141 samples of raw ewes' milk was determined, before and after storage for 72 h at 4 degrees and 7 degrees C. Penicillin-resistant bacteria represented ca 61% of 1760 psychrotrophic isolates from refrigerated milk samples. Pseudomonas fluorescens and Pseudomonas fluorescent group-related strains predominated (ca 86%) in the Gram negative psychrotrophic microflora. Leuconostoc dextranicum was the most frequent Gram positive psychrotrophic species isolated.

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One hundred forty-four samples of Burgos and Villalón cheeses collected during April and July 1983 from retail outlets in Madrid, Spain, were analyzed for microbiological quality. Geometric mean counts were 6.8 × 105 staphylococci/g, 1.0 × 105 coliforms/g and 7.9 × 103 yeasts/g for Burgos cheese, whereas the respective mean counts for Villalón cheese were 1.4 × 106/g, 8.7 × 104/g and 1.9 × 104/g Coagulase-positive staphylococci represented in April and July, 7.9 and 42.2%, respectively, of the isolates from egg-yolk tellurite glycine agar plates. Fecal coliforms accounted for 5.6 and 26.9% of the isolates from violet red bile agar plates in April and July, respectively. Presence of alkaline phosphatase activity in cheese was not significantly related to numbers of staphylococci or coliforms and should only be regarded as a presumptive test. The use of pH values or total counts as indicators of the microbiological quality of Burgos and Villalón cheeses is also discussed.

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Derivatives of the lactose miniplasmid pMG820 were constructed in which a staphylococcal erm gene was inserted and in which this was accompanied by subsequent deletion of the lactose genes. The resulting plasmids were thus marked with both erythromycin resistance and lactose utilization genes in pFI132 or solely erythromycin resistance in pFI133. These plasmids retained the normal conjugation properties characteristic of lactose plasmid pLP712, including the generation by intermolecular rearrangement of high-frequencytransfer Clu+ derivatives which exhibited cell aggregation. The use of such Clu+ plasmids in a variety of mating experiments between different lactococcal strains and the observation of cell aggregation when particular mating mixtures were made led to the discovery of a new component of this conjugation system named Agg. A chromosomal gene agg was postulated to be present in some but not all strains of lactococci. High-frequency conjugation and cell aggregation thus depend on the presence of both Agg and Clu, although in a mating pair these components can be in the same or in separate strains. The Agg and Clu components may be analogous to the binding substance and aggregation substance that are involved in the hemolysin plasmid transfer system of Enterococcus faecalis, although control of their expression is different.