SuMlMARYSeveral antibiotics were tested against a range of actinomycetes, bacteria and fungi representing types found in soil. From these tests four antibiotics, nystatin (50 pg./ml.), actidione (50 pg./ml.), polymyxin B sulphate (5.0 pg./ml.) and sodium penicillin (1.0 ,ug./ml.), were selected for incorporation into a starch +casein medium to achieve selective growth of actinomycetes on soil dilution plates. This mixture of antibiotics was tested with a number of soils and its efficiency compared with several other methods for selective development of actinomycete colonies. The most suitable mixture for the enumeration of soil actinomycete colonies was starch + casein medium with the two antifungal antibiotics (nystatin, actidione) ; for isolation of actinomycete colonies the same medium with all four antibiotics was the most satisfactory.

Conjugal transfer of the Streptococcus lactis 712 lactose plasmid was found to occur at a low frequency. Variants of this plasmid were selected which had much greater donor abilities and which also exhibited an unusual cell aggregation phenotype.

Plasmid DNA from lactic streptococci was subjected to electrophoresis on agarose gels. The plasmid profiles so obtained were strain specific and sufficiently stable to suggest their use in strain differentiation. A group of Streptococcus lactis strains, 712. 763 (ML3), 505 (C2) and 2031 (C2), found to have similar plasmid profiles, were shown to be closely related. Gene transfer by transduction and conjugation occurred between members of this group at frequencies comparable to those in homologous systems and temperate phages cross plated readily between their prophage cured derivatives. Minor variations were, however, found between these four strains; slight differences in plasmid profiles, lysogenic status, prophage curability and temperate phage morphology were detected and it is suggested that these have evolved as a result of maintenance in different environments.

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SUMMARYOrganisms belonging to several genera of the order Actinomycetales were examined using the ' Stereoscan ' electron microscope of the Cambridge Instrument Company. Unlike the transmission electron microscope, this instrument provides surface views of fine structures without the need for preparation of replicas. Using a simple preparation procedure, surface views of intact sporing structures of several actinomycetes were observed over a range of magnifications. The potential value of using scanning electron microscopes for the examination of actinomycetes was assessed.