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Antisera were raised against rods of 17 named Arthrobacter and Aureobacterium strains. Antigenic relationships between these strains, other soil bacteria and new Arthrobacter isolates from several soils were studied, using agglutination, immunodiffusion, immunofluorescence and ELISA techniques. Many of the named Arthrobacter species had common antigens, and there were also common antigens amongst named Arthrobacter strains and many fresh Arthrobacter isolates. Agglutination, ELISA and immunofluorescence tests revealed greater antigenic differences between the named strains than did immunodiffusion tests. Serological similarities between the 17 named strains and the fresh Arthrobacter isolates were calculated using SJ coefficients. The occurrence of named strains in serogroups based on immunodiffusion data supported the taxonomic scheme for arthrobacters in Bergey's Manual of Systematic Bacteriology. The distribution of soil isolates in serogroups resembled that in groups based on numerical analysis of diverse characters. This makes it possible to use serological tests to locate particular species in soil samples. Arthrobacter atrocyaneus was serologically distinct from other members of the Ar. globiformis/Ar. citreus group and did not cluster with them phenetically. Serological data suggest that Ar. aurescens, Ar. ureafaciens and Ar. histidinolovorans constitute a single species. Although A. simplex and A. tumescens have been placed in the genus Pimelobacter, they are serologically distinct from one another and more closely resemble Ar. globiformis. Aureobacterium strains were serologically distinct from arthrobacters. Micrococcus roseus showed many cross-reactions with several antisera, supporting the placement of micrococci in the same family as arthrobacters.

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The fluorescent-antibody technique was used to identify cells and spores of Bacillus subtilis and cells of B. circulans from soil. From cells grown in three broth media of different nutrient status, i.e., a cold extracted soil medium (CSE), an unamended autoclaved soil extract (HSE), and nutrient broth (NB), antisera were produced with both quantitative and qualitative differences in antibody content. The specificities of antisera to two strains of each of the Bacillus species were determined. Antisera for B. subtilis O antigens were species-specific and showed no cross-reactions, whereas those for the B. circulans O antigens were strain-specific and in some cases showed cross-reactions with B. alvei. This cross-reaction was removed by absorption of the antiserum with B. alvei O antigen. Fluorescein isothiocyanate gamma-globulin conjugates prepared from these antisera showed the same specificity reactions. A method for staining bacteria on soil particles was developed, by use of small staining troughs. By mounting stained soil particles on slides and irradiating them with transmitted and incident ultraviolet blue light, bacteria on both mineral and organic particles, taken directly from soil, could be observed. Fluorescent antibodies against cells grown in CSE gave brighter fluorescence of stained bacteria on soil particles than did fluorescent antibodies against cells grown in either HSE or NB. Colonies of both Bacillus species were generally small and localized. Spore antisera, though not rigorously tested for specificity, were used to identify spores of B. subtilis on soil particles. The uses and implications of the technique in soil bacteriology are discussed.

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Details of the surface structure of microorganisms growing in soil and the complex topography of individual soil particles were observed with the Stereoscan electron microscope. Because it was not necessary to remove the microorganisms from the soil particles, it was possible to establish their occurrence in different microhabitats. This technique can provide useful ecological information about the soil microflora.

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