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The effect of enzymatic hydrolysate of casein, NH4+, a mixture of volatile fatty acids (acetic, n-valeric, isovaleric, 2-methylbutyric, and isobutyric), hemin, and ruminal fluid on growth of 89 freshly isolated strains of predominant culturable ruminal bacteria was studied, using basal media containing glucose, cellobiose, or maltose as energy source, minerals, cysteine, and S= as reducing agents, and H2CO3-HCObuffer. Of these strains, 13% (four morphological groups) grew poorly or not at all in defined medium plus casein hydrolysate; 6% (one morphological group) required casein hydrolysate; 56% (four morphological groups) grew with either NH4+ or casein hydrolysate as the main source of nitrogen; and NH4+, but

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The 5S rRNA sequences of eubacteria and mycoplasmas have been analyzed and a phylogenetic tree constructed. We determined the sequences of 5S rRNA from CMstridium innocuum, Acholeplasma laidlawii, Acholeplasma modicum, Anaeroplasma bactoclasticum, Anaeroplasma abactoclasticum, Ureaplasma urealyticum, Mycoplasma mycoides mycoides, Mycoplasma pneumoniae, and Mycoplasma gallisepticum. Analysis of these and published sequences shows that mycoplasmas form a coherent phylogenetic group that, with C. innocuum, arose as a branch of the low G+C Gram-positive tree, near the lactobacilli and streptococci. The initial event in mycoplasma phylogeny was formation of the Acholeplasma branch; hence, loss of cell wall probably occurred at the time of genome reduction to lO000 MDa. A subsequent branch produced the Spiroplasma. This branch appears to have been the origin of sterol-requiring mycoplasmas. During development of the Spiroplasma branch there were several independent genome reductions, each to -500 MDa, resulting in Mycoplasma and Ureaplasma species. Mycoplasmas, particularly species with the smallest genomes, have high mutation rates, suggesting that they are in a state of rapid evolution.The wall-less prokaryotes are grouped in the class Mollicutes and are referred to as mycoplasmas (1). Mycoplasma genera (Table 1) are diverse in terms of their biochemistry, ecological niches, and genome structure. In addition, some of these organisms are significant animal, plant, and insect pathogens. Two aspects of mycoplasma biology are of particular interest. First, some mycoplasmas (i.e., Spiroplasma, Mycoplasma, Ureaplasma, and some Anaeroplasma species) are the only prokaryotes known to require cholesterol for growth; and, second, mycoplasmas have the smallest amounts of genetic information of any free-living organisms. Mycoplasma DNAs contain a low percentage of G+C.Mycoplasma genomes fall into two size classes. Acholeplasma, Spiroplasma, and Thermoplasma genomes are "1000 MDa, a size that is rare but not unknown among eubacteria (reviewed in ref.2). Mycoplasma and Ureaplasma genomes are "500 MDa, the smallest reported cellular genomes. This is only slightly larger than the theoretical minimal amount of genetic information for a cellular system (4), indicating that the complexity of mycoplasma cells must be constrained by their limited genome content.The origin and phylogenetic relationships of mycoplasmas have been of interest because these cells are the result of natural selection operating within the constraint of limited genome complexity. In the past decade, a number of studies indicated that particular biochemical properties of some Mycoplasma and Acholeplasma species are closer to those of Gram-positive than to Gram-negative eubacteria (reviewed in refs. 2 and 3). A more complete view of mycoplasma evolution came from a recent comparative analysis of 16S rRNA oligonucleotide catalogs of Acholeplasma laidlawii, Spiroplasma citri, Mycoplasma capricolum, Mycoplasma gallisepticum, and Thermoplasma acidophilum with ...

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