than 1 millimeter in diameter. Colony variants within one species may range from rough to smooth (matt to glossy) to mucoid. Subsurface colonies are usually lenticular. Carbohy- drate fermentation is homofermentative with dextro rotatory lactic acid as the dominant end-product. Carbon dioxide is produced in very small quantities or not at all from sugar fermentation. Ethanol, acetic acid and formic acid may be produced in appreciable quanti- ties from glucose if allowed to ferment in alkaline media (Gunsalus and Niven, Jour. Biol. Chem., 1I^5, 1942, 131). Many of the streptococci oxidize a number of the alcohols, glycols and short-chain fatty acids (Gunsalus and Wood, Jour. Bact., 44, 1942, 523; Gunsalus and Umbreit, Jour. Bact., 49, 1945, 347; Niven, Evans and White, Jour. Bact., ^9, 1945, 105; Wolin, Evans and Niven, Jour. Bact., 6^, 1952, 531), but the energy derived from these respiratory mechanisms is not utilized directly for growth. However, some enterococcus cultures may show moderately superior growth under aerobic conditions (Seeley and Van- Demark, Jour. Bact., 61, 1951, 27). The cytochrome systems are absent; catalase-negative. Do not reduce nitrate to nitrite. Facultative with respect to oxygen. Not soluble in ten per cent bile. Proteolytic strains found only in the enterococcus group. All streptococci are fastidious with respect to their nutritional requirements, thereby requiring a number of the B vitamins and amino acids for growth. The nutritional requirements are, in general, species specific, but some variations may be noted within one species. Some strains require an unsaturated fatty acid or increased carbon dioxide tension for growth. In some in- stances, the nutritional requirements may be of aid in the identification of a species. The reader is referred to Dubos (Bacterial and Mycotic Infections of Man. Lippincott, 1948, 240) for a general summary of the nutritional requirements for most of the species. Sero- logical tests, specifically the Lancefield precipitin technique, have proved to be of distinct value in classifjang the streptococci. In general, all streptococci except those in the viridans group possess a serologically active, group specific "C" substance (polysaccharide), thus allowing them to be placed into serological groups by the precipitin technique. One sero- logical group may comprise more than one species. Also, one serological group may possess a number of established serological types based upon the presence of capsular type-specific antigens and detected by the agglutinin or precipitin technique. Thus far, little relationship has been noted between serological type specificity and species specificity as established by their respective physiological characteristics. Several instances are known in which certain strains of streptococci possess common type antigens but belong to different serological groups. Members of the viridans group possess demonstrable type antigens but, for the most part, they have been of little taxonomic value. The streptococci are commonly found wherever organic matter containing sugars is accumulated. They occur regularly in the mouth and intestine of man and other animals, in dairy and other food products and in fermenting plant juices. Some species are pathogenic.
The arrangement of the species presented is in harmony with the suggestions of Sherman (Bact. Rev., 1, 1937, 3) in which the facultative streptococci are separated into four general groups: the pyogenic, the viridans, the enterococcus and the lactic groups. The arrange- ment of the species into these four categories was based upon a series of common physio- logical characteristics possessed by the respective groups, the most important of which was their temperature limits for growth. However, as new species are recognized, the indi- viduality of each group naturally becomes less distinct, and the difficulties of placing the new species, which may possess characteristics of more than one group, are increased. For example. Streptococcus tiberis Diernhofer, a member of the viridans group, possesses certain characteristics that would tempt one to place it in the enterococcus group, while Strepto- coccus acidominimus Ayers and Mudge, also a member of the viridans group, might be considered a member of the pyogenic group.
The type species is Streptococcus pyogenes Rosenbach.
It is difficult to draw up a workable key for streptococci. Keys, of necessity, emphasize single characters or, at most, only a few characters, and these are not necessarily the most
