Except in the case of abscesses and certain skin infections, early studies of staphylococci furnished no definite information concerning their relation to disease. The term "skin cocci," used to designate albus varieties found in the skin, was in common use and even today persists in such trinomials as "Staphylococcus epidermidis albus." On the other hand, it was recognized early that cultures from pyogenic foci usually revealed pigmented types. This gave rise to a separation of the species into varieties based on their pigment production. It was generally considered that the white strains were not pathogenic. In this paper we will outline methods which we consider of value for the differentiation of pathogenic from non-pathogenic staphylococci. Many authors have shown that Staph. albus, and many of the intermediate-colored varieties, could be derived from Staph. aureus. Pinner and Voldrich (1932) also demonstrated that, of the strains which they examined, the yellow were more pathogenic than the white. Duran-Reynals (1933) showed that the albus variants differed from aureus parent strains in hemolysin production and the liquefaction of gelatin. He also found that all the very invasive strains were of the aureus variety while about 82 per cent of his non-invasive strains were of the albus variety. While a number of investigators have studied the pathogenicity of staphylococci, the results have been somewhat confusing as the following survey of the literature will demonstrate.
Although numerous in vitro tests have been proposed for differentiating staphylococci on the basis of pathogenicity the subject is confusing to many workers. This paper describes attempts to determine the significance of certain tests. The following seem to have been the more common sources of confusion: (1) Most investigators have used a series of cultures which was too small for statistical analysis; (2) groups of cultures from different sources, or obtained at different times from the same type of source, may not yield the same proportion of positive results; (3) results based on the source of the strains usually ignore the possibility of error; (4) differences in technic, or in the interpretation of results, may be sufficient to cause two investigators to arrive at opposite conclusions; only infrequently has an author described tests, or their interpretation, with sufficient precision to permit others to duplicate them; (5) the dissociation of a strain, resulting in loss of one or more properties in some cells of the culture, causes it to become a mixture of variants with different pathogenic properties; and (6) pathogenic cultures degenerate or dissociate at different rates and in different ways, depending upon a number of poorly understood factors.
Although methods are available for testing the gastroenterotoxic properties of staphylococci, it is generally agreed that food-poisoning strains cannot be diEerentiated with certainty from other types of staphylococci. For this reason, preliminary diagnosis of staphylococcus food poisoning depends upon finding this organism in increased numbers in the suspected food or vomitus. Where only a small number is present, the interpretation is d B c u l t and it becomes necessary to rely on various tests, most of which have not proved entirely satisfactory.It will be shown in this paper that food-poisoning staphylococci possess properties in common with other pathogenic staphylococci which make it possible to isolate them by the method of Chapman, Lieb, and Curcio. I n addition, food-poisoning staphylococci possess characteristic properties which are useful for making a rapid presumptive diagnosis.Several factors have complicated the investigation of food-poisoning staphylococci: (1) Only a small number of strains has been available for investigation. ( 2 ) There is no assurance that all "strains" of staphylococci which have been considered the cause of food-poisoning outbreaks represent the original gastroenterotosic strains o r that they are genetically related to them. ( 3 ) Stock strains have been treated as if their properties remain unaltered indefinitely. It will be shown that many of them contain dissociants which may be regarded as degenerate forms of the parent strain and which, when present in excess, may alter the properties of the strain. (4) Testing for gastroenterotoxic properties has not been entirely satisfactory and has been confused by factors mentioned in ( 3 ) . (5) I t is possible that other pathogenic staphylococci, not involved in food-poisoning outbreaks, may possess gastroenterotoxic properties.The 56 food-poisoning cultures used in this investigation were obtained through courtesy of the following investigators, to whom the authors are deeply indebted for their helpful cooperation :
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