CULTURAL REACTIONS 609 



The differentiation between these two types morphologically, therefore, is not 

 always easy ; on a single medium alone it may be impossible, but if the appear- 

 ance of the coccus is studied in both liquid and solid media, not much difficulty 

 will be experienced. The differential points are (1) that the chains formed by 

 staphylococci rarely contain more than four members and (2) that the clusters 

 formed by streptococci generally consist of aggregations of chains ; the chain is 

 the fundamental unit of the streptococcus. 



Staining Reactions. — The staphylococci stain well with most of the aniline dyes, 

 and are uniformly Gram-positive. It is not uncommon, however, to see them 

 described as being sometimes negative. Winslow, Rothberg and Parsons (1920), 

 in a study of 180 strains, encountered 5 Gram-negative strains, and the same 

 authors make the generalization that, whereas the orange and white cocci are 

 Gram-positive, the yellow and red ones, including the Sarcince, are Gram-negative. 

 This discrepancy can be explained by the facts, firstly that different types of 

 staphylococci do vary in their resistance to decolorization, and secondly that 

 many strains which are Gram-positive in an 18 or 24 hours' culture become Gram- 

 negative as they grow older. To obtain uniformity, therefore, it is essential to 

 use a young culture — never more than a day old — and not to prolong unduly the 

 process of decolorization. If these precautions are taken, it will be found that 

 almost without exception the staphylococci, at any rate on first isolation, as also 

 the sarcinae, react positively to Gram's stain. 



Cultural Reactions. — The staphylococci are among the easiest of micro-organisms 

 to cultivate in vitro. Though some develop more slowly than others, particularly 

 Staphylococcus citreus, they all give abundant growths. 



In nutrient broth after 24 hours at 37° C. there is a moderate to dense turbidity, 

 with a moderate deposit of a powdery nature, which, on shaking, swirls up and 

 disappears completely, increasing the turbidity. This is the usual picture. But 

 some types, the salivary staphylococci, for example, form a thick, weedy, glutinous 

 deposit, leaving the supernatant fluid clear. After 2 days' incubation, a surface 

 ring growth is generally present. In no case is 

 there any distinctive pigment formation in fluid ''; '~~ 



media. 



On nutrient agar, there is produced within 1 to 



2 days a moderately thick, raised, confluent growth, 



with a moist, glistening, smooth, or somewhat 



honey-combed surface — due to the imperfect fusion 



of individual colonies ; in most cases it is of 



butyrous consistency and easy to emulsify, but in 



the case of the salivary staphylococci, it is glutinous, 



adherent to the medium, and more difficult to 



emulsify. Pigment production is most obvious on ''" ~^ureus^ ococcus 



agar at 22° C. ^ t ^ »* 



° . o ^ 1 1 -1 bunace colony on agar, 24 



On gelatin plates at 22 C. development is slower, hours, 37° C. ( x 8). 



there being often no visible growth for 2 to 3 days ; 



the colonies which are then formed are small and relatively unpigmented ; later 



a zone of liquefaction may appear around them. 



In gelatin stab cultures there is a filiform growth reaching to the bottom of 



the tube, and a surface growth of variable degree. Liquefaction may or may not 



occur ; when it does, the usual type given by the white and golden cocci is 



P.B. X 



