148 MANUAL OF MICROBIOLOGICAL METHODS 



have been made and, if possible, have been shown to bear some relation 

 to the phase indicated by colony type. 



MICROMETHODS 



Attention is called to the type of methods for determining biochemical 

 characteristics known as ''quick" methods or micromethods. The 

 methods in question depend on making mass inoculations into media pre- 

 heated to 37°C and making readings after very brief periods of incubation. 

 The principle involved is that the enzymes produced act so quickly that if 

 mass inoculations of vigorous cultures are made, action of the enzymes 

 can be studied almost like a chemical reaction without waiting for further 

 bacterial growth to occur. A method depending on this principle, for 

 determining rennet production (see page 166), was given in several editions 

 of the old manual, but it is not considered to be so well standardized as 

 those more recently worked out by Weaver in the United States and 

 Cowan in England, together with their respective associates. The quick 

 tests prove quite effective for determining nitrate reduction, indole, H2S, 

 and acetyl-methyl-carbinol production and in some cases for sugar 

 fermentations as well as for the methyl red te^t. Studies by Cowan 

 (1953a and h) have shown, however, that some micromethods are not 

 readily suited to routine work. Determinations such as the methyl red 

 test and fermentations of carbohydrates give variable results unless fac- 

 tors such as buffer concentrations, substrate concentration, and quantity 

 of cells are carefully controlled. In addition there is always the possi- 

 bility that a cultural test, micro or macro, may be made so sensitive that 

 the property being tested loses its diagnostic significance. An illustration 

 of this is the Batty-Smith procedure for detecting acetylmethyl carbinol 

 (Shaw et al., 1951) and several methods for the determination of H2S 

 (Clarke, 1953). This point is discussed by Clarke and Cowan (1952). 



CULTURAL CHARACTERISTICS 



Space is provided on both charts for recording appearance of colonies, 

 growth on agar stroke, in broth and gelatin stab. In addition to the 

 space provided for sketches, various terms are listed in order that those 

 which apply may be underlined. The meaning of all the terms is given in 

 the glossary (Chap. XIII). 



As some of the terms, especially in regard to shape and structure of 

 colonies, are more easily described graphically than verbally, the diagram 

 in Fig. 4 is included here to assist the student in understanding the 

 appropriate terms. 



