106 ACTIVITIES OF BACTERIA. 



periment is repeated with more dilute serum if it results 

 positively in the first instance. 



More accurate than the macroscopic is the microscopic 

 method, also suggested by Gruber and Durham. In the 

 examination of human blood rarely is there more serum 

 at hand than is required for the microscopic examination. 

 After the serum has been obtained by centrifugation in 

 two glass capillaries 1 cm. in length (see above), the serum 

 from one tube is blown out into a cell by means of a fine 

 tube placed above it, and then has added to it bouillon from 

 5 segments, each 10 cm. long, of a tube of similar size to that 

 which contained the serum. In a hanging drop, by means of 

 the immersion lens, it is observed whether agglutination 

 of introduced bacteria occurs. This follows, if the reaction 

 is strong, in a few seconds; if the action is weaker, in 

 from ten minutes to one hour. It is observed that the 

 organisms suffer a loss of motion, become someAvhat 

 swollen (rarely seen) and cemented together in irregular 

 bunches and clumps. Single bacilli often remain longer 

 motile. If the reaction is not promptly positive, the 

 preparation is kept in the incubator and examined after 

 half an hour and one hour. Positive results after two 

 hours are not of much value. Control preparations with- 

 out serum, but with bouillon only, should always be pre- 

 pared by the beginner, so as not to mistake a sedimenta- 

 tion, etc., for agglutination. x 



If the action occurs with a dilution of -£$, then it is a 

 positive reaction, and it can then be determined whether 



rio> tfo, *iv> two o> and mnro are also active, Unnecessary 

 dilutions being prepared preferably by further dilutions of 

 the first sample. If no result occurs with the dilution of 

 -fa, then the reserved centimeter tube of serum is diluted 

 -fa; and if still no reaction is obtained, the diagnosis is ab- 

 solutely negative. In general it is customary to attach no 

 value to reactions with higher concentrations than -fa to fa. 

 (Compare further under Bact. typhi and Vibrio cholera?. ) 

 The reaction is in a great degree specific (see below). 



1 Cultures killed with chloroform vapor are likewise agglutinated ; 

 also some non-motile varieties, as Streptococcus lanceolatus, Bacterium 

 pestis, and Bact. pneumoniae, have been caused to agglutinate by spe- 

 cific sera. 



