This form, discovered by Neisser, is now acknowledged to be the active agent in the causation of gonorrhoea. It is a large micrococcus, which is generally found united in twos (diplococcus), the two surfaces facing each other being usually concave.

The gonococcus is not to be detected by Gram's method, as the iodine decolorizes it. It may be stained by the ordinary aniline dyes, but it is best demonstrated as follows: - A cover glass which has been smeared with the pus is placed for a few minutes in a concentrated alcoholic solution of eosine (best with slight heating). The eosine is then removed with blotting paper and the preparation treated for a very short time (about a quarter of a minute) with a concentrated alcoholic solution of methylblue, and then washed in water. The cocci are now seen to be blue, whilst those enclosed in pus corpuscles have a red background, as the pus corpuscles take up the eosine.

.

The principal peculiarity of the gonococcus is that it is found in the substance of the pus cells (Fig. 140), often filling the protoplasm but leaving the nucleus free. It is exceedingly difficult to cultivate, success having been obtained by the use of human blood serum, and other media, but the growth has been comparatively slight. The identity of the coccus and its connection with gonorrhoea was proved by Bumm, who inoculated a culture of the third generation in the female urethra, and produced a typical gonorrhoea in three days. In certain animals, especially in white mice and guinea-pigs, inoculation of the abdominal cavity produces a suppurative peritonitis.