Prontosil, also called sulfamido chrysoidine, trade name of the first synthetic drug used in the treatment of general bacterial infections in humans.

Prontosil resulted from research, directed by German chemist and pathologist Gerhard Domagk, on the antibacterial action of azo dyes. A red azo dye of low toxicity, prontosil was shown by Domagk to prevent mortality in mice infected with Streptococcus bacteria.

The dye was also effective in controlling staphylococcus infections in rabbits. Within a relatively short period, it was demonstrated that prontosil was effective not only in combating experimental infections in animals but also against Streptococcal disease in humans, including meningitis and puerperal sepsis. Structural formula of prontosil is

From the structure of prontosil, it is very clear that it has $-\mathrm{N}=\mathrm{N}$ - linkage. It was discovered that the part of the structure of prontosil molecule shown in box, i.e., p -aminobenzenesulphonamide has antibacterial activity.

Salvarsan is also known as arsphenamine. It was introduced at the beginning of 1910s as the first effective treatment for syphilis. It is an organoarsenic molecule and has $-$As $=$ As$-$ double bond.

Salvarsan and prontosil show similarity in their structure. Both of these drugs are antimicrobials. Salvarsan contains $-\mathrm{As}=\mathrm{As}$ — linkage whereas prontosil has $-\mathrm{N}=\mathrm{N}$ linkage.

Prontosil (a red azo dye) and azo dye both have $-\mathrm{N}=\mathrm{N}-$ linkage.