Robe Koch assistant advised him to use agar instead of gelatin for solidifying culture media for cultivation of bacteria because

Gelatin was the substance originally used to gel culture media. However, gelatin is liquid at 35oC., the optimum temperature for many bacteria. In addition, some bacteria are capable of liquifying the gelatin, making it an unsuitable solidifying agent. Agar has neither of these drawbacks. Agar is preferred over gelatin, as it is bacteriologically ine, melts at 98degC and usually solidifies at 42degC.