There are many chemical mutagens that have been used successfully to make bacterial mutants. In such cases its important to have a good screening method to detect which bacteria have the desired mutation and you must appreciate that multiple mutations may be generated in a single bacterium, complicating further analysis. Examples of chemical mutagens include nitrosoguanidine (MNNG, NTG), ethyl methane sulfonate (EMS), methyl methane sulfonate (MMS), diethylsulfate (DES), hydroxylamine and Nitrous acid. You may also wish to consider other options though, such as irradiation with UV, or transposon mutagenesis. The latter has been popular for gene knockouts, since it tends to produce "cleaner", single genomic location, mutants.