Streptomycin resistance, when caused by a chromosomal mutation, has been
shown to be due to an alteration of specific protein on the 30S ribosomal
subunit to which streptomycin binds in the sensitive cell. Another class of
streptomycin resistant bacteria owes their resistance to the presence of plasmid
which mediates either the acetylation, adenylation or phosphorylation of the
drug molecule, itself. According to Holt et al., (1994) spontaneous mutants for
resistance to most antibiotics are a component of all "wild type" strain.