Abstract: Recently, the emergence of new strains of bacteria resistant to most antibiotics, like Enterobacteriaceae, has killed dozens of people in different parts of the world. Here, two types of non-modified chitosan (CS), nominated CSA (High Molecular Weight, 85% deacetylation degree or Dd) and CSB (Medium Molecular Weight, 90.43% Dd) were tested for their antimicrobial activity against potentially pathogenic bacteria. The microorganisms tested were Staphylococcus aureus, Escherichia coli and Lactobacillus casei. The inhibitory effects on growth were determined by the agar-diffusion test; among the strains analyzed, CSB showed an inhibitory effect against E. coli and L. casei. The statistical analysis of antimicrobial activity show that it was more evident with E. coli during the first five hours of incubation with CSB ranging from 0.5 to 2.0% but CSB 1.0% showed the maximum inhibitory effect until 72 h. For L.casei, CSB concentrations ranging from 0.5 to 2.0%, showed inhibitory effects in the 72 h study but 1.5% give the greatest inhibitory effects. The antimicrobial activity to E. coli was confirmed by Transmission Electron Microscopy (TEM) which indicates that damage to cell wall and membranes of E. coli with loss of cell contents was the main inhibitory action of CSB.