Impact of sodium chloride on Escherichia coli O157:H7 and Staphylococcus aureus analysed using transmission electron microscopy

Abundant literature information is available on sodium chloride, NaCl, as an antimicrobial and a preservative, however, information on NaCl effects on bacterial cell morphology is lacking. The effect of NaCl, on Escherichia coli O157:H7 and Staphylococcus aureus cells individually grown in a laboratory medium was examined using transmission electron microscopy (TEM). Cultures were grown in brain heart infusion (BHI) broth containing dissolved 0%, 5%, or 10% (w/v) commercially obtained fine (FN) and extra coarse (EC) grade granular NaCl. The pathogens were incubated at 35 degrees C for 12 and 24 h. Then, a mixture of five strains of each pathogen per treatment was prepared. Samples were centrifuged, pellets collected, fixed immediately with glutaraldehyde, and prepared for TEM examination. Cells morphology on TEM micrographs verified that the magnitude of morphological damage to E. coli O157:H7 cells was significantly greater than that of S. aureus cells. More cell injury occurred as NaCl concentration increased from 5% to 10%. Generally, S. aureus maintained its cellular structure and no severe cell wall or plasma membrane damage and/or shrinkage was observed. At 10% NaCl, the damage to E. coli O157:H7 cells was extensive, and the pathogen seemed to have lost its cellular integrity. Although NaCl affected the morphology of E. coli O157:H7 and S. aureus, the coarse grade of NaCl seemed to have a milder effect with respect to cell damage, especially on S. aureus. The 24 h-old cultures were more susceptible to NaCl treatment compared to the 12 h-old cells. Thus, the age of the cells has an impact on their resistance to salt--the environmental stressor.