Effects of ascorbic acid, citric acid, lactic acid, NaCl, potassium sorbate and Thymus vulgaris extract on Staphylococcus aureus and Escherichia coli

Staphylococcus aureus and some strains of Escherichia coli are frequently implicated in foodborne diseases. This study examined the effects of some compounds (ascorbic acid, citric acid, lactic acid, sodium chloride, potassium sorbate and Thymus vulgaris extract) on growth of S. aureus and E. coli. Lactic acid (0.03% or 0.1%) alone nearly completely inhibited growth of S. aureus or E. coli, respectively. Citric acid (0.03%) reduced growth and ascorbic acid (0.1%) nearly completely inhibited the growth of S. aureus; the percentages of inhibition after 24 h incubation in nutrient broth were 33 and 91%, respectively. Citric acid (0.03%) and ascorbic acid (0.1%) did not inhibit growth of E. coli, but a lag occurred before increase in number could be observed. NaCl (5%) significantly reduced growth of both strains; the percentages of inhibition of S. aureus and E. coli after 24 h incubation were 55 and 64%, respectively. Thymus vulgaris extract (0.3%) alone or potassium sorbate (0.09%) alone reduced growth of both strains. A combination of citric acid (0.03%) and potassium sorbate (0.05%) or citric acid (0.03%) and NaCl (5%) nearly completely or completely inhibited, respectively, the growth of S. aureus. For E. coli, combination of citric acid (0.03%) and potassium sorbate (0.05%) together completely inhibited the growth. A combination of citric acid (0.03%) and NaCl (3%) or T. vulgarisextract (0.3%) and NaCl (3%) greatly reduced the growth of E. coli strains and the percentages of inhibition after 24 h incubation were 65 and 70%, respectively.   Key words: Staphylococcus aureus, Escherichia coli, inhibition, chemicals.