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. vulgaris extract (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.

Citrobacter freundii has been implicated in food spoilage and food poisoning outbreaks. This study examines the effects of some compounds (e.g. citric acid, ascorbic acid, lactic acid, sodium chloride, and Thymus vulgaris extract) on growth of two strains of Citrobacter freundii at 31 ºC and 5 ºC. At 31 ºC, lactic acid (0.2%) or ascorbic acid (0.2%) alone completely inhibited growth of the tested strains, as there was 100% reduction in growth of the strains after 24 h incubation in nutrient broth containing these compounds. Thymus vulgaris extract (0.3%) reduced the growth rate (p < 0.05), the percentages of inhibition after 24 h incubation were about 60% for both strains. NaCl (5%) greatly reduced growth, the percentages of inhibition were about 84% for both strains. Combination of T. vulgaris extract (0.3%) and NaCl (4%) together completely inhibited growth of C. freundii species tested. Ascorbic acid (0.1%) or citric acid (0.03%) did not affect growth of the strains (p > 0.05), but a lag occurred before increase in number could be observed. In chicken and fish homogenates, combination of NaCl (4%) and ascorbic acid (0.1%) reduced the growth (p < 0.05) (growth inhibition was 40%). At 5 ºC, lactic acid (0.1%) alone greatly reduced the growth (p < 0.05). The activity of NaCl, or ascorbic acid alone against the tested strains was greatly increased (p < 0.05). For C. freundii 4, the percentage of growth inhibition after 6 days incubation in broth containing 3% NaCl or 0.1% ascorbic acid were 88% and 72%, respectively. For C. freundii 38, the percentage of growth inhibition after 6 days incubation in broth containing these compounds were 60% and 54%, respectively.

Previous studies reported that, at neutral pH (e.g. 7.4), many preservatives do not have antimicrobial effect against Pseudomonas aeruginosa and some strains of E. coli . This study investigated the effect of combination of low pH and some preservatives (e.g. sodium chloride, potassium sorbate or Thymus vulgaris extract) on the growth and caseinase production of Pseudomonas aeruginosa and E. coli at 31 degrees C and 7 degrees C. At 31 degrees C, although growth of the strains was not affected by low pH (5.7) alone, caseinase production by both strains was decreased. A combination of low pH and NaCl, potassium sorbate or thyme extract significantly reduced (p < 0.05) growth and caseinase production by E. coli 28 at 31 degrees C. For P. aeruginosa , addition of NaCl or thyme extract to nutrient broth (low pH) did not affect the growth but reduced caseinase production of the tested strain. Combination of low temperature (7 degrees C) and growth in nutrient broth (low pH) plus sodium chloride or nutrient broth (low pH) plus potassium sorbate was the most effective treatment in reducing or inhibiting growth and caseinase production of the tested strains.

Coloring agents such as allura red and tartrazine are frequently used in food and pharmaceutical industries. Also, many types of preservatives are added to food and pharmaceutical products to prevent their spoilage. This study examined the effect of pretreatment with nutrient broth containing allura red (AR) or tartrazine (TA) on survival of E. coli and Staphylococcus aureus in media containing preservatives (eg ascorbic acid, citric acid, or sodium chloride). Pretreatment of E. coli for 4 days with AR or TA before exposure for 3 days to nutrient broth containing NaCl (20%) significantly increased its resistance to sodium chloride (20%). Pretreatment of E. coli for 4 days with AR, TA, or nutrient broth alone before exposure for 3 days to nutrient broth containing ascorbic acid (1%) or to nutrient broth containing citric acid (0.5%) killed all cells. For S. aureus, pretreatment with AR or TA before exposure for 3 days to nutrient broth containing citric acid (0.3%) significantly increased its resistance to citric acid (0.3%). Pretreatment of S. aureus for 4 days with AR, TA, or nutrient broth alone before exposure for 3 days to nutrient broth containing citric acid (0.3%) reduced number of cells by 4.37, 2.54, and 6.97 logs, respectively. Pretreatment of S. aureus for 4 days with AR, TA, or nutrient broth alone before exposure for 3 days to nutrient broth containing ascorbic acid (0.5%) reduced number of viable cells by 3, 3.6, and 3.07 logs, respectively. Pretreatment of S. aureus for 4 days with AR, TA, or nutrient broth alone before exposure for 3 days to nutrient broth containing NaCl (20%) increased number of cells by 0.74, 0.7 and 1.1logs, respectively. These results have implications in food and pharmaceutical industries.

The effects of citric acid, Thymus vulgaris extract and NaCl on the heat sensitivity of Aeromonas spp. were examined in three different situations in this study. First, the effects of pretreatment with nutrient broth plus 0.03% citric acid, nutrient broth plus 0.3% T. vulgaris extract, nutrient broth plus 2.5% NaCl or nutrient broth plus 3% NaCl on the survival and caseinase production by heated A. caviae 166 and A. sobria 74 were investigated. Pretreatment of Aeromonas spp. with these preservatives for 1 or 6 days significantly increased their resistance to subsequent heating at 54C. However, pretreatment of Aeromonas strains with nutrient broth plus 2.5% NaCl or nutrient broth plus 3% NaCl before heating at 54C significantly (P < 0.05) decreased the production of caseinase by the heated cells. Second, the effect of post‐treatment with preservatives after heating of Aeromonas strains was examined. Post‐treatment of Aeromonas strains with the tested preservatives for 7 days after heating at 54C for 20 min prevented a complete recovery of cells and decreased the caseinase production compared with Aeromonas cells that were incubated in nutrient broth alone for 7 days after heating at 54C. Third, the effect of the type of the heating menstruum on the heat sensitivity of Aeromonas strains was investigated in this study. Heating in NaCl (0.85%) containing citric acid (0.03%) was the most effective treatment in killing Aeromonas spp. Heating in NaCl (0.85%) containing T. vulgaris extract (0.3%) or in NaCl (3.85%) slightly increased the resistance of cells to heat. PRACTICAL APPLICATIONS The results obtained in this study can be applied in the food industry, where combination of mild heat treatment and addition of low doses of chemical preservatives to food is nowadays frequently used. This study determined the heat sensitivity and caseinase production by A. caviae and A. sobria at three different conditions that may be encountered during processing of food industrially or at home. Firstly, the effect of pretreatment with some preservatives on the heat sensitivity and caseinase production by the tested Aeromonas. spp. was studied. Secondly, the effect of post‐treatment with preservatives on growth and caseinase production by the heated (54C) Aeromonas cells was investigated. Thirdly, effect of presence of preservatives in the heating menstruum on the heat sensitivity of Aeromonas spp. was studied.