Determination of the efficacy of chemical disinfectants against some gram positive and gram negative bacteria in dairies under standardized conditions [DVG-guidelines]

ABSTRACT

In this study the bactericidal efficacy of four reference disinfectants used as standards in the recently published DVG-guidelines was assessed against Gram-positive and Gram-negative bacteria in dairies in the presence of organic matter [milk] by using two test methods according to the DVG-guidelines [2007] and European Standards which specify a test methods and minimum requirements for bactericidal activity of chemical disinfectants and antiseptics that are used in the dairies. This test methods are based on European standards [EN] which were prepared by the Technical Committee CEN/TC 216 [Chemical Disinfectant and Antiseptic]. The results showed that when we used suspension test which was the limiting test method for the listing of disinfectants for the food industries in the former DVG-guideline [2000] for determining the bactericidal efficacy of the tested reference disinfectants against tested organisms, Staphylococcus aureus [ATCC 6538] and Pseudomonas aeruginosa [ATCC 15442] were highly sensitive to formic acid while, Escherichia coli [ATCC 10536] and Enterococcus hirae [ATCC 10541] were more resistant. With application of peracetic acid the most resistant microorganisms were Staphylococcus aureus and Escherichia coli. While, the other two bacterial strains were highly susceptible. With glutaraldehyde the highly sensitive microorganisms were Enterococcus hirae and Escherichia coli. Benzyl-alkyl-dimethyl ammonium chloride showed higher bactericidal effect against Enterococcus hirae and Pseudomonas aeruginosa than against Staphylococcus aureus and Escherichia coli which needed longer exposure times at the same concentration. So, the limiting test organism when using formic acid as reference substance was Enterococcus hirae. While, with peracetic acid application was Staphylococcus aureus. Both Staphylococcus aureus and Pseudomonas aeruginosa appear to be the limiting test organisms with glutaraldehyde. When using benzyl-alkyl-dimethyl ammonium chloride were Staphylococcus aureus and Escherichia coll. Higher concentration and prolonged exposure times where necessary when test organisms were dried onto the surface of steel disks [carrier tests] as they were when the organisms were placed in suspension [suspension test] mainly with Gram negative organisms. This appears when using formic acid as reference substance against Gram negative test organisms we need higher concentrations in the same contact time. Also, with peracetic acid and benzyl-alkyl-dimethyl ammonium chloride applications higher concentrations respectively prolonged exposure time were required. This also was observed with Gram positive test organisms when using peracetic acid as reference substance. Differences in the disinfectant susceptibility were noticed between the four strains of microorganisms where, Escherichia coli was highly resistant to formic acid, while Pseudomonas aeruginosa was the most resistant strain to peracetic acid. Glutaraldehyde gave the same bactericidal effect against all tested strains. With benzyl-alkyl-dimethyl ammonium chloride the highly sensitive microorganism was Enterococcus hirae. These findings emphasize the need for caution in selecting an appropriate disinfectant for use on contaminated surfaces in dairies and dairy industry particularly in the presence of organic material [milk] as well as the need to include reference substances in the disinfectant testing procedure to be able to compare the activity of different products and check the susceptibility of the test organisms used