AATCC 147-2011 (2016e). Test Method for Antibacterial Activity of Textile Materials: Parallel Streak.

Test not accredited in our laboratory.

The AATCC 147 standard describes a qualitative, quick, and easily executed test to determine antibacterial activity of diffusible microbial agents in treated textile materials. For the quantitative assessment of the antibacterial activity of textile materials, the test described by the AATCC 100 standard must be performed. The test described by the AATCC 147 standard is suitable for the evaluation of textile materials with diffusible antibacterial agents, which can migrate in the agar, and is not suitable for materials that prevent the diffusion of the antibacterial agent or that contain neutralizing antibacterial substances.

The test described by the AATCC 147 standard is based on the parallel streak method to evaluate the bacteriostatic activity of the test material. The standard states the following test bacteria to demonstrate activity against Gram positive and negative bacteria: Staphylococcus aureus and Klebsiella pneumoniae, but it also stablishes that, other suitable strains can be used depending on the intended end use of the test material. In this test, 25 x 50 mm rectangular specimens of the test material, a material of the same composition and finish, but not treated with the antibacterial agent (negative control), and a material with known bacteriostatic activity (positive control) are used. Each one of the specimens is placed in intimate contact with the surface of an agar plate in which the inoculum of one of the test bacteria has been transferred to the central area, in 5 parallel streaks or lines 60 mm long, spaced 10mm apart. The inoculating loop that is loaded only once with the test inoculum to make the 5 parallel lines or streaks, which will result in decreasing inoculum from one end of each streak to the other and between one streak and the next. The specimen is placed in the inoculated Petri dish, transversely across to the inoculum streaks, so that it covers the 5 parallel streaks in its central part, but not along its entire length. The procedure is repeated for each test microorganism. Plates with test material specimens and controls are incubated at 37 ± 2°C for 18-24 hours. After incubation, the plates with the test specimens are examined and it is determined: 1) the presence or absence of bacterial growth along the inoculum streaks under the specimen; and 2) the presence or absence of zones of inhibition in the streaks of inoculum around the specimen, clear areas of non-growth of the microorganism due to the diffusion of the microbial agent beyond the edges of the specimen, and, if positive, the size of the zones of inhibition. The evaluation of the bacterial activity of the test material must be qualitative, by means of the comparison with the positive and negative control of the test. The standard does not define a criterion for passing the test, which must be established by the client and the interested parties according to the intended use of the product. Nevertheless, the standard states that for a test material to exhibit acceptable antibacterial activity, no bacterial colonies directly under the test specimen of the test material must be detected.

Although a quantitative evaluation cannot be made under this test, AATCC 147 indicates that a rough estimate of activity can be obtained when evaluating the persistence of antibacterial activity in textiles with antibacterial agents after multiple washings. Because the growth of the inoculum organism decreases from one end of each streak to the other and from one streak to the next, increasing degrees of sensitivity are obtained which, together with the size of the zone of inhibition and the narrowing of the streaks, may estimate residual antibacterial activity after multiple washes.

The client must provide at least 4 specimens of test material and 4 specimens of negative control material per test microorganism. That is, to perform the test with Staphylococcus aureus and Klebsiella pneumoniae, at least 8 specimens of test material and 8 specimens of negative control material must be provided. The standard indicates as recommended size of the test specimens, rectangular pieces of 25 x 50 mm. But, if it is not possible to obtain these specimens, pieces of a similar size can be evaluated.

It is important that the client provides, in addition to the test material, the negative control, a material with the same composition and finish as the test material, but without the antibacterial agent. The negative control allows to determine if the antibacterial activity is due to the antibacterial agent. Many standard textile finishing chemicals, especially wrinkle resistant and permanent press reagents, will give strong antibacterial activity even after many washes.