ASTM G22 Resistance of Plastics to Bacteria

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The ASTM G 22 test method, titled “Standard Practice for Determining Resistance of Plastics to Bacteria” is designed to test the resistance of synthetic polymeric materials (molded materials, surface treated articles or similar objects) to bacterial attack.

The main component of synthetic polymeric materials, resin, is usually resistant to bacteria, as it does not serve as a carbon source for the growth of bacteria. What does serve as the carbon source are the other components that are added to the resin, like lubricants, colorants, and stabilizers. When plastics are under conditions that are favorable to bacterial attack, such as high temperature and humidity, it is important to determine the plastic’s resistance to such attack. The ASTM G 22 test is essentially the bacterial version of the ASTM G21 with some modification.

Summary of the ASTM G 22 Test

  • Test microorganism, Pseudomonas aeruginosa, is grown in nutrient broth and before testing, is centrifuged and washed with saline solution.
  • The washed test culture is then used to inoculate nutrient-salts agar.
  • After inoculation, the nutrient-salts agar is poured into a petri dish and allowed to harden.
  • Once the nutrient-salts agar is hardened, the test sample is placed on top.
  • Another layer of nutrient-salts agar is then poured on top of the test sample and allowed to harden.
  • Test sample is incubated under high humidity for 21 days.

Strengths of the ASTM G 22

  • The method and parameters are designed so that every condition for growth is present (i.e., salts, minerals, water and a humid environment) other than a sufficient carbon source, thus testing a materials true ability to withstand bacterial attack.
  • Having the test substance sandwiched between inoculated agar medium enables maximum exposure and efficacy to test microorganism.

Weaknesses of the ASTM G 22

  • Since nutrient-salts agar is used primarily for fungal testing, samples that are not sterile and have fungal spores can skew test results.
  • Only one species of bacteria is used in the method.

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