Evaluation of kappa carrageenan as a substitute for agar in microbiological media

Seventy-one samples of the colloid kappacarrageenan extracted from 12 seaweed species were subjected to a number of standard physical demands of solid bacteriological culture media. All samples had a lower melting temperature (less than 67° C) than agar and a gelling (setting) temperature between 16° C and 51° C, some the same and others lower or higher than agar. Temperature spreads were narrow (ca 10° C) to broad (ca 30° C), depending on the seaweed source, but none were as broad as that of agar (ca 40° C). The majority of commercially prepared samples held a slant when incubated at 37° C, but California seaweed colloids were best at 28° C in this test. The majority of samples released little to no water of syneresis in slant tests as well as in plates. Some plates prepared with the colloid were crystal clear as compared to agar plates. All test microorganisms grew as well on kappa-carrageenan media as on agar media. Some media responses could be attributable to the seaweed species, but others could be traced to chemical extraction methods and modification of the colloid.