The Preservation of Pseudomonas Under Mineral Oil

SUMMARY The use of mineral oil for the preservation of species of Pseudomonas is described. Eighty‐three out of 91 (91·2%) Pseudomonas isolates remained viable for at least 66 months; 156 out of 168 (92·8%) isolates were still living after at least 46 months, and only 12 out of 170 (2·3%) isolates were non‐viable after three years. An explanation is offered for variation in ability to grow and produce acid in chemically defined media containing galactose or sucrose. The ability to hydrolyze tributyrin and to reduce nitrate was stable over 66 months, regardless of whether the stock cultures were maintained under mineral oil or in yeast‐extract peptone broth. However, the ability to digest milk or gelatin was frequently impaired or lost when cultures were maintained in peptone broth, although the mineral oil‐preserved stock cultures remained stable in this respect. Over a nineteen month period six diagnostic biochemical characters of the oil‐covered stock cultures remained stable within the limits of the experimental methods used. Data from the literature and information from other workers reveal that more complex methods of preserving pseudomonads are not particularly advantageous in regard either to viability or stability. The mechanism of the survival of bacteria under mineral oil is discussed. The taxonomic significance of the loss of proteolytic activity by the ‘bench‐broth’ cultures of Pseudomonas spp. is illustrated by a single example.