Firm agar media containing 3-6% agar-three times the concentration of agar in routine media-inhibited the swarming of 167 strains of and 14 strains incubated in air. The swarming of was also prevented by firm agar when incubation was anaerobic, provided that a negative pressure of 600-650 mm of mercury was generated in the removal of air from jars before admitting hydrogen for the palladium-catalysed removal of residua] traces of oxygen. When a negative pressure of only 300 mm of mercury was generated there was a tendency for colonies of to develop outgrowths after incubation for 2 days. Firm agar inhibited the swarming of and

As judged by a viable-count method, firm agar medium permitted the growth of maximal numbers of colonies for all organisms tested except the organisms included and

Precautions necessary for the preparation of firm agar media are defined. Colonies of a number of bacterial species on firm agar are described and illustrated. Firm agar media, being applicable to all bacteria with swarming colonies and practically non-inhibitory to other bacteria, are recommended for the isolation of bacteria from clinical specimens that may contain swarmers.


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