Effect of firm agar on the swarming of Proteus and Clostridium species and on the colonies of clinically important bacteria (Plates IX-XI)

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Abstract
Firm agar media containing 3.6% agar.sbd.3 times the concentration of agar in routine media.sbd.inhibited the swarming of 167 strains of P. mirabilis and 14 strains of P. vulgaris incubated in air. The swarming of P. mirabilis was also prevented by firm agar when incubation was anaerobic, provided that a negative pressure of 600-650 mm of Hg was generated in the removal of air from jars before admitting H for the Pd-catalyzed removal of residual traces of O2. When a negative pressure of only 300 mm of Hg was generated there was a tendency for colonies of P. mirabilis to develop outgrowths after incubation for 2 days. Firm agar inhibited the swarming of C. tetani and C. septicum. As judged by a viable-count method, firm agar medium permitted the growth of maximal numbers of colonies for all organisms tested except C. chauvoei; the organisms included C. oedematiens, Haemophilus influenzae, Neisseria gonorrhoeae, Streptococcus pneumoniae and S. pyogenes. 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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