39. Cells coccoid only at pH 7.0 on peptone yeast extract acetate agar; develop into multi- cellular rods with peritrichous flagella under other conditions; do not fix atmospheric nitrogen Caryophanon p. 831 Cells grow in nitrogen-free mineral salts media, fixing atmospheric nitrogen Azotobacter p. 283 40. Aerobic 41 Anaerobic 42 41. Gram -positive; cells occur in irregular clusters Micrococcus p. 455 Gram-negative; fix atmospheric nitrogen. The coccoid form is only part of a cycle of morphological forms, the initial stage of which is a large rod Azotobacter p. 283 42. Large cocci, 3 to 4 microns wide, sometimes bearing rod-shaped protuberances on op- posite sides and at an obtuse angle to one another — a pleomorphic stage of a rod- shaped cell 0.8 by 2.4 to 10 microns; produce copious gas from peptone Sphaerophorus -p. 44i {S. ridiculosis) Spherical cells; pleomorphic, ranging in diameter from 0.7 to 2.5 microns; occurring in pairs, short chains and in irregular groups; dependent on glycine for growth in or- ganic media. Glycine is decomposed to CO2 , NH3 and acetic acid Peptococcus p. 474 {P. (jlycinophilus) 43. Large, cj'lindrical, pear-shaped or slightly curved rods 3 to 14 microns wide; activelj^ motile by means of single polar flagella; contain large spherules of calcium carbonate and may also contain sulfur Macromonas p. 80 Not as above 44 44. Curved rods 45 Straight rods 46 45. Curved rods with a bunch of flagella inserted laterally in the concave part of the cell; anaerobic; recorded from the cecum of the guinea pig, the buccal cavity of man and the rumen of the herbivora Selenomonas p. 258 Curved rods with polar flagella; 1.7 to 2.4 by 6.6 to 14.0 microns; contain small globules of sulfur in the center of the cell and a single large volutin granule at each end Thiospira p. 82 46. Cells 1.4 to 2.0 by 4.0 to 5.0 microns; motile by means of polar flagella; anaerobic to microaerophilic; ferments glucose, producing ethyl alcohol, carbon dioxide and lactic acid Zymonionas p. 199 Motile by means of peritrichous flagella; grow in a nitrogen-free mineral salts medium, fixing atmospheric nitrogen Azotobacter p. 283
