SECTION F
1. Pear-shaped cells; 1.0 to 2.0 by 4.0 to 5.0 microns; non-motile; grow attached to each other or to solid surfaces by a holdfast secreted from the narrow end; sessile; multiply by longitudinal fission and by budding at the free end Pasteuria p. 279 Not as above 2 2. Curved or straight rods which produce a well -differentiated stalk by which they may attach to a surface; a single cell occurs at the end of each stalk except during the process of multiplication 3 Not as above 6 3. Cells curved; stalk produced as a continuation of the cell wall parallel to the long axis of the cell; reproduction by transverse fission of the cell from the free end, the daugh- ter cell developing a single polar flagellum, then breaking away and producing a stalk from the end of the cell at which the flagellum is located Caulobacter -p. 213 Not as above; cells produce a stalk at right angles to the main axis of the cell 4 4. Large rods, 2.0 by 6.0 to 12.0 microns, borne on the ends of lobose, dichotomously branched stalks composed of gum, forming a gummy colony which floats on water; may become attached Nevskia p. 216 Not as above; stalks composed of or impregnated with ferric hydroxide; dissolve com- pletely in hydrochloric acid; cells curved with the stalk secreted from the concave side ; reported only from iron-bearing waters 5 5. Stalks ribbon-like and usually twisted; cells located terminally Gallionella p. 2U Stalks horn-shaped; not twisted; round in cross section Siderophacus p. 216 6. Obligate autotrophes which oxidize ferrous to ferric iron at low pH in mineral synthetic media ; Gram-negative rods 7 Not as above 8 7. Thiosulfate is oxidized Thiobacillus p. 83 {T. ferrooxidans) Thiosulfate is not oxidized Ferrobacillus p. 227 (F. ferrooxidans) 8. Organisms which store oxides of manganese or iron either in the cell membrane, in the cell wall or in the surrounding capsules; found in water and mud 9 Note: In the absence of further information, these organisms are identified on the basis of their iron-depositing characteristics. Most iron organisms studied in pure culture metabolize the organic compound which forms the iron chelate, and the liberated iron then chelates with some cell component. Citrate-utilizing organisms will, for example, release iron from ferric ammo- nium citrate. Accumulation of the iron in or on the cell may depend only on the nature of the cell substance. Pure-culture studies may place these or- ganisms in more commonly recognized genera. They should also be treated as non-iron-depositing cells and should be followed through the key. Not as above 13 9. Cells encapsulated or embedded in mucus or surrounded by a torus of iron 10 Not as above; iron or manganese stored in cell membrane or cell wall Siderobacter p. 226 10. Encapsulated cells occurring singly or in short chains, each capsule being completely surrounded by a ring (torus) heavily impregnated with iron or manganese giving the general appearance of links in a chain Naumanniella p. 223 Not as above 11 11. Cells surrounded by a ring (torus) which is open at one end; cells motile by means of
two unequal polar flagella Ochrobium p. 225
1005
