;U THE MORPHOLOGY OF BACTERIA 
(e) Sarcina. Planes of septation alternate, at right angles, in three planes of 
space. Form packets. 
(/) Planococcus. Motile cocci, provided with flagella. 
(g) Planosarcina. Motile sarcina, provided with flagella. 
2. BaciUacem. Cells elongated and cylindrical; straight. 
(a) Bacterium. Non-motile. No flagella. 
{h) Bacillus. Cells motile. Peritrichic flagellation. 
(c) Pseudomonas. Cells motile. Polar flagellation. Single flagellum or tufts of 
flagella at one or both poles of the organism. 
3. SpirillacecE. Cells elongated and cylindrical; spirally twisted about the long axis. 
(a) Spirosoma. Cells rigid and slightly curved; without flagella. 
(b) Microspira. Cells rigid and slightly curved; with one, rarely several, polar 
flagella. 
(c) Spirillum. Cells rigid, loosely coiled; with tuft of polar flagella. 
id) Spirochseta. Cells flexuous, closely coiled; flagellation unknown, 
(e) Treponemata. Organisms cylindrical with pointed ends. 
(/) Spironemata. Axial filament, doubly contoured membrane, 
(fir) Leptospirata. Semicircular bending of one or both ends. 
The Higher Bacteria. 
4. Chlamydobacteriacew. Cells enclosed in a sheath. 
(a) Streptothrix. Cell division always in one plane 
(b) Phragmidothrix. Cell division in three planes of space; very delicate sheath. 
(c) Crenothrix. Cell division in three planes of space; sheath well developed. 
(d) Cladothrix. Cells more or less branched. 
5. Beggiatoacece (Thiothrix) . Cells contain sulphur granules. 
H. MUTATION: CONSTANCY OF TYPES. i 
True mutation or discontinuous variation is not commonly observed 
among bacteria, although some few instances are on record which have 
been subjected to satisfactory scrutiny. Mutation must be carefully 
differentiated from the loss of one or more characteristics of bacteria 
during cultivation: the loss or suppression of one or more characteris- 
tics is fairly commonly observed among bacteria. Smooth and rough 
colonies seem to appear spontaneously in bacterial cultures, and these 
often show differences in virulence for experimental animals.^ Pigment 
production, and proteolytic activity— as for example the ability to 
liquefy gelatin— are frequently lost to cultures of bacteria during 
prolonged cultivation, but these properties may be regained when the 
organisms are placed once more in a suitable environment. Similarly, 
strains of fermenting bacteria may temporarily, or even permanently, 
become unable to decompose certain carbohydrates. Change in 
virulence, or loss of virulence, is rather commonly noticed among path- 
ogenic bacteria grown outside the animal body. It is even possible so 
to parasitize organisms by prolonged cultivation upon one medium that 
they will develop not at all, or slowly at best, on other media. Thus, 
a strain of B. proteus has been grown continuously upon agar with 
frequent transfers for several years, and the organism will no longer 
grow in broth. Similarly, B. bulgaricus is an obligate milk parasite. 
1 Eisenberg: Ueber Mutationen bei Bakterien und anderen Midroorganismen in 
Ergebnisse d. Immunitatsforsch. experimentellen Therapie, Bakteriologie und Hygiene, 
Berlin, 1914, pp. 28-142, for summary. 
2 See Hadley: Jour. Inf. Dis., 1927, 40, 1, for excellent discussion. 
