22 ] The Classification of Lower Organisms 



The nine genera listed first occur normally in animals, mostly in the gut and 

 mostly as commensals; exceptions are important pathogens. Most of them produce 

 acid, and many of them produce gas, from sugar. These genera are the traditional 

 colon-typhoid-dysentery group. 



Escherichia coli, the colon bacillus, and Aerohacter aerogenes, the gas bacillus, are 

 common commensals which produce acid and gas from dextrose and lactose. The 

 standard method of testing waters for contamination is essentially a test for the 

 presence of these organisms. 



Klebsiella also produces acid and gas from sugars. It inhabits the respiratory 

 tract. The cells are heavily capsulated and non-motile. The type species K. pneumo- 

 niae is an important pathogen, the pneumobacillus of Friedlander. 



Proteus vulgaris (this is at least the third genus to bear the name Proteus, but the 

 first in this kingdom) produces acid and gas from dextrose but not lactose, and 

 liquefies gelatine. It is usually isolated from spoiled meat. 



Salmonella is distinguished from Proteus by non-liquefaction of gelatine. Many 

 of its species are harmless commensals; others cause paratyphoid fevers. Immunologi- 

 cal study of cultures of Salmonella from cases of disease and from waters have re- 

 sulted in the distinction of fully 150 races, mostly unnamed and identifiable only by 

 immunological reactions. Eberthella includes motile rods producing acid but not 

 gas from sugars, and belonging to the same immunological system as the various 

 races of Salmonella. Eberthella typhi causes typhoid fever. 



Shigella is distinguished from Eberthella by non-motility. The Shiga bacillus, 

 S. dystenteriae, is the cause of dystentery. 



Bacteroides is a numerous group of acid-producing gut bacteria, motile or non- 

 motile, generally harmless.^ distinguished from the foregoing as strictly anaerobic. 



Alcaligenes fecalis, an apparently harmless organism isolated from intestinal con- 

 tents, does not produce acid from sugars; grown in milk, it produces an alkaline 

 reaction. 



Numerous races of bacteria which have been isolated from soil and are capable 

 of attacking cellulose are assigned to the genus Cellulomonas. Bacteria which produce 

 an extracellular red pigment are Serratia (one of the oldest generic names for bac- 

 teria); those which produce yellow pigment are Flavobacterium; those which produce 

 blue, black, or violet growths are Chromobacterium. Cultures which lack the distinc- 

 tive characters of all of the above named genera (most such cultures have been 

 isolated from water) are called Achromohacter. 



Family 8. Pasteurellacea nom. nov. Family Parvobacteriaceae Rahn; there is no 

 corresponding generic name. Minute non-motile Gram negative rods, pathogenic, 

 requiring special media for cultivation. Pasteurclla avicida is the cause of chicken 

 cholera, upon which Pasteur made important studies. Of greater direct importance 

 to man is Pasteurella pestis, the cause of plague. Hemophilus includes the agents 

 of whooping cough, soft chancre, and conjunctivitis. Brucella includes the organisms 

 which cause Malta fever, undulant fever. Bang's disease, contagious abortion. Pfeif- 

 ferella mallei is the cause of glanders. 



Family 9. Rhizobiacea [Rhizobiaceae] Conn in Jour. Bact. 36: 321 (1938). Gram 

 negative rods, atrichous or peritrichous, parasites on plants. Cultured in the presence 

 of sugars, these organisms produce acid; they are evident allies of the colon group. 



Erwinia commemorates Erwin F. Smith, the discoverer of many bacteria pathogenic 

 to plants. Typical species cause blights, wilts, or dry necroses. The discovery by 

 Burrill, 1882, of Erwinia amylovora, the cause of the fire blight of pears, should 



