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Phylum Firmicutes, Class Clostridia, Order Clostridiales, Family Clostridiaceae, Genus Clostridium, Cluster I (Clostridium sensu
stricto),
Clostridium fallax  (Weinberg and Seguin 1915) Bergey, Harrison, Breed, Hammer and Huntoon 1923.

Historical synonyms:
Bacillus fallax Weinberg and Séguin 1915, Vallorillus fallax (Weinberg and Séguin) Heller 1922.
Gram-positive, straight to curved rods, 0.5-1.4 x 1.6-15.4 µm, occuring singly or in
pairs. Motility is variable (in young cultures, cells are motile and peritrichous, but both
motility and flagella may be lost on subsequent transfer). Spores are oval, central to
subterminal, swelling the cell; sporulation of most strains occurs most readily in
chopped-meat broth cultures.
Surface colonies on blood agar plates are 1-5 mm in diameter, hemolytic, raised or
convex, translucent, gray, shiny and smooth, circular to slightly irregular, with entire to
slightly erose margins, often with a mottled or granular internal structure.  
Poor growth in nutrient broth; good growth in broth with fermentable carbohydrate
cultures in PYG broth are turbid with a smooth sediment and have a pH of 4.8
-5.3  
after incubation for one week.
Growth is inhibited by 6.5% NaCl,  20% bile, or a pH of 8.5. Optimum temperature for
growth is 37 ºC; weak growth at 25 ºC. Can grow at 45 ºC.
Products in PYG broth:  acetic, lactic and butyric  acids  and hydrogen. Abundant gas
is produced in PYG deep agar cultures.
Isolated from soil, marine sediments, animal wounds, clinical specimens from soft tissue infections in humans and human feces.
Susceptible to chloramphenicol, clindamycin, erythromycin, penicillin G, and tetracycline.
Pathogenic for laboratory animals (mice, guinea pigs), but pathogenicity is quickly lost.
  1. N.A. Logan and P. De Vos, 2009. Genus I. Clostridium Prazmowski 1880. In: (Eds.) P.D. Vos, G. Garrity, D. Jones, N.R. Krieg, W.
    Ludwig, F.A. Rainey, K.-H. Schleifer, W.B. Whitman. Bergey’s Manual of Systematic Bacteriology, Volume 3: The Firmicutes,
    Springer, 738-828.
  2. Smith L.D.S. and Hobbs G., 1974. Genus III. Clostridium Prazmowski 1880. In: (Eds.) Buchanan R.E. and Gibbons N.E., Bergey’s
    Manual of Determinative Bacteriology, Eighth Edition, The Williams & Wilkins Company, Baltimore, 551-572.
Milk reaction is positive (curd production). Meat digestion is negative.

Positive results for H
2 production, ammonia production, DN-ase, esculin hydrolysis, H2S production, substrate utilized and/or acid
produced from: fructose, galactose, glucose, maltose, mannose, ribose & starch.

Negative results for indole production, lecithinase, lipase, neuraminidase, starch hydrolysis, substrate utilized and/or acid produced
from: amygdalin, arabinose, dulcitol, glycogen, glycerol, inositol, inulin, mannitol, melezitose, melibiose, raffinose, rhamnose, salicin,
sorbitol, sucrose, trehalose & xylose.

Variable results for nitrate reduction, substrate utilized and/or acid produced from: cellobiose (weak), lactose (weak) & sorbose.
Clostridium fallax
(c) Costin Stoica
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