ABSTRACT
Propionic acid bacteria (PAB) are characterized as gram-positive, nonsporing, nonmotile
pleomorphic rods. Although some strains may be relatively aerotolerant, they are basically
anaerobes that produce propionic acid, acetic acid, and CO2 as their main fermentation
products. Their optimal growth temperature is between 30 and 378C. PAB were first described by Freudenreich and Jensen in 1906 and given the name Propionibacterium
by Jensen in 1909.[1] PAB are divided into two principal groups: the classical or dairy
PAB and the cutaneous PAB (Table 1). The main habitat of the classical PAB are dairy
Table 1 Two Groups of Propionibacterium Species
Classical (dairy) strains Cutanous strains
P. acidipropionici P. acnes
P. australiense P. avidum
P. cyclohexanicum P. granulosum
P. freudenreichii subsp. freudenreichii P. lymphophilum
P. freudenreichii subsp. shermanii P. propionicus
P. jensenii
P. thoenii
P. microaerophilum
products, in particular Swiss-type cheeses. However, they have also been isolated from
rumen contents, spoiled olives, and spoiled orange juice. The main habitat of the
cutaneous PAB is the human skin, where they are part of the normal microbiota. They
have also been isolated from the feces of humans, chickens, and pigs. Cutaneaus PAB
have occasionally also been isolated from pathological material. Based on 16S rRNA
studies, it appears that the classical and dairy PAB are also phylogenetically distinct.[2,3]
II. METABOLISM
The major means of energy generation of PAB is the production of CO2, propionic acid,
and acetic acid through fermentation:
3 glucose ! 4 propionateþ 2 acetate þ 2CO2 (1)
This yields 4 ATP per glucose molecule and is thereby more energy efficient than lactic or
acetic acid fermentation. The ATP is generated not only through substrate phosphoryl-
ation, but also by oxidative phosphorylation.
