Propionibacteria  are  Gram-positive nonmotile  rods, which when first isolated tend  to  be irregular and sometimes with short branching .  They show typical coryneform appearance which has led to problems in taxonomy.  During  laboratory culturing their  cells became more regular  and smaller  .   Isolation  requires  seven   days'  incubation anaerobically at 35-37°C. However, these bacteria are not strict anaerobes and procedures used for manipulation under anaerobic conditions are  not required. The  colonies  are buff to pink depending on the species, and generally domed.

 

  From their first discovery in  1896 by Unna these bacteria have acquired different generic names, from Bacillus, to  Corynebacterium, to Propionibacterium,  as well as being referred to as anaerobic diphtheroids and anaerobic coryneforms. Up to 1946,  when Douglas and Gunter3 recommended the generic name Propionibacterium, Corynebacterium was used. Since then both names have appeared in the medical literature, though Propionibacterium is now accepted by bacterial taxonomists as correct.

 

  Johnson  and  Cummins  recognized three species, Propionibacterium acnes, Propionibacterium avidum and Propionibacterium granulosum, which  are  still recommended to date. Other species described by Prevot and Fredette5, Corynebacterium liquifaciens, Corynebacterium pyogenes,  Corynebacterium  parvum,  Corynebacterium diphtheroides  and  Corynebacterium  anaerobium  would appear to be P. acnes according to Zierdt et al6 and Cummins and Johnson.

 Selected information of tests used to identify ropionibacteria to the species level. Cell wall analysis of sugars is the most  reliable method of dividing both P.  acnes and P. avidum  into biovars (biotypes). The presence or absence of galactose in the cell walls of P. acnes and P. avidum  corresponds to biovars I and II. However, this is a lengthy procedure requiring culturing, concentration and washing of cells, acid treatment  and thin-layer chromatography. 

   Various  investigations have shown that species may be divided into serovars,  biovars and phagovars. There  is  no  universal agreement  on  these  typing schemes apart  from the simple serotyping  of P. acnes and P.  avidum  into, in  each  case,  types I  and II. Investigators wishing to pursue strain typing should consult the references given above. Biotyping should at present be viewed with caution because Zierdt et al6 and the present authors have shown variation in results on repeated tests.  Serotyping, relying on simple agglutination tests, has technical difficulties in that many isolates self-agglutinate, and to overcome the problem an indirect fluorescent  antibody test may be used. Bacteriophage typing offers a future means of typing P. acnes for epidemiological investigations.