Gram Positive Bacilli

Medstudents' Homepage

Medstudents thanks Dr. Martin Nachbar, M.D. from New York University, because this section can only be here with permission of the Hippocrates Project of the New York University School of Medicine

Organisms to be considered in this group are nonsporulating pleomorphic rods of the genus Corynebacterium, the much larger sporulating rods of the facultative anaerobic genus Bacillus, and the obligate anaerobic genus Clostridium.

Note the size differences in the following two images: one a gram stain of a Corynebacterium and the other a spore stain of a Bacillus.

Corynebacterium

Bacillus

Among the organisms of these groups those of medical importance include

• Corynebacterium diphtheriae, (diphtheria)
• Clostridium perfringens (gas gangrene)
• Clostridium tetani, (tetanus),
• Clostridium botulinum,(bolulism)
• Clostridium difficile - the causative organism of pseudomembanous colitis which is a life threatening condition arising in patients treated with antibiotics for other infections
• Bacillus anthracis (anthrax)

Identification Scheme for Gram positive bacilli

Properties useful In distinquishing among the Corynebacterium

• growth on chocolate tellurite agar-Corynebacteria can take up the tellurite in the medium and reduce it to the metal, tellurium, which precipitates out resulting in the observed gray to black colonies.

  Corynebacterium diphtherium, mitis, on chocolate tellurite agar

  

  Corynebacterium diphtheriae, mitis grown on chocolate agar without tellurite

  

  Corynebacterium pseudodiphtheriticum grown on chocolate tellurite agar

  

• colony morphology on chocolate tellurite agar - three cultural variants (biotypes) of Corynebacterium diphtheriae have been described. These can be distinquished on the basis of their colony morphology on chocolate tellurite agar. The biotypes are gravis, mitis, and intermedius. Gravis colonies are large, dull, flat, and gray to black; mitis colonies are smaller, glossy, and more domed shaped; intermedius colonies are very small and may or may not be glossy. The manifestations of diphtheria bare no constant relationship to the colonial types.

  Corynebacterium diphtheriae, gravis

  

  Corynebacterium diphtheriae, mitis

  

  Corynebacterium diphtheriae, intermedius

  

• growth on Tinsdale agar - this medium, in addition to containing tellurite, also contains cystine. On this medium some Corynebacterium species are capable of producing hydrogen sulfide from the cystine which reacts with the tellurite to produce a smoky brown halo around the black tellurium containing bacterial colonies

  Corynebacterium diphtheriae, mitis, a hydrogen sulfide producer

  

  Corynebacterium xerosis, a hydrogen sulfide non-producer

  

   

Properties useful in distinquishing the Bacillus/ Clostridium

• aerobic vs anaerobic growth requirements - Bacillus species are facultative anerobes whereas most species of Clostridium are obligate anaerobes, although a few like Clostridium perfringens are aerotolerant (i.e. their metabolism is fermentative even in the presence of oxygen).

  Bacillus subtilis grown in air on sheep blood trypticase soy agar

  

  Clostridium perfringens grown anaerobically on sheep blood trypticase soy agar

  

   

Properties useful in distinquishing among Clostridium species

• lecithinase activity - when a culture is grown on a medium rich in phospholipids such as egg yolk agar, lecithinase activity is detected as a opalescence which develops around the colonies as the phospholipids are split into insoluble diglycerides.

  Clostridium perfringens grown on egg yolk agar

  

Back to Basic Sciences

Back to Medstudents' Homepage