The Classification and Identification
of Bacteria of Medical Importance

The Gram-positive rods can be divided according to their ability or otherwise to produce spores. Spores of Gram-positive rods are highly resistant structures that may add considerably to their pathogenic capacity. Sporing Gram-positive rods that are confined to the (somewhat confusingly named) genus Bacillus. Important members of this genus include Bacillus anthracis the cause of anthrax, and Bacillus cereus a cause of food poisoning. (Ceres was a Roman goddess of the harvest). The genus Bacillus also has members that produce clinically useful antibiotics, like Bacillus polymyxa, the source of polymyxin.

Obligately anaerobic sporing Gram-positive rods are placed in the genus Clostridium. These include Clostridium perfringens, a principal cause of gangrene, Clostridium tetani, the cause of tetanus, and Clostridium botulinum the cause of the fatal food poisoning - botulism (Botulus is the Latin for a sausage). Clostridium perfringens used to be known as Clostridium welchii, and you may find reference to this name in older text books. In Gram films prepared from cultures it is very rare to see the spores of Clostridium perfringens, but Clostridium tetani produces large spores at the ends of the bacilli, giving them the characteristic appearance of drumsticks.

The motility of the non-sporing Gram-positive rods is an important attribute in distinguishing coryneform bacteria and lactobacilli from listeria. Listeria monocytogenes is an important human pathogen, and it is capable of a characteristic tumbling motility seen at 25 degrees C but not at 37 degrees C. Lactobacilli appear microscopically as long, slender rods that often grow in chains. They may appear "Gram-variable" with some parts of the cell appearing blue-black and other portions looking red. They tend to make their immediate environment too acid for other bacteria to tolerate. Some lactobacilli are important members of the vaginal commensal flora of women of child-bearing age. These are sometimes referred to as Döderlein bacilli. The lactobacilli are catalase-negative, and can thus be distinguished from the coryneform bacteria that do produce catalase. The most infamous of the coryneform bacteria is Corynebacterium diphtheriae , toxigenic strains of which cause diphtheria. This gives the coryneform bacteria their alternative name - diphtheroids. They appear somewhat irregular in shape, and tend to cluster in Gram films. Some microbiologists think that this gives them the microscopic appearance of Chinese letters. Propionibacteria are coryneforms that cannot grow in the presence of air. A notable example is Propionibacterium acnes, associated with acne.