Gram-Positive Spore-Forming Bacilli
Gram-positive bacilli which form endospores are common in soil and water environments. The genera Bacillus and Clostridium include pathogens whose factors are toxins. Cl. tetani and Cl. botulinum produce the most deadly toxins known. B. anthracis toxin is also deadly in systemic infections. Microbial control of these pathogens must include methods which destroy their highly resistant spores.

Species in the genus Bacillus are all aerobic, although some are facultative anaerobes. Most are also motile. Significant pathogens are B. anthracis, the agent of anthrax, and B. cereus, the agent of food poisoning. Important commercial species of Bacillus produce antibiotics. B. thuringiensis is a commercial source of biological insecticides.

Species in the genus Clostridium are strict anaerobes. Significant pathogens are Cl. tetani, the agent of tetanus, Cl. botulinum, the agent of botulism, Cl. perfringens and other species, the agents of gas gangrene, and Cl. difficile, an opportunistic agent of intestinal infection following antibiotic therapy. Commercially important Clostridia are industrial producers of organic acids and alcohols.

Gram-Positive Regular Non-Spore-Forming Rods
Regular, non-spore-forming gram-positive rods are so named because of their consistent shape and staining properties. The seven genera of this group are otherwise diverse in habitat, biochemical properties, and size. Significant pathogens in this group of Listeria monocytogenes, the agent of listeriosis, and Erysipelothrix rhusiopathiae, agent of erysipelas in both humans and animals. Lactobacillus species are important commercial producers of many dairy products.

Gram-Positive Irregular Non-Spore-Forming Rods
Irregular, non-spore-forming gram-positive rods are so named because of irregularities in both morphology and staining properties. They inhabit soil and water environments but three genera contain pathogens: Corynebacterium, Propionibacterium, and Mycobacterium.

C. diphtheriae is the agent of diphtheria, an upper respiratory infection. The primary virulence factor is diphtherotoxin, which is toxic to upper respiratory tissue, the PNS, and the heart. Immunization provides effective protection.

Propionibacterium acnes is the agent of acne vulgaris, an infection of pilosebaceous glands exacerbated by biochemical changes of adolescence.

Mycobacteria: Acid-Fast Bacilli
The genus Mycobacteria is distinguished by its acid-fast cell wall composed of mycolic acids and its slow but persistent rate of growth. Significant pathogens are M. tuberculosis and M. leprae.

M. tuberculosis is an ancient pathogen that annually kills millions of individuals worldwide. It infects the lungs (pulmonary tuberculosis), and other organs of the body (extrapulmonary tuberculosis). Its virulence factors include high infectivity, resistance to drying, cord factor, ability to escape phagocytosis, ability to stimulate host delayed hypersensitivity response, and the recent development of multiple drug resistance. Reduction of tuberculosis control programs has also contributed to its status as a major pathogen worldwide.

M. leprae is the causative agent of leprosy (Hansen's disease). Its infectivity appears to be either direct contact or droplet transmission. Genetic susceptibility and nutritional status of the host may also be factors. There are two forms of leprosy, both of which destroy epithelial tissue and peripheral nerves. Lepromatous leprosy is the malignant, disfiguring form. Tuberculoid leprosy is the milder, secondary form. Intermediate infections also occur and combine characteristics of both forms. The degree of virulence is related to the growth rate of M. leprae and its ability to suppress the host immune response. Treatment with combination drug therapy is effective if begun early. Research for an effective vaccine is underway, but is not yet available.