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The genus Mycobacterium is represented by bacilli covered with a mycolic acid coat. This waxy coat protects the bacteria from some antibiotics, prevents them from drying out, and blocks penetration by Gram stain reagents (see Staining Microscopic Specimens ). Because of this, a special acid-fast staining procedure is used to visualize these bacteria. The genus Mycobacterium is an important cause of a diverse group of infectious diseases. M. tuberculosis is the causative agent of tuberculosis , a disease that primarily impacts the lungs but can infect other parts of the body as well. It has been estimated that one-third of the world’s population has been infected with M. tuberculosis and millions of new infections occur each year. Treatment of M. tuberculosis is challenging and requires patients to take a combination of drugs for an extended time. Complicating treatment even further is the development and spread of multidrug-resistant strains of this pathogen.
Another pathogenic species, M. leprae , is the cause of Hansen’s disease ( leprosy ), a chronic disease that impacts peripheral nerves and the integrity of the skin and mucosal surface of the respiratory tract. Loss of pain sensation and the presence of skin lesions increase susceptibility to secondary injuries and infections with other pathogens.
Bacteria in the genus Corynebacterium contain diaminopimelic acid in their cell walls, and microscopically often form palisades , or pairs of rod-shaped cells resembling the letter V. Cells may contain metachromatic granules , intracellular storage of inorganic phosphates that are useful for identification of Corynebacterium . The vast majority of Corynebacterium spp. are nonpathogenic; however, C. diphtheria is the causative agent of diphtheria , a disease that can be fatal, especially in children ( [link] ). C. diphtheria produces a toxin that forms a pseudomembrane in the patient’s throat, causing swelling, difficulty breathing, and other symptoms that can become serious if untreated.
The genus Bifidobacterium consists of filamentous anaerobes, many of which are commonly found in the gastrointestinal tract, vagina, and mouth. In fact, Bifidobacterium spp. constitute a substantial part of the human gut microbiota and are frequently used as probiotics and in yogurt production.
The genus Gardnerella , contains only one species, G. vaginalis . This species is defined as “gram-variable” because its small coccobacilli do not show consistent results when Gram stained ( [link] ). Based on its genome, it is placed into the high G+C gram-positive group. G. vaginalis can cause bacterial vaginosis in women; symptoms are typically mild or even undetectable, but can lead to complications during pregnancy.
[link] summarizes the characteristics of some important genera of Actinobacteria. Additional information on Actinobacteria appears in Appendix D .
| Actinobacteria: High G+C Gram-Positive | ||
|---|---|---|
| Example Genus | Microscopic Morphology | Unique Characteristics |
| Actinomyces | Gram-positive bacillus; in colonies, shows fungus-like threads (hyphae) | Facultative anaerobes; in soil, decompose organic matter; in the human mouth, may cause gum disease |
| Arthrobacter | Gram-positive bacillus (at the exponential stage of growth) or coccus (in stationary phase) | Obligate aerobes; divide by “snapping,” forming V-like pairs of daughter cells; degrade phenol, can be used in bioremediation |
| Bifidobacterium | Gram-positive, filamentous actinobacterium | Anaerobes commonly found in human gut microbiota |
| Corynebacterium | Gram-positive bacillus | Aerobes or facultative anaerobes; form palisades; grow slowly; require enriched media in culture; C. diphtheriae causes diphtheria |
| Frankia | Gram-positive, fungus-like (filamentous) bacillus | Nitrogen-fixing bacteria; live in symbiosis with legumes |
| Gardnerella | Gram-variable coccobacillus | Colonize the human vagina, may alter the microbial ecology, thus leading to vaginosis |
| Micrococcus | Gram-positive coccus, form microscopic clusters | Ubiquitous in the environment and on the human skin; oxidase-positive (as opposed to morphologically similar S. aureus ); some are opportunistic pathogens |
| Mycobacterium | Gram-positive, acid-fast bacillus | Slow growing, aerobic, resistant to drying and phagocytosis; covered with a waxy coat made of mycolic acid; M. tuberculosis causes tuberculosis; M. leprae causes leprosy |
| Nocardia | Weakly gram-positive bacillus; forms acid-fast branches | May colonize the human gingiva; may cause severe pneumonia and inflammation of the skin |
| Propionibacterium | Gram-positive bacillus | Aerotolerant anaerobe; slow-growing; P. acnes reproduces in the human sebaceous glands and may cause or contribute to acne |
| Rhodococcus | Gram-positive bacillus | Strict aerobe; used in industry for biodegradation of pollutants; R. fascians is a plant pathogen, and R. equi causes pneumonia in foals |
| Streptomyces | Gram-positive, fungus-like (filamentous) bacillus | Very diverse genus (>500 species); aerobic, spore-forming bacteria; scavengers, decomposers found in soil (give the soil its “earthy” odor); used in pharmaceutical industry as antibiotic producers (more than two-thirds of clinically useful antibiotics) |
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