Bacteria: The High G+C Gram Positives

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Chapter Overview

This chapter surveys the general characteristics of the actinomycetes and other organisms that are classified as high G + C gram-positives in the 2nd edition of Bergey's Manual. The actinomycetes are filamentous bacteria that form branching hyphae and asexual spores.

Chapter Objectives

After reading this chapter you should be able to:

• describe the filamentous actinomycetes
• describe the morphology and arrangement of spores, cell wall chemistry, and the types of sugars present in cell extracts within the actinomycetes
• discuss the roles of actinomycetes in the mineralization of organic compounds and in the production of antibiotics
• describe the important human pathogens contained in the genera Corynebacterium and Mycobacterium

These are the most important concepts you are learning in this chapter:

1. Volume 4 of the first edition of Bergey’s Manual contains aerobic, gram-positive bacteria — the actinomycetes — that form branching hyphae and asexual spores.
2. The morphology and arrangement of spores, cell wall chemistry, and the types of sugars present in cell extracts are particularly important in actinomycete taxonomy and are used to divide these bacteria into different groups.
3. The second edition of Bergey’s Manual will classify high G+C gram-positive bacteria using 16S rRNA data. They will be placed in the class Actinobacteria, which will contain the actinomycetes in volume 4 of the first edition plus gram-positive bacteria from sections 12, 15, 16, and 17 of volume 2.
4. Actinomycetes have considerable practical impact because they play a major role in the mineralization of organic matter in the soil and are the primary source of most naturally synthesized antibiotics. The genera Corynebacterium and Mycobacterium contain important human pathogens.

Study Outline

1. General Properties of the Actinomycetes
1. Exhibit filamentous growth
1. Form substrate mycelia
2. 2 Septa divide the mycelia into long cells (20 um and longer), each containing several nucleoids
3. Some form a tissue-like mass called a thallus
4. They may have aerial mycelia that form conidospores at the ends of filaments or that form sporangiospores within a sporangium; spores are not heat resistant but withstand desiccation
2. Actinomycetes are generally nonmotile, but spores may be flagellated
3. Cell wall types vary and can be distinguished by the amino acid in position 3 of the tetrapeptide, the presence of glycine in the interpeptide bridge, and the sugar content; four types are known
4. Cell wall type, sugars in extracts, morphology and color of mycelia and sporangia, G + C content, membrane phospholipid composition, and heat resistance of the spores are all important in classifying these organisms, as is comparison of 16S rRNA sequences and pulse-field electrophoresis of large DNA fragments produced by restriction endonuclease digestion
5. Are of considerable practical importance
1. Those in soil degrade a number of organic compounds and are important in the mineralization processes; also produce most of the medically important, naturally synthesized antibiotics
2. A few species are pathogenic in humans, other animals, and plants
2. High G + C Gram-Positive Bacteria in Bergey?s Manual
1. The 1st edition of Bergey?s Manual divides the actinomycetes into 7 sections, primarily based on cell wall type, conidia arrangement, and the presence or absence of a sporangium
2. The 2nd edition uses 16S rRNA sequences to create a large phylum, Actinobacteria, containing one class Actinobacteria, five subclasses, six orders, 14 suborders and 40 families
3. This chapter focuses on the subclass Actinobacteridae and the order Actinomycetales; the order Bfidobacteriales is also briefly described
3. Suborder Actinomycineae
1. Most genera are irregularly shaped, nonsporing rods with aerobic or facultative metabolism
2. Genus Actinomyces
1. Straight or slightly curved rods and slender filaments with true branching
2. Facultative or obligate anaerobes; require CO2 for best growth
3. Cell walls contain lysine but not diaminopimelic acid
4. Normal inhabitants of mucosal surfaces of warm-blooded animals; some cause disease in their hosts
4. Suborder Micrococcineae
1. Contains 10 families and many genera
2. Genus Micrococcus
1. Aerobic, catalase-positive cocci that occur in pairs, tetrads or irregular clusters; usually nonmotile
2. Often, yellow, red, or orange pigmented
3. Widespread in soil, water, and on mammalian skin; usually not pathogenic
3. Genus Arthrobacter
1. Aerobic, catalase-positive rods with respiratory metabolism and lysine in peptidoglycan
2. Exhibit a rod-coccus growth cycle
1. When growing in exponential phase they are rods that reproduce by a snapping division
2. In stationary phase they change to a coccoid form
3. Upon transfer to fresh medium, coccoid cells produce outgrowths and resume active reproduction as rods
3. Most important habitat is soil
1. Resistant to desiccation and nutrient deprivation
2. Very flexible nutritionally; able to degrade some herbicides and pesticides
4. Genus Dermatophilus
1. Forms packets of motile spores with tufts of flagella
2. Facultative anaerobe
3. Mammalian parasite responsible for a skin infection called streptothrichosis
5. Suborder Corynebacterineae
1. Contains seven families with several important genera
2. Genus Corynebacterium (family Corynebacteriaceae)
1. Aerobic and facultative species; catalase-positive; straight to slightly curved rods, often with tapered ends
1. Remain partially attached after snapping division resulting in angular arrangements
2. Form metachromatic granules
3. Cell walls contain meso-diaminopimelic acid
2. Some species are harmless soil and water saprophytes; many are animal and human pathogens (e.g., C. diphtheriae-causative agent of diphtheria in humans)
3. Genus Mycobacterium (family Mycobacteriaceae)
1. Straight or slightly curved rods that sometimes branch or form filaments
2. Aerobic and catalase-positive; grow very slowly
3. Cell walls contain waxes with 60-90 carbon mycolic acids-make them acid-fast (i.e. basic fuchsin dye cannot be removed with acid-alcohol treatment)
4. Some are free-living saprophytes; but they are best known as human and animal pathogens
1. M. bovis-tuberculosis in cattle and other ruminants
2. M. tuberculosis-tuberculosis in humans
3. M. leprae-causes leprosy in humans
4. Genera Nocardia and Rhodococcus (family Nocardiaceae)
1. These and related species are collectively called nocardioforms
2. Develop a substrate mycelium that readily breaks into rods and coccoid elements; some develop aerial mycelia
3. Most are strict aerobes
4. They are found in soil and aquatic habitats
1. Nocardia degrade hydrocarbons and waxes and are involved in biodegradation of rubber joints in water and sewage pipes; most are free-living saprophytes, but some species (e.g., N. asteroides) are opportunistic pathogens causing nocardiosis
2. Rhodococcus can degrade a wide variety of molecules, including those found in toxic wastes
6. Suborder Micromonosporineae
1. Contains many genera that are often referred to as Actinoplanetes
1. Extensive substrate mycelia; aerial mycelia are absent or rudimentary
2. Form conidiospores within a sporangium that extends above the surface of the substratum; spores can be motile or nonmotile
3. Genera vary in arrangement and development of spores
2. Found in soil and freshwater habitats and occasionally in the ocean
1. Soil dwellers play an important role in plant and animal decomposition
2. Some produce antibiotics such as gentamicin
7. Suborder Propionibacterineae
1. Contains two families and 10 genera
2. Genus Propionibacterium
1. Pleomorphic, nonmotile rods that are often club shaped; cells may also be coccoid or even branched; single cells, short chains, or in clumps
2. Facultatively anaerobic or aerotolerant; ferment sugars to produce propionic acid
3. Found on skin and in the digestive tract of animals; also in dairy products such as cheese; contribute to the production of Swiss cheese; P. acne is involved in the development of body odor and acne vulgaris
3. Suborder Streptomycineae
4. Only one family, Streptomycetaceae and three genera
1. Have aerial mycelia that divide in a single plane to form chains of nonmotile conidiospores
2. Commonly called streptomycetes
5. Genus Streptomyces
1. An enormous genus with around 500 species 2 Strict aerobes
2. Form nonmotile spores within a thin sheath
6. Streptomycetes are ecologically and medically important
1. Natural habitat is soil where they represent from 1-20% of the organisms present (impart the characteristic odor of moist earth by producing volatile substances such as geosmin)
2. Metabolically flexible; major contributors to mineralization
3. Best known for the synthesis of a vast array of antibiotics useful in medicine and research
4. Only S. somaliensis is known to be pathogenic in humans; it causes actinomycetoma, an infection of subcutaneous tissues that produces swelling, abscesses and bone destruction
7. Genus Streptoverticillium-has aerial hyphae in a whorl of three to six short branches
8. Suborder Streptosporangineae
1. Contains 3 families and 14 genera
2. Many referred to as maduromycetes because the sugar madurose (3-O-methyl-D-galactose) is found in their cell extracts; have aerial mycelia that produce pairs or short chains of spores; substrate mycelia are branched; some genera form sporangia
3. Genus Thermomonospora-produce single spores on the aerial mycelium or on both the aerial and the substrate mycelium; isolated from high temperature habitats such as compost piles and hay
9. Suborder Frankineae
1. Genera Frankia and Geodermatophilus
1. Form clusters of spores
2. The genus Geodermatophilus has motile spores and is an aerobic soil organism
3. The genus Frankia
1. Forms nonmotile sporangiospores in a sporogenous body
2. Grows in symbiotic relationship with at least 8 families of higher nonleguminous plants
3. Microaerophilic and able to fix atmospheric nitrogen
2. Genus Sporichthya-lack a substrate mycelium but use holdfasts to anchor to the substratum; grow upward to form aerial mycelia that release motile, flagellated conidia in the presence of water
10. Order Bfidobacteriales
1. Contains one true family and 8 genera
2. Genera Falcivibrio and Gardnerella are found in the human genitourinary tract; Gardnerella may be a major cause of vaginitis
3. Genus Bfidobacterium is best studied
1. Nonmotile, nonsporing, gram-positive rods of varied shapes that are slightly curved and clubbed; often they are branched; rods can be single cells, in clusters or in V-shaped pairs
2. Anaerobic and ferment carbohydrates to produce acetic and lactic acids but no carbon dioxide
3. Found in the mouth and intestinal tract of warm-blooded animals, in sewage, and in insects
1. B.bfidus is a pioneer colonizer of the human intestinal tract, particularly when babies are breast-fed
2. Some infections of humans have been reported but does not appear to be a major cause of disease