High G + C Gram-Positive Bacteria: The
This chapter surveys the general characteristics of members of the phylum Actinobacteria. All are high
G + C gram positives and many are actinomycetes. The actinomycetes are filamentous bacteria that form
branching hyphae and asexual spores.
After reading this chapter you should be able to:
describe actinomycetes
list and describe the phenotypes that are important to the taxonomy of actinobacteria
discuss the roles of actinomycetes in the mineralization of organic compounds and in the production of
describe the important human pathogens in Actinobacteria
General Properties of the Actinomycetes
A. Many of the high G + C gram-positive bacteria are actinomycetes; they are aerobic bacteria that
exhibit filamentous growth
1. Nearly all form substrate mycelia made of hyphae; septa divide the mycelia into long cells (20
µm and longer), each containing several nucleoids
2. They may have aerial mycelia that form conidiospores at the ends of filaments or
sporangiospores within a sporangium; spores are not heat resistant but withstand desiccation
B. Actinomycetes are generally nonmotile, but spores may be flagellated
C. 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 major types are known
D. 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
E. Considerable practical importance
1. Those in soil degrade a number of organic compounds and are important in the mineralization
process; also produce many of the medically important, natural antibiotics and drugs
2. A few species are pathogenic in humans, other animals, and plants
F. The high G + C gram positives were characterized primarily based on cell wall type, conidia
arrangement, and the presence or absence of a sporangium, but now 16S rRNA sequences have been
used to create a large phylum, Actinobacteria, containing one class, five subclasses, six orders, 14
suborders, and 44 families
Suborder Actinomycineae
A. Most genera (one family, five genera) are irregularly shaped, nonsporing rods (straight or slightly
curved; usually with swellings or other deviations from rod shape), with aerobic or facultative
B. Genus Actinomyces
Straight or slightly curved rods and slender filaments with true branching; may have swollen,
clubbed, or clavate ends
2. Facultative or obligate anaerobes; require CO2 for best growth
3. Cell walls contain lysine but not meso-diaminopimelic acid
4. Normal inhabitants of mucosal surfaces (often oral) of warm-blooded animals; some cause
disease in their hosts
III. Suborder Micrococcineae
A. Contains 14 families and many genera
B. Genus Micrococcus
1. Aerobic, catalase-positive cocci that occur in pairs, tetrads, or irregular clusters; usually
nonmotile; often have yellow, red, or orange pigmentation
2. Widespread in soil, water, and on mammalian skin; usually not pathogenic
C. Genus Arthrobacter
1. Aerobic, catalase-positive rods with respiratory metabolism and lysine in peptidoglycan
2. Exhibit a rod-coccus growth cycle
a. When growing in exponential phase, they are rods that reproduce by snapping division
b. In stationary phase, they change to a coccoid form
c. Upon transfer to fresh medium, coccoid cells produce outgrowths and resume active
reproduction as rods
3. Most important habitat is soil; also is isolated from fish, sewage, and plant surfaces
a. Resistant to desiccation and nutrient deprivation
b. Very flexible nutritionally; able to degrade some herbicides and pesticides
D. 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
IV. Suborder Corynebacterineae
A. Contains seven families with several important genera
B. Genus Corynebacterium (family Corynebacteriaceae)
1. Aerobic or facultative; catalase-positive; straight to slightly curved rods, often with tapered
ends; club-shaped forms also are seen
2. Remain partially attached after snapping division resulting in angular arrangements
3. Form metachromatic granules and their cell walls contain meso-diaminopimelic acid
4. Some species are harmless soil and water saprophytes; many are animal and human pathogens
(e.g., C. diphtheriae is the causative agent of diphtheria in humans)
C. Genus Mycobacterium (family Mycobacteriaceae)
1. Straight or slightly curved rods that sometimes branch or form filaments that readily fragment
2. Aerobic and catalase positive; grow very slowly
3. Cell walls contain waxes with 60 to 90 carbon mycolic acids; these make them acid-fast (i.e.,
basic fuchsin dye cannot be removed with acid-alcohol treatment)
4. Some are free-living saprophytes; but many are animal pathogens
a. M. bovis—tuberculosis in cattle and other ruminants
b. M. tuberculosis—tuberculosis in humans
c. M. leprae—leprosy in humans
d. M. avium—common mycobacterial disease
D. Genera Nocardia and Rhodococcus (family Nocardiaceae)
1. These and related genera 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. Most have peptidoglycan with meso-diaminopimelic acid and no peptide interbridge; mycolic
acids are present
5. They are found in soil and aquatic habitats
Members of 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
b. Rhodococcus can degrade a wide variety of molecules, including those found in toxic
V. Suborder Micromonosporineae
A. Contains one family and many genera; are often referred to as actinoplanetes
1. Extensive substrate mycelia; aerial mycelia are absent or rudimentary; have type IID cell walls
2. Form spores 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
B. 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
VI. Suborder Propionibacterineae
A. Contains two families and 14 genera
B. Genus Propionibacterium
1. Pleomorphic, nonmotile rods that are often club shaped; cells also may be coccoid or even
branched; arranged as single cells, short chains, or in clumps
2. Facultatively anaerobic or aerotolerant; can 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
VII. Suborder Streptomycineae
A. Contains 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
B. Genus Streptomyces
1. An enormous genus with around 150 species, but all are strict aerobes, have wall type I, and
form nonmotile spores within a thin sheath; one of the largest prokaryotic genomes with many
regulatory genes to control its complex life cycle
2. Species are distinguished based on morphological and physiological characteristics
C. 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,
including streptomycin, neomycin, tetracycline, erythromycin, amphotericin B,
chloramphenicol, and nystatin
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
VIII. Suborder Streptosporangineae
A. Contains three families and 16 genera
B. Many of the genera are referred to as maduromycetes because the sugar madurose (3-O-methyl-Dgalactose) 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
C. Genus Thermomonospora produces 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
IX. Suborder Frankineae
A. Genera Frankia and Geodermatophilus
1. Form clusters of spores and have type III cell walls
2. The genus Geodermatophilus has motile spores and is an aerobic soil organism
3. Genus Frankia
Forms nonmotile sporangiospores in a sporogenous body
Grows in symbiotic relationship with at least eight families of higher nonleguminous
c. Microaerophilic and able to fix atmospheric nitrogen within nodules formed in host plant
B. Genus Sporichthya lacks a substrate mycelium but uses holdfasts to anchor to the substratum; grows
upward to form aerial mycelia that release motile, flagellated conidia in the presence of water
Order Bifidobacteriales
A. Contains one family and 10 genera
B. Genera Falcivibrio and Gardnerella are found in the human genitourinary tract; Gardnerella may
be a major cause of vaginitis
C. Genus Bifidobacterium 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
a. B. bifidus is a pioneer colonizer of the human intestinal tract, particularly when babies
are breast-fed
b. Some infections of humans have been reported, but genus does not appear to be a major
cause of disease and are sold as probiotic agents to promote health
Place the letter of each term in the space next to the definition or description that best matches it.
____ 1.
____ 2.
____ 3.
____ 4.
____ 5.
____ 6.
____ 7.
A tissuelike mass of cells formed by actinomycetes
Asexual spores held on the ends of filaments
Asexual spores located within a structure at the end of
the filament
A volatile substance produced by members of the
genus Streptomyces that imparts the characteristic odor
of moist earth
An infection of subcutaneous tissues that produces
lesions and leads to swelling, abscesses, and bone
destruction if untreated
Complex fatty acids with a hydroxyl group on the βcarbon and an aliphatic chain attached to the α-carbon
conidiospores (conidia)
mycolic acids
Aerobic, gram-positive bacteria that form branching, usually nonfragmenting, hyphae and asexual spores
are called
Actinomycetes form asexual spores called
or ____________, if they are at the tips of hyphae,
if they are located in a sporangium.
The actinomycetes called
have hyphae that readily fragment into rods and coccoid
elements. Some also have
in their cell walls, as do mycobacteria.
Members of the genus Arthrobacter exhibit a type of reproduction in which the inner layer of the cell
grows inward to generate a transverse wall dividing the new cells. As the transverse wall thickens, it puts
pressure on the outer layer of the original cell wall, eventually causing the outer layer to rupture at its
weakest point. This type of reproduction is called
are composed of
and their high G + C relatives.
Members of the genus Mycobacterium are said to be ____________ (i.e., basic fuchsin dye cannot be
removed with acid-alcohol treatment). This is because their cell walls contain waxes with 60 to 90 carbon
____________ acids.
The maduromycetes, members of the suborder Streptosporangineae, all have type III cell walls
containing the sugar
Members of the suborder Streptomycineae are often called
For each of the questions below select the one best answer.
Which of the following is NOT a reason for
studying the actinomycetes?
a. They contribute to the mineralization of
organic material.
b. They produce the majority of the
medically useful, natural antibiotics.
c. Some members are pathogenic.
d. All of the above are reasons for
studying actinomycetes.
Which of the following is characteristic of
the members of the suborder
a. They lack a substrate mycelium.
b. Some species produce antibiotics such
as gentamicin.
c. Both (a) and (b) are correct.
d. Neither (a) nor (b) is correct.
Which of the following colors is frequently
observed in the colonies of members of the
genus Micrococcus?
a. red
b. yellow
c. orange
d. all of the above.
Which of the following is correct about the
genus Arthrobacter?
a. They are irregular, branched rods
during exponential growth.
b. They are coccoid in stationary phase.
c. Both (a) and (b) are correct.
d. Neither (a) nor (b) is correct.
A member of which of the following genera
causes diphtheria?
a. Corynebacterium
b. Mycobacterium
c. Propionibacterium
d. none of the above
Which of the following diseases is
caused by members of the genus
a. tuberculosis
b. leprosy
c. Both (a) and (b) are correct.
d. Neither (a) nor (b) is correct.
Which of the following is correct about
members of the genus Propionibacterium?
a. They contribute to the production of
Swiss cheese.
b. They cause acne vulgaris and contribute
to the development of body odor.
c. Both (a) and (b) are correct.
d. Neither (a) nor (b) is correct.
Which of the following is characteristic of
the streptomycetes?
a. They have type I cell walls.
b. They synthesize a vast array of
c. They play a major role in mineralization
d. All of the above are characteristic of the
Which of the following is NOT characteristic
of the Frankineae?
a. They produce clusters of spores at
hyphal tips.
b. They have type III cell walls.
c. Some form symbiotic associations with
leguminous plants.
d. Some can fix nitrogen.
Important Characteristics
(structural, physiological, ecological)
____ 1.
____ 2.
____ 3.
____ 4.
____ 5.
____ 6.
____ 7.
____ 8.
Most actinomycetes are not motile, but when present, motility is confined to flagellated spores.
Unlike most other actinomycetes, the nocardioforms have filaments that readily fragment into rods
and coccoid elements.
No streptomycetes are known to be pathogenic for humans.
In addition to being opportunistic pathogens, some species of Nocardia cause biodeterioration of
rubber joints in water and sewer pipes.
Streptomycetes can constitute up to 20% of the culturable microorganisms found in the
Of the nearly 500 species in the genus Streptomyces, only S. somaliensis is known to be pathogenic
in humans.
In the actinomycetes, six major types of cell walls can be distinguished.
Members of the genus Bifidobacterium are among the first colonizers of the intestinal tract of
nursing infants.
Discuss the major ecological and medical contributions of the genus Streptomyces.
Terms and Definitions
1. g, 2. b, 3. f, 4. c, 5. a, 6. d, 7. e
Fill in the Blank
1. actinomycetes 2. conidia; conidiospores; sporangiospores 3. nocardioforms; mycolic acids 4. snapping
division 5. actinobacteria; actinomycetes 6. acid-fast; mycolic 7. madurose 8. streptomycetes
Multiple Choice
1. d, 2. b, 3. d, 4. c, 5. a, 6. c, 7. c, 8. d, 9. c
1. T, 2. T, 3. F, 4. T, 5. T, 6. T, 7. F, 8. T