2 Septa divide the mycelia into long cells (20 um and longer), each
containing several nucleoids
Some form a tissue-like mass called a thallus
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
Actinomycetes are generally nonmotile, but spores may be flagellated
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
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
Are of considerable practical importance
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
A few species are pathogenic in humans, other animals, and plants
High G + C Gram-Positive Bacteria in Bergey's Manual
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
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
This chapter focuses on the subclass Actinobacteridae and the order
Actinomycetales; the order Bfidobacteriales is also briefly described
Suborder Actinomycineae
Most genera are irregularly shaped, nonsporing rods with aerobic or
facultative metabolism
Genus Actinomyces
Straight or slightly curved rods and slender filaments with true
branching
Facultative or obligate anaerobes; require CO2 for best growth
Cell walls contain lysine but not diaminopimelic acid
Normal inhabitants of mucosal surfaces of warm-blooded animals;
some cause disease in their hosts
Suborder Micrococcineae
Contains 10 families and many genera
Genus Micrococcus
Aerobic, catalase-positive cocci that occur in pairs, tetrads or
irregular clusters; usually nonmotile
Often, yellow, red, or orange pigmented
Widespread in soil, water, and on mammalian skin; usually not pathogenic
Genus Arthrobacter
Aerobic, catalase-positive rods with respiratory metabolism and
lysine in peptidoglycan
Exhibit a rod-coccus growth cycle
When growing in exponential phase they are rods that reproduce
by a snapping division
In stationary phase they change to a coccoid form
Upon transfer to fresh medium, coccoid cells produce outgrowths
and resume active reproduction as rods
Most important habitat is soil
Resistant to desiccation and nutrient deprivation
Very flexible nutritionally; able to degrade some herbicides
and pesticides
Genus Dermatophilus
Forms packets of motile spores with tufts of flagella
Facultative anaerobe
Mammalian parasite responsible for a skin infection called streptothrichosis
Suborder Corynebacterineae
Contains seven families with several important genera
Genus Corynebacterium (family Corynebacteriaceae)
Aerobic and facultative species; catalase-positive; straight to
slightly curved rods, often with tapered ends
Remain partially attached after snapping division resulting
in angular arrangements
Form metachromatic granules
Cell walls contain meso-diaminopimelic acid
Some species are harmless soil and water saprophytes; many are
animal and human pathogens (e.g., C. diphtheriae-causative agent of
diphtheria in humans)
Genus Mycobacterium (family Mycobacteriaceae)
Straight or slightly curved rods that sometimes branch or form
filaments
Aerobic and catalase-positive; grow very slowly
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)
Some are free-living saprophytes; but they are best known as human
and animal pathogens
M. bovis-tuberculosis in cattle and other ruminants
M. tuberculosis-tuberculosis in humans
M. leprae-causes leprosy in humans
Genera Nocardia and Rhodococcus (family Nocardiaceae)
These and related species are collectively called nocardioforms
Develop a substrate mycelium that readily breaks into rods and
coccoid elements; some develop aerial mycelia
Most are strict aerobes
They are found in soil and aquatic habitats
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
Rhodococcus can degrade a wide variety of molecules, including
those found in toxic wastes
Suborder Micromonosporineae
Contains many genera that are often referred to as Actinoplanetes
Extensive substrate mycelia; aerial mycelia are absent or rudimentary
Form conidiospores within a sporangium that extends above the surface
of the substratum; spores can be motile or nonmotile
Genera vary in arrangement and development of spores
Found in soil and freshwater habitats and occasionally in the ocean
Soil dwellers play an important role in plant and animal decomposition
Some produce antibiotics such as gentamicin
Suborder Propionibacterineae
Contains two families and 10 genera
Genus Propionibacterium
Pleomorphic, nonmotile rods that are often club shaped; cells may
also be coccoid or even branched; single cells, short chains, or in
clumps
Facultatively anaerobic or aerotolerant; ferment sugars to produce
propionic acid
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
Suborder Streptomycineae
Only one family, Streptomycetaceae and three genera
Have aerial mycelia that divide in a single plane to form chains
of nonmotile conidiospores
Commonly called streptomycetes
Genus Streptomyces
An enormous genus with around 500 species 2 Strict aerobes
Form nonmotile spores within a thin sheath
Streptomycetes are ecologically and medically important
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)
Metabolically flexible; major contributors to mineralization
Best known for the synthesis of a vast array of antibiotics useful
in medicine and research
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
Genus Streptoverticillium-has aerial hyphae in a whorl of three to
six short branches
Suborder Streptosporangineae
Contains 3 families and 14 genera
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
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
Suborder Frankineae
Genera Frankia and Geodermatophilus
Form clusters of spores
The genus Geodermatophilus has motile spores and is an aerobic
soil organism
The genus Frankia
Forms nonmotile sporangiospores in a sporogenous body
Grows in symbiotic relationship with at least 8 families of
higher nonleguminous plants
Microaerophilic and able to fix atmospheric nitrogen
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
Order Bfidobacteriales
Contains one true family and 8 genera
Genera Falcivibrio and Gardnerella are found in the human genitourinary
tract; Gardnerella may be a major cause of vaginitis
Genus Bfidobacterium is best studied
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
Anaerobic and ferment carbohydrates to produce acetic and lactic
acids but no carbon dioxide
Found in the mouth and intestinal tract of warm-blooded animals,
in sewage, and in insects
B.bfidus is a pioneer colonizer of the human intestinal tract,
particularly when babies are breast-fed
Some infections of humans have been reported but does not appear
to be a major cause of disease
