Actinobacteria & Actinomyces

Phylum: Actinobacteria

• Gram-positive, aerobic bacteria commonly found in soil
• Some are pathogens, like Mycobacterium
• Certain species form branching filaments and resemble mycelia of fungi
• High G+C bacteria have unusually large amounts of guanine and cytosine
• Actinomyces israelii is a slightly anaerobic bacterium usually found as oral flora that can spread
• It produces sulfur granules in anaerobic conditions

Genus: Actinomyces

• Exhibit diversity in form, ranging from straight to slightly curved rods to slender filaments with branching
• Irregular forms display diphtheroid arrangements like "V", "Y", and "T" shapes
• Mature colonies are often soft to mucoid, or rough and crumbly, lack aerial filaments
• Pigmentation is usually white to gray or creamy, with some being red pigmented
• Frequently isolated from animal and human bodies
• A. bovis causes actinomycosis in cattle
• A. israelii causes cervicofacial, thoracic, and abdominal actinomycosis, and dental plaque in humans
• A. odontolyticus causes dental plaque and calculus

Genus: Micrococcus

• Cells are spherical, typically in pairs, tetrads, or irregular clusters
• Produce yellow carotenoid pigments
• Grow in the presence of 5% NaCl and produce acid from glucose without gas formation
• Found in aerobic habitats, including mammalian skin, dairy products, soils, and water

Genus: Arthrobacter

• Characterized by irregular shapes (club or V-shaped) due to snapping division during growth
• Cell walls are two-layered, with only the inner layer involved in cross wall formation
• Daughter cells remain attached by the outer cell wall layer, which leads to cell bending and V-shape development
• These are non-pathogenic soil microorganisms that convert from rod to sphere and back to rod during their life cycle
• Exhibit resistance to desiccation and starvation without endospore formation
• Demonstrate nutritional versatility, with some strains decomposing herbicides, caffeine, nicotine, phenol, and other organic compounds
• Play a major role in the mineralization of organic matter in soil

Genus: Dermatophilus and Geodermatophilus

• Do not form mycelia like other actinomycetes, forming multilocular sporangia
• High G+C content but lack mycolic acids
• Characterized by a life cycle with two growth phases
• R-form consists of motile flagellated rods that reproduce by budding.
• C-form consists of non-motile aggregates of coccoid cells increasing in size before reproducing through cleavage
• Interconversion between R and C forms is nutritionally controlled, particularly by the presence or absence of peptones

Genus: Corynebacterium

• Gram-positive, aerobic, non-motile, club-shaped rods
• Divide by snapping division, similar to Arthrobacter
• Composed of a diverse group of animal and plant pathogens, and saprophytes
• C. diphtheriae causes diphtheria

Genus: Gordonia

• Members are Gram-positive to Gram-variable, slightly acid-fast, non-motile, nocardioform actinomycetes
• Mycelial growth occurs through fragmentation into rod-shaped or coccoid elements
• Colonies range from slimy, smooth, and glossy, to irregular and rough, depending on the growth medium
• Colony color ranges from white, yellow, and tannish to orange, red, and pink
• Capable of degrading xenobiotics, environmental pollutants, and slowly biodegradable natural polymers
• Some Gordonia species are opportunistic pathogens, which may restrict their environmental application

Genus: Nocardia

• Gram-positive aerobic bacteria that form a fungal mycelium that breaks into rod-shaped or coccoid elements and are pleomorphic
• Rudimentary to extensively branched vegetative hyphae grow on the surface or penetrate agar
• Some strains are pathogenic to humans and animals

Genus: Micromonospora

• Form septate hyphae which are branched and non-fragmenting; Aerial mycelium rarely develops
• Compact substrate mycelium composes the colored vegetative colonies
• No sheath is formed
• Brown spores are produced singly at the tips of the subsurface mycelium
• Changes in colony color are due to sporulation

Genus: Mycobacterium

• Rod-shaped cells sometimes pleomorphic, may form filaments that fragment into rods and cocci

• Exhibit acido-alcohol fastness due to unique lipid components called mycolic acids on the cell surface

• Acido-alcohol staining property is taxonomically significant in defining the genus

• Considered Gram-positive, but can convert if the cell's lipoidal components is removed with alkaline ethanol

• Cell walls contain peptidoglycan covalently bound to arabinose-galactomycolic acid polymer

• The lipid-polysaccharide peptidoglycan complex confers hydrophobic characteristics

• On solid media, colonies are tight, compact and wrinkled, clumping together because high lipid content and hydrophobic nature

• Produce yellow carotenoid pigments and are classified into three groups depending on presence of light

  • Non-pigmented colonies
  • Those that produce pigments only in the light (photochromogenesis)
  • Those that produce pigments even in the dark (scotochromogenesis)

Genus: Propionibacterium

• First discovered in Swiss cheese, where fermentative production of CO2 creates characteristic holes
• Propionic acid produces the cheese's unique flavor
• Gram-positive, pleomorphic, non-sporulating, non-motile, and anaerobic
• Ferment lactic acid, carbohydrates, and polyhydroxy alcohols to produce propionic, succinic, and acetic acids, and CO2
• Require complex nutrients
• Important in the manufacture of dairy products

Genus: Streptomyces

• Form distinct mycelial masses like fungi that gives bacteria a large surface area
• Produce exospores and specialized filaments to support sporulation
• Exospores have a dual role, as a method of reproduction and a survival mechanism in harsh conditions
• Mainly soil organisms, also found in aquatic habitats
• Characteristic earthy odor of soil is due to geosmins, a series of streptomycetes metabolites

Genus: Frankia

• Nitrogen-fixing actinomycete
• Inhabits nodules of non-leguminous woody plants
• Cells are endosymbionts of many trees
• The root nodules are about the size of tennis balls
• Root nodules consist of densely packed coral-like branching roots that have ceased to grow
• Vegetative hyphae are branched, without aerial mycelium
• Multi-locular sporangia borne terminally, laterally, or on vegetative hyphae

Phylum: Chlamydiae

• Obligate parasites
• Cause diseases in humans; C. trachomatis causes trachoma, an Egyptian eye disease
• Diseases are transmitted by contact, with birds as main hosts
• Energy parasites that depend on the host cell metabolism

Similarities of Rickettsias and Chlamydias to Viruses

• Obligate parasites need living cells for growth and multiplication
• Same size as viruses
• Pass through bacterial filters

Similarities of Rickettsias and Chlamydias to Bacteria

• Cell envelope structure is the same as Gram-negative eubacteria
• Cells are cocco-bacillary
• Possess a single chromosome made of unbounded fibrillar nuclear material DNA
• Possess ribosomes, a trilaminar CM, and a bacterial-type cell wall
• Contain RNA and DNA in a ratio characteristic of bacteria

Genus: Chlamydia

• Gram-negative, non-motile coccoid organism
• Multiplies within membrane-bounded vacuoles in the cytoplasm of a host cell during its unique cycle
• Elementary bodies = infectious with electron-dense material, few ribosomes, rigid walls
• Reticulate bodies = larger, less dense, thin flexible walls, non infective
• Cycle completes when reticulate bodies condense into new infectious elementary bodies.

Phylum: Spirochaetes and Genus: Spirochaeta

• Gram-negative enteric bacteria with a unique morphology and motility
• Aerobic, facultative and/or anaerobic chemoheterotrophs
• Typically slender, flexuous, helical (coiled) cells
• Protoplasmic cylinder, of the cell, contains cytoplasmic membrane and cell wall
• Fibrils or axial filaments attach to both cell poles and wrap the cylinder
• Axial fibrils and the cylinder have three-layered outer sheath
• Axial fibrils support motility; with a free sheath, the cell rotates and moves; with a solid surface, the cell creeps forward

Genus: Treponema

• Anaerobic parasites of humans and animals
• T. pallidum causes syphilis
• T. pallidum are sensitive to increased temperatures and rapidly killed at 41-42°C
• Kills rapidly by drying, transmission typically requires direct contact, usually sexually

Phylum: Bacteroidetes

• Composed of Bacteroides, Flavobacteria, and Sphingobacteria classes
• Related by similarity in the composition of the small 16S subunit of their ribosomes
• Bacteroides are human commensals and sometimes pathogens
• The other classes are rarely pathogenic to humans

Class Bacteroides and Genus Bacteroides

• Anaerobic, Gram-negative bacilli or coccobacilli, saccharolytic, produce acetate and succinate
• DNA base composition ranges from 40-48 mol% GC
• The anaerobic counterpart of E. coli, but somewhat smaller
• Non-spore-forming but produce large capsules
• Limited pathogenicity due to a lack of endotoxin in their cell membrane
• Infection typically follows severe trauma to the abdominal region, can cause, abscesses, and fever
• Antibiotic treatment usually involves metronidazole or clindamycin

Class Flavobacteria and Genus Flavobacterium

• Found in soil, water, plants, food and water systems, anesthetics, eyes, urine, and stool samples
• Most species are yellow pigmented
• Some fix nitrogen and/or produce acid
• Highly antibiotic resistant
• Fastidious microbes; some strains are pathogenic to humans and animals
• 312 strains exist
• 250 of Flavobacterium are acid-producing
• Acid-producing strains include meningosepticum, the strain which causes meningitis

Class Sphingobacteria, Genus Cytophaga

• Long, slender rods with pointed ends
• Chemoorganotrophs; aerobic or facultative anaerobic
• Able to degrade bio-macromolecules like proteins, starch, cellulose, pectin, agar, or chitin
• Widespread in soil and water
• Cellulose decomposers can be easily isolated
• Digest cellulose filters, forming transparent, spreading, yellow/orange colonies
• Aligned cellulases on cell surfaces that adhere to cellulose fibrils
• Degradation of agar can be recognized by the sinking of the colonies into the agar, resulting in zones of clearance around colonies

Sporocytophaga and Lysobacter

• Sporocytophaga organisms are similar in morphology and physiology to Cytophaga, but form resting spherical structures called microcepts
• Lysobacter are gliding, rod-shaped bacteria with the ability to lyse both bacteria and fungi
• This occurs through the action of an array of proteases, chitinases, and other lytic enzymes excreted into the medium