The phylum Firmicutes contains cell wall-less bacteria (mycoplasmas) and the low G+C gram-positive bacteria
Firmicutes is divided into three classes: Mollicutes, Clostridia, and Bacilli
Class Mollicutes (The Mycoplasmas)
Has five orders and six families having the following characteristics:
Lack cell walls and cannot synthesize peptidoglycan precursors; therefore are penicillin resistant and susceptible to lysis by osmotic shock and detergent treatment
Are smallest bacteria capable of self-reproduction
Most are nonmotile but some can glide along liquid-covered surfaces
Most species require sterols (unusual for bacteria)
Usually facultative anaerobes but a few are strict anaerobes
Have some of the smallest genomes observed in procaryotes; G + C content ranges from 23 to 41%
Can be saprophytes, commensals or parasites
Metabolism is not particularly unusual
Are deficient in several biosynthetic pathways
Some produce ATP by the Embden-Meyerhoff pathway and lactic acid fermentation; others catabolize arginine to urea
Pentose phosphate pathway functions in some; none have a complete TCA cycle
Widespread
Can be isolated from plants, animals, soil, and compost piles
Serious contaminants of mammalian cell cultures; difficult to detect; difficult to eliminate
In animals, they colonize mucous membranes and joints and are often associated with diseases of the respiratory and urogenital tracts
Pathogenic species include:
M. mycoides-bovine pleuropneumonia in cattle
M. gallisepticum-chronic respiratory disease in chickens
M. pneumoniae-primary atypical pneumonia in humans
M hominis and Ureaplasma urealyticum-pathogenic in humans
Spiroplasmas-pathogenic in insects, ticks, and a variety of plants
Low G + C Gram-Positive Bacteria in Bergey's Manual
First edition treats low G + C gram positives phenotypically
Classified on the basis of cell shape, clustering and arrangement of cells, presence or absence of endospores, oxygen relationships, fermentation patterns, peptidoglycan chemistry, etc.
Peptidoglycan structure varies considerably
Some contain meso-diaminopimelic acid cross-linked through its free amino group to the carboxyl group of the terminal D-alanine of the adjacent chain
Others contain lysine cross-linked by interpeptide bridges
Others contain L,L-diaminopimelic acid and have one glycine as the interpeptide bridge
Others use ornithine to cross-link between positions 2 and 4 of the peptide chains rather than positions 3 and 4 as used by the other forms
Other cross-links and differences in cross-link frequency also contribute to variation in structure
These variations are characteristic of particular groups and are therefore taxonomically useful
Bacterial endospores are complex structures that allow survival under adverse conditions; sporeformers are distributed widely but found mainly in soil
Second edition takes a phylogenetic approach dividing the low G + C gram positives into two classes: Clostridia and Bacilli; endospore-formers are found in both groups
Class Clostridia
Contains three orders and 11 families
The largest genus is Clostridium
Obligate anaerobes, sporeformers, do not carry out dissimilatory sulfate reduction
Over 100 species in distinct phylogenetic clusters
Practical impact
Responsible for many cases of food spoilage, even in canned foods (e.g., C. botulinum)
C. perfringens-gas gangrene
C. tetani-tetanus
Some are of industrial value (e.g., C. acetobutylicum-used to manufacture butanol)
Genus Desulfotomaculum
Anaerobic, endospore-forming bacteria that reduce sulfate and sulfite to hydrogen sulfide during anaerobic respiration
Stains gram-negative but actually has a gram-positive type cell wall with a lower than normal peptidoglycan content
Genera Heliobacterium and Heliophilum
Are anaerobic, photosynthetic bacteria that use bacteriochlorophyll g; have a photosystem like the green sulfur bacteria, but lack intracytoplasmic photosynthetic membranes (pigments are in the plasma membrane)
Stain gram negative but have gram-positive type cell wall with lower than normal peptidoglycan content
Genus Veillonella (family Veillonellaceae)
Anaerobic, chemoheterotrophic cocci
Usually diplococci
Have complex nutritional requirements; ferment carbohydrates, lactate and other organic acids, and amino acids; produce gas and a mixture of volatile fatty acids
Parasites of homeothermic animals; part of the normal microflora of the mouth, the gastrointestinal tract, and urogenital tract of humans and other animals
Class Bacilli
Order Bacilliales
Genus Bacillus
Largest genus in the order
Gram-positive, endospore-forming, chemoheterotrophic rods that are usually motile with peritrichous flagella
Usually aerobic, sometimes facultative, and catalase positive
Many species are of considerable importance: some produce antibiotics, some cause disease (e.g., B. cereus-causes food poisoning and B. anthracis-causes anthrax), and some are used as insecticides (e.g., B. thuringiensis and B. sphaericus)
Genus Thermoactinomyces
Thermophilic; form single spores on both aerial and substrate mycelia
Commonly found in damp haystacks, compost piles, and other high-temperature habitats
The spores are very heat-resistant and thus are true bacterial endospores-can survive 90oC for 30 minutes
T. vulgaris-causative agent for farmer's lung disease, an allergic respiratory disease in agricultural workers
Genus Caryophanon-strict aerobe, catalase positive, motile by peritrichous flagella; lives in cow dung; disk-shaped cells that join together to form rods
Genus Staphylococcus (family Staphylococcaceae)
Facultatively anaerobic, nonmotile cocci that form irregular clusters
Normally associated with skin, skin glands, and mucous membranes of warm-blooded animals
Cause many human diseases (e.g., endocarditis, wound infections, surgical infections, urinary tract infections, various skin infections, pneumonia, toxic shock syndrome, and food poisoning
Genus Listeria (family Listeriaceae)-short rods that are peritrichously flagellated; aerobic or facultative, catalase positive; L. monocytogenes is a human pathogen that causes listeriosis, an important food infection
Largest genus is Lactobacillus with nearly 80 species
Can be rods and sometimes coccobacilli; lack catalase
Can carry out heterolactic or homolactic acid fermentation
Grow optimally between pH 4.5 and pH 6.4
Found on plant surfaces and in dairy products, meat, water, sewage, beer, fruits, and many other materials
Normal microflora of mouth, intestinal tract, and vagina; usually not pathogenic
Used in the production of fermented vegetable foods, beverages, sour dough, hard cheeses, yogurt, and sausages
Responsible for spoilage of beer, milk, and meat
Genus Leuconostoc (family Leuconostocaceae)
Facultatively cocci that may be elongated or elliptical shape; clustered in pairs or chains
Lack catalase; carry out heterolactic fermentation
Isolated from plants, silage, and milk
Important in wine production, fermentation of vegetables such as cabbage and cucumbers, manufacture of buttermilk, butter, cheese, and dextrans; involved in food spoilage
Genus Streptococcus (family Streptococcaceae)
Most are facultative anaerobes; catalase negative; a few are obligate anaerobes
Form pairs or chains in liquid media; do not form endospores; nonmotile
Homolactic fermentation; produces lactic acid but no gas
The many species of this genus are distinguished by hemolysis reactions (b-hemolysis-incomplete with greenish zone or b-hemolysis-complete with clear zone but no greening), serologically, and by a variety of biochemical and physiological tests
Members of the genera Enterococcus, Streptococcus, and Lactococcus have great practical importance:
S. pyogenes-causes streptococcal sore throat, acute glomerulonephritis and rheumatic fever
S. pneumonia-causes lobar pneumonia
S. mutans-associated with dental caries
E. faecalis-opportunistic pathogen that can cause urinary tract infections and endocarditis
L. lactis-used in the production of buttermilk and cheese
