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Low G + C Gram-Positive Bacteria: The Firmicutes
CHAPTER OVERVIEW
This chapter describes the low G + C gram-positive bacteria. These bacteria are placed in the phylum
Firmicutes, which is divided into three classes: Mollicutes, Clostridia, and Bacilli. For each class, important
genera are discussed.
CHAPTER OBJECTIVES
After reading this chapter you should be able to:
•
•
•
discuss the various roles of these organisms and their pathogenic members
discuss the metabolism and special features of these organisms
describe the important taxa of the Firmicutes
CHAPTER OUTLINE
I.
II.
The Firmicutes
A. Divided into three classes (Mollicutes, Clostridia, and Bacilli), 10 orders, 34 families, 255 genera,
and over 1,300 species
B. All have low G + C content, and with the exception of the Mollicutes, which lack a cell wall; all
have gram-positive cell walls
Class Mollicutes (The Mycoplasmas)
A. Has five orders and six families with the following characteristics:
1. Lack cell walls and cannot synthesize peptidoglycan precursors; therefore are penicillinresistant, pleomorphic, and susceptible to lysis by osmotic shock and detergent treatment
2. Most are nonmotile, but some can glide along liquid-covered surfaces
3. Most species require sterols (unusual for bacteria)
4. Usually facultative anaerobes, but a few are strict anaerobes
5. Are smallest bacteria capable of self-reproduction; have some of the smallest genomes
observed in free-living prokaryotes; G + C content ranges from 23 to 41%
6. Can be saprophytes, commensals, or parasites
B. Metabolism is not particularly unusual
1. Deficient in several biosynthetic pathways
2. Some produce ATP by the Embden-Meyerhof pathway and lactic acid fermentation; others
catabolize arginine to urea; the pentose phosphate pathway functions in some; none has a
complete TCA cycle
C. Widespread
1. Can be isolated from plants, animals, soil, and compost piles
2. Serious contaminants of mammalian cell cultures, where they are difficult to detect and
difficult to eliminate
3. In animals, they colonize mucous membranes and joints and are often associated with diseases
of the respiratory and urogenital tracts; pathogenic species include:
a. M. mycoides—bovine pleuropneumonia in cattle
b. M. gallisepticum—chronic respiratory disease in chickens
c. M. pneumoniae—primary atypical pneumonia in humans
d. M. hominis and Ureaplasma urealyticum—pathogenic in humans
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e. Spiroplasmas—pathogenic in insects, ticks, and a variety of plants
III. Class Clostridia
A. Contains three orders and 11 families
B. The largest genus is Clostridium, having over 100 species in distinct phylogenetic clusters
1. Obligate anaerobes; form endospores; do not carry out dissimilatory sulfate reduction; can
produce ATP via fermentation of amino acids using other amino acids as electron sinks
(Strickland reaction)
2. Practical impact
a. Responsible for many cases of food spoilage, even in canned foods (e.g., C. botulinum,
botulism)
b. C. perfringens—gas gangrene and food poisoning
c. C. tetani—tetanus
d. Some are of industrial value (e.g., C. acetobutylicum, which is used to manufacture
butanol)
C. Genus Desulfotomaculum
1. Anaerobic, endospore-forming bacteria that reduce sulfate and sulfite to hydrogen sulfide
during anaerobic respiration
2. Stains gram negative but has a gram-positive type cell wall with a lower than normal
peptidoglycan content
D. Genera Heliobacterium and Heliophilum
1. 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)
2. Stain gram negative but have gram-positive type cell wall with lower than normal
peptidoglycan content
E. Genus Veillonella
1. Anaerobic, chemoheterotrophic cocci; usually diplococci
2. Have complex nutritional requirements; ferment carbohydrates, lactate and other organic acids,
and amino acids; produce gas and a mixture of volatile fatty acids; unable to ferment glucose
or other carbohydrates
3. Parasites of homeothermic animals; part of the normal microflora of the mouth, the
gastrointestinal tract, and urogenital tract of humans and other animals
IV. Class Bacilli
A. Consists of two orders, 17 families, and over 70 genera
B. Order Bacillales
1. Genus Bacillus (family Bacillaceae)
a. Largest genus in the order; includes the type species B. subtilis
b. Gram-positive, endospore-forming, chemoheterotrophic rods that are usually motile with
peritrichous flagella
c. Usually aerobic, sometimes facultative, and catalase positive
d. 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)
2. Genus Thermoactinomyces (family Thermoactinomycetaceae)
a. Thermophilic; form single spores within the hyphae of both aerial and substrate mycelia;
the spores are very heat-resistant and thus are true bacterial endospores
b. Commonly found in damp haystacks, compost piles, and other high-temperature habitats
c. T. vulgaris—causative agent for farmer’s lung disease, an allergic respiratory disease in
agricultural workers
3. Genus Sporosarcina—only coccoid bacterium to form endospores; very tolerant of alkali and
degrades urea
4. Family Staphylococcaceae
a. Contains five genera including Staphylococcus
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b.
C.
Facultatively anaerobic, nonmotile cocci that form irregular clusters; have teichoic acids
in their cell walls
c. Catalase positive; oxidase negative; ferment glucose anaerobically; respire aerobically
and some can reduce nitrate to nitrite
d. Normally associated with skin, skin glands, and mucous membranes of warm-blooded
animals
e. Cause many human diseases (e.g., endocarditis, wound infections, surgical infections,
urinary tract infections, various skin infections, pneumonia, toxic shock syndrome, and
food poisoning); those with coagulase or hemolysins most virulent
f.
Major health concern since they are often antibiotic resistant including methicillinresistant S. aureus (MRSA) and vancomycin-resistant S. aureus (VRSA)
5. Family Listeriaceae—short rods that are peritrichously flagellated; aerobic or facultative, and
catalase positive; Listeria monocytogenes is a human pathogen that causes listeriosis, an
important food-borne infection
Order Lactobacillales
1. Lactic acid bacteria—nonsporing; nonmotile; lack cytochromes; fermentative (lactic acid
fermentation); nutritionally fastidious; facultative or aerotolerant anaerobes
2. Largest genus is Lactobacillus with around 100 species
a. Can be rods and sometimes coccobacilli; lack catalase and cytochromes, and are
facultative or microaerophilic
b. Can carry out heterolactic or homolactic acid fermentation
c. Grow optimally between pH 4.5 and pH 6.4
d. Found on plant surfaces and in dairy products, meat, water, sewage, beer, fruits, and
many other materials; are also normal microflora of mouth, intestinal tract, and vagina;
usually not pathogenic
e. Used in the production of fermented vegetable foods, beverages, sour dough, hard
cheeses, yogurt, and sausages
f.
Responsible for spoilage of beer, milk, and meat
3. Genus Leuconostoc (family Leuconostocaceae)
a. Facultative cocci that may be elongated or elliptical shape; arranged in pairs or chains
b. Lack catalase; carry out heterolactic fermentation
c. Isolated from plants, silage, and milk
d. Important in wine production, fermentation of vegetables such as cabbage and
cucumbers, manufacture of buttermilk, butter, cheese, and dextrans; involved in food
spoilage
4. Genus Streptococcus (family Streptococcaceae)
a. Most are facultative anaerobes; catalase negative; a few are obligate anaerobes
b. Form pairs or chains in liquid media; do not form endospores; nonmotile
c. Homolactic fermentation; produces lactic acid but no gas
d. The many species of this genus are distinguished by hemolysis reactions (α-hemolysis—
incomplete with greenish zone or β-hemolysis—complete with clear zone but no
greening), serologically (e.g., Lancefield grouping system), and a variety of biochemical,
genetic, and physiological tests
5. Members of the genera Enterococcus, Streptococcus, and Lactococcus have great practical
importance
a. S. pyogenes—causes streptococcal sore throat, acute glomerulonephritis, and rheumatic
fever
b. S. pneumonia—causes lobar pneumonia
c. S. mutans—associated with dental caries
d. E. faecalis—opportunistic pathogen that can cause urinary tract infections and
endocarditis
e. L. lactis—used in the production of buttermilk and cheese
6. Streptococci can be divided into three groups:
a. Pyogenic streptococci—pus-producing pathogens; includes S. pyogenes
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b.
c.
Oral streptococci—normal flora of the mouth
Other streptococci—used in food production
TERMS AND DEFINITIONS
Place the letter of each term in the space next to the definition or description that best matches it.
____ 1.
____ 2.
____ 3.
____ 4.
____ 5.
Contains protein toxins that are insecticidal
An enzyme that causes blood plasma to clot
Incomplete lysis of red blood cells with formation of a
greenish ring around the bacterial colony
Complete lysis of red blood cells with no greenish
zone formed
Fermentation of amino acids, using other amino acids
as electron acceptors
a.
b.
c.
d.
e.
coagulase
α-hemolysis
β-hemolysis
parasporal body
Strickland reaction
FILL IN THE BLANK
1.
2.
3.
4.
Staphylococcus aureus is the most important staphylococcal human pathogen. It differs from other
staphylococcal species in that it is positive for the enzyme ____________, which causes blood plasma to
clot.
One way of distinguishing streptococci is by determining their effect on red blood cells. Some cause
____________ , which is only partially lysis of red blood cells, leading to the formation of a
____________ ring around the growing colony. Others cause ____________, which is characterized by a
zone of complete killing or lysis. Streptococci are often identified using the
, which is based on differences in the teichoic acids and
polysaccharides of streptococcal species.
The lactobacilli are
bacteria and are of importance in the food industry. They
carry out either a homolactic fermentation using the Embden-Meyerhof pathway or a
fermentation with the pentose phosphate pathway.
Some Firmicutes (called the
) actually stain gram negative because they lack a cell
wall. They are the smallest bacteria capable of self-reproduction.
MULTIPLE CHOICE
For each of the questions below select the one best answer.
1.
2.
Lactobacilli are normal inhabitants of the
human body. Where are they found?
a. mouth
b. intestines
c. vagina
d. all of the above
Which of the following can be attributed
to members of the genus Bacillus?
a. They produce certain useful
antibiotics.
b. They are the causative agents of
some food poisonings.
c. They have been used as a biological
insecticide.
d. All of the above are properties
attributed to members of the genus
Bacillus.
3.
4.
5.
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Which of the following is caused by Clostridium
perfringens?
a. gas gangrene
b. tetanus
c. botulism
d. None of the above is caused by this
organism.
Which genus produces endospores?
a. Staphylococcus
b. Streptococcus
c. Bacillus
d. Lactobacillus
Which of the following is produced by the
action of lactobacilli?
a. sauerkraut and pickles
b. hard cheeses
c. yogurt
d.
6.
7.
8.
All of the above involve lactobacilli
in their production.
Which of the following processes does
NOT involve members of the genus
Leuconostoc?
a. fermentation of vegetables like
cabbage (sauerkraut) and cucumbers
(pickles)
b. manufacturing of buttermilk, butter,
and cheese
c. manufacturing of beer
d. All of the above involve
Leuconostoc.
Which of the following is a characteristic
of the heliobacteria (genera
Heliobacterium and Heliophilum)?
a. They contain
bacteriochlorophyll g.
b. They have two photosystems.
c. They have a higher than normal
peptidoglycan content in their cell
walls.
d. all of the above
Which genus contains gram-positive cocci
in pairs or chains that are usually
facultative and carry out homolactic
fermentation?
a. Streptococcus
b. Enterococcus
c. Lactococcus
d. all of the above
9.
10.
11.
12.
13.
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Which of the following causes primary
atypical pneumonia in humans?
a. Streptococcus pneumoniae
b. Mycoplasma pneumoniae
c. Clostridium pneumoniae
d. none of the above
Which genus contains facultatively anaerobic,
nonmotile, gram-positive cocci that form
irregular clusters?
a. Streptococcus
b. Staphylococcus
c. Enterococcus
d. all of the above
Which of the following is a reason why
mycoplasmas are a major problem when
working with mammalian cell cultures?
a. They are difficult to detect.
b. They are difficult to eliminate.
c. Both (a) and (b) are correct.
d. Neither (a) nor (b) is correct.
Which of the following is characteristic of the
genus Desulfotomaculum?
a. Its members are aerobic.
b. Its members do not form endospores.
c. Its members use sulfate as a terminal
electron acceptor for anaerobic respiration.
d. Its members lack cell walls.
Which of the following is characteristic of the
family Veillonellaceae?
a. Its members have a higher than normal
peptidoglycan content in their cell walls.
b. Many of its members are parasites of
homeothermic animals.
c. Its members stain gram positive.
d. Its members are aerobic.
Firmicutes
Class
Mollicutes
Representative Genera
Important Characteristics
(structural, physiological, ecological)
Clostridia
Bacilli
Bacillales
Lactobacillales
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TRUE/FALSE
____ 1.
____ 2.
____ 3.
____
____
____
____
4.
5.
6.
7.
____ 8.
When the members of the gram-positive cocci are examined both phenetically and phylogenetically,
it is found that the two ways of classifying these organisms show a close match.
The streptococci carry out homolactic fermentation, producing lactic acid but no gas.
Leuconostoc will tolerate a higher sugar content (unlike most organisms) and is, therefore, a major
problem in sugar refineries. It can also cause food spoilage of foods that are baked in heavy syrup.
Lactobacilli are alkalophilic and prefer conditions that are slightly alkaline for optimal growth.
Mycoplasmas are serious contaminants of mammalian cell cultures.
Clostridium botulinum is responsible for cases of food spoilage, even in canned foods.
The division of gram-positive bacteria into groups having low G + C and high G + C content is not
supported by 16S rRNA analysis.
Members of the genus Thermoactinomyces are thermophilic and produce mycelia and endospores.
One species causes a disease called farmer’s lung.
CRITICAL THINKING
1.
In the production of yogurt, two organisms are used. The first to grow is Streptococcus thermophilus and
the second is Lactobacillus bulgaricus. This is true even though the two are inoculated into the milk
simultaneously. Explain. In your explanation be sure to consider the properties of lactobacilli and how
these may contribute to the sequence of events.
2.
Tetanus, which is caused by Clostridium tetani, is of serious concern in deep puncture wounds. However,
it is seldom a problem with surface lacerations. Using your knowledge of the properties of the genus
Clostridium, explain these observations.
3.
Many endospore-forming bacteria are inhabitants of soil. Speculate why this characteristic is
advantageous in soil environments.
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ANSWER KEY
Terms and Definitions
1. d, 2. a, 3. b, 4. c, 5. e
Fill in the Blank
1. coagulase 2. α-hemolysis; greenish; β-hemolysis; Lancefield grouping system 3. lactic acid; heterolactic
4. mycoplasmas
Multiple Choice
1. d, 2. d, 3. a, 4. c, 5. d, 6. c, 7. a, 8. d, 9. b, 10. b, 11. c, 12.c, 13. b
True/False
1. F, 2. T, 3. T, 4. F, 5. T, 6. T, 7. F, 8. T
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