Prescott’s Microbiology, 9th Edition
23
Firmicutes: The Low G 1 C Gram-Positive Bacteria
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.
LEARNING OUTCOMES
After reading this chapter you should be able to:
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identify several Clostridia spp. that form endospores
outline the reaction by which some Clostridia can ferment amino acids and relate this carbon substrate to their
environmental distribution and pathogenic potential
identify the terminal electron acceptors used by Desulfotomaculu,
distinguish phototrophy in the heliobacteria from that of other anoxygenic phototrophic bacteria
discuss the importance of Veillonella to human disease
list terminal electron acceptors and fermentation products produced by Bacillus spp. and staphylococci
discuss reasons that make Bacillus subtilis an important model organism
list three reasons Bacillus spp. are of practical importance
summarize the evolution and emergence of MRSA, and list at least two diseases caused by Staphylococcus
aureus
describe the structural and medical importance of Listeria monocytogenes
identify the genera that are considered lactic acid bacteria, and discuss their importance in the food industry
distinguish between enterococci and streptococci
name bacterial genera capable of hemolysis, and differentiate between hemolysis and hemolysis
CHAPTER OUTLINE
I.
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
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Prescott’s Microbiology, 9th Edition
1.
II.
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
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
b. 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 methicillin-resistant 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
C. 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
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manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
Prescott’s Microbiology, 9th Edition
d.
3.
4.
5.
6.
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
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
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
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 (tooth decay)
d. E. faecalis—opportunistic pathogen that can cause urinary tract infections and endocarditis
e. L. lactis—used in the production of buttermilk and cheese
Streptococci can be divided into three groups:
a. Pyogenic streptococci—pus-producing pathogens; includes S. pyogenes
b. Oral streptococci—normal flora of the mouth
c. Other streptococci—used in food production
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.
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manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
Prescott’s Microbiology, 9th Edition
3.
Many endospore-forming bacteria are inhabitants of soil. Speculate why this characteristic is advantageous in
soil environments.
4.
Are all the endospore forming bacteria Gram positive rods? If not, what variations on a theme are seen?
CONCEPT MAPPING CHALLENGE
Construct a concept map using the following terms and adding your own linking words.
Clostridia
Bacilli
spore forming
Streptococcaceae
pathogenic
Listeriaceae
obligate anaerobe
Staphylococcaceae
facultative anaerobe
hemolysis
mesophile
fermentation
catalase
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© 2014 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution in any
manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.