BACTERIA

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 Bacteria:  The Proteobacteria

Chapter Overview

This chapter presents the diverse group of eubacteria known as the proteobacteria. The distinguishing characteristics of these gram-negative bacteria: morphology, physiology, metabolism, and ecology are presented. The phylogenetic relationships are mengetahuied and representative species are examined

Chapter Objectives

After reading this chapter you should be able to:

1. discuss the importance of this diverse group of organisms
2. describe the diverse life styles and metabolism of members of this group of organisms
3. discuss the complex structures (prosthecae, stalks, buds, sheaths, or complex fruiting bodies produced by some members of this group
4. discuss the ecological impact by chemolithotrophic bacteria
5. discuss the dependence of parasitic bacteria, such as Bdellovibrio and the rickettsia, on their hosts for energy and/or cell constituents

These are the most important concepts you are learning in this chapter:

1. The proteobacteria of the second edition of Bergey’s Manual come from volumes 1 and 3 of the first edition. In the first edition bacteria are placed in a particular section based on a few major phenotypic properties such as general shape, nutritional type, motility, oxygen relationships, and so forth. The second edition uses nucleic acid sequences, particularly 16S rRNA sequence comparisons, to place bacteria in phylogenetic groupings.
2. Many of these gram-negative bacteria are of considerable importance, either as disease agents or because of their effects on the habitat. Others, such as E. coli, are major experimental organisms studied in many laboratories.
3. Although many of these bacteria do not vary drastically in general appearance, they often are very diverse in their metabolism and life-styles, which range from obligately intracellular parasitism to a free-living existence in soil and aquatic habitats.
4. Bacteria do not always have simple, unsophisticated morphology but may produce prosthecae, stalks, buds, sheaths, or complex fruiting bodies.
5. Chemolithotrophic bacteria obtain energy and electrons by oxidizing inorganic compounds rather than the organic nutrients employed by most bacteria. They often have substantial ecological impact because of their ability to oxidize many forms of inorganic nitrogen and sulfur.
6. Many bacteria that specialize in predatory or parasitic modes of existence, such as Bdellovibrio and the rickettsias, have relinquished some of their metabolic independence through the loss of metabolic pathways. They depend on the prey’s or host’s energy supply and/or cell constituents.

Study Outline

1. Introduction
1. The phylum Firmicutes contains cell wall-less bacteria (mycoplasmas) and the low G+C gram-positive bacteria
2. Firmicutes is divided into three classes: Mollicutes, Clostridia, and Bacilli
2. Class Mollicutes (The Mycoplasmas)
1. Has five orders and six families having the following characteristics:
1. Lack cell walls and cannot synthesize peptidoglycan precursors; therefore are penicillin resistant and susceptible to lysis by osmotic shock and detergent treatment
2. Are smallest bacteria capable of self-reproduction
3. Most are nonmotile but some can glide along liquid-covered surfaces
4. Most species require sterols (unusual for bacteria)
5. Usually facultative anaerobes but a few are strict anaerobes
6. Have some of the smallest genomes observed in procaryotes; G + C content ranges from 23 to 41%
7. Can be saprophytes, commensals or parasites
2. Metabolism is not particularly unusual
1. Are deficient in several biosynthetic pathways
2. Some produce ATP by the Embden-Meyerhoff pathway and lactic acid fermentation; others catabolize arginine to urea
3. Pentose phosphate pathway functions in some; none have a complete TCA cycle
3. Widespread
1. Can be isolated from plants, animals, soil, and compost piles
2. Serious contaminants of mammalian cell cultures; difficult to detect; difficult to eliminate
3. In animals, they colonize mucous membranes and joints and are often associated with diseases of the respiratory and urogenital tracts
4. Pathogenic species include:
1. M. mycoides-bovine pleuropneumonia in cattle
2. M. gallisepticum-chronic respiratory disease in chickens
3. M. pneumoniae-primary atypical pneumonia in humans
4. M hominis and Ureaplasma urealyticum-pathogenic in humans
5. Spiroplasmas-pathogenic in insects, ticks, and a variety of plants
3. Low G + C Gram-Positive Bacteria in Bergey?s Manual
1. First edition treats low G + C gram positives phenotypically
1. Classified on the basis of cell shape, clustering and arrangement of cells, presence or absence of endospores, oxygen relationships, fermentation patterns, peptidoglycan chemistry, etc.
2. Peptidoglycan structure varies considerably
1. 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
2. Others contain lysine cross-linked by interpeptide bridges
3. Others contain L,L-diaminopimelic acid and have one glycine as the interpeptide bridge
4. 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
5. Other cross-links and differences in cross-link frequency also contribute to variation in structure
6. These variations are characteristic of particular groups and are therefore taxonomically useful
3. Bacterial endospores are complex structures that allow survival under adverse conditions; sporeformers are distributed widely but found mainly in soil
2. 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
4. Class Clostridia
1. Contains three orders and 11 families
2. The largest genus is Clostridium
1. Obligate anaerobes, sporeformers, do not carry out dissimilatory sulfate reduction
2. Over 100 species in distinct phylogenetic clusters
3. Practical impact
1. Responsible for many cases of food spoilage, even in canned foods (e.g., C. botulinum)
2. C. perfringens-gas gangrene
3. C. tetani-tetanus
4. Some are of industrial value (e.g., C. acetobutylicum-used to manufacture butanol)
3. Genus Desulfotomaculum
1. Anaerobic, endospore-forming bacteria that reduce sulfate and sulfite to hydrogen sulfide during anaerobic respiration
2. Stains gram-negative but actually has a gram-positive type cell wall with a lower than normal peptidoglycan content
4. Genera Heliobacterium and Heliophilum
1. 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)
2. Stain gram negative but have gram-positive type cell wall with lower than normal peptidoglycan content
5. Genus Veillonella (family Veillonellaceae)
1. Anaerobic, chemoheterotrophic cocci
2. Usually diplococci
3. Have complex nutritional requirements; ferment carbohydrates, lactate and other organic acids, and amino acids; produce gas and a mixture of volatile fatty acids
4. Parasites of homeothermic animals; part of the normal microflora of the mouth, the gastrointestinal tract, and urogenital tract of humans and other animals
5. Class Bacilli
1. Order Bacilliales
1. Genus Bacillus
1. Largest genus in the order
2. Gram-positive, endospore-forming, chemoheterotrophic rods that are usually motile with peritrichous flagella
3. Usually aerobic, sometimes facultative, and catalase positive
4. 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
1. Thermophilic; form single spores on both aerial and substrate mycelia
2. Commonly found in damp haystacks, compost piles, and other high-temperature habitats
3. The spores are very heat-resistant and thus are true bacterial endospores-can survive 90oC for 30 minutes
4. T. vulgaris-causative agent for farmer's lung disease, an allergic respiratory disease in agricultural workers
3. Genus Caryophanon-strict aerobe, catalase positive, motile by peritrichous flagella; lives in cow dung; disk-shaped cells that join together to form rods
4. Genus Staphylococcus (family Staphylococcaceae)
1. Facultatively anaerobic, nonmotile cocci that form irregular clusters
2. Catalase positive, oxidase negative; ferment glucose anaerobically
3. Normally associated with skin, skin glands, and mucous membranes of warm-blooded animals
4. Cause many human diseases (e.g., endocarditis, wound infections, surgical infections, urinary tract infections, various skin infections, pneumonia, toxic shock syndrome, and food poisoning
5. 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
2. Order Lactobacilliales
1. Lactic acid bacteria-nonsporing, nonmotile, fermentative (lactic acid fermentation), nutritionally fastidious, facultative or aerotolerant anaerobes;
2. Largest genus is Lactobacillus with nearly 80 species
1. Can be rods and sometimes coccobacilli; lack catalase
2. Can carry out heterolactic or homolactic acid fermentation
3. Grow optimally between pH 4.5 and pH 6.4
4. Found on plant surfaces and in dairy products, meat, water, sewage, beer, fruits, and many other materials
5. Normal microflora of mouth, intestinal tract, and vagina; usually not pathogenic
6. Used in the production of fermented vegetable foods, beverages, sour dough, hard cheeses, yogurt, and sausages
7. Responsible for spoilage of beer, milk, and meat
3. Genus Leuconostoc (family Leuconostocaceae)
1. Facultatively cocci that may be elongated or elliptical shape; clustered in pairs or chains
2. Lack catalase; carry out heterolactic fermentation
3. Isolated from plants, silage, and milk
4. 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)
1. Most are facultative anaerobes; catalase negative; a few are obligate anaerobes
2. Form pairs or chains in liquid media; do not form endospores; nonmotile
3. Homolactic fermentation; produces lactic acid but no gas
4. 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
5. Members of the genera Enterococcus, Streptococcus, and Lactococcus have great practical importance:
1. S. pyogenes-causes streptococcal sore throat, acute glomerulonephritis and rheumatic fever
2. S. pneumonia-causes lobar pneumonia
3. S. mutans-associated with dental caries
4. E. faecalis-opportunistic pathogen that can cause urinary tract infections and endocarditis
5. L. lactis-used in the production of buttermilk and cheese