Bergey’s Manual of Systematic Bacteriology

 The definitive work on the taxonomic organization of bacteria is the latest edition of Bergey’s Manual of Systematic Bacteriology. First published in 1923, this publication taxonomically classifies, in the form of a key, known bacteria that have or have not been cultured or well described.

A companion volume, Bergey’s Manual of Determinative Bacteriology, serves as an aid in the identification of bacteria that have been described and cultured. The major bacteria that cause infectious diseases, as categorized in Bergey’s Manual, are listed in Table 1. Because it is likely that emerging information concerning phylogenetic relationships will lead to further modifications in the organization of bacterial groups within Bergey’s Manual, its designations must be regarded as a work in progress.

Table1. Major Categories and Groups of Bacteria That Cause Disease in Humans as Part of an Identification Scheme Described in Bergey’s Manual of Determinative Bacteriology, 9th Ed.

Table1. Major Categories and Groups of Bacteria That Cause Disease in Humans as Part of an Identification Scheme Described in Bergey’s Manual of Determinative Bacteriology, 9th Ed. (Continued)

As discussed in Chapter 2, there are two different groups of prokaryotic organisms: eubacteria and archaebacteria. Both are small unicellular organisms that replicate asexually. Eubacteria refer to classic bacteria as science has historically understood them. They lack a true nucleus, have characteristic lipids that make up their membranes, possess a peptidoglycan cell wall, and have a protein and nucleic acid synthesis machinery that can be selectively inhibited by antimicrobial agents. In contrast, archaebacteria do not have a classic peptidoglycan cell wall and have many characteristics (eg, protein synthesis and nucleic acid replication machinery) that are similar to those of eukaryotic cells.

The Eubacteria

 A. Gram-Negative Eubacteria

This is a heterogeneous group of bacteria that have a com plex (Gram-negative type) cell envelope consisting of an outer membrane, a periplasmic space containing a thin peptidoglycan layer, and a cytoplasmic membrane. The cell shape (Figure 1) may be spherical, oval, straight or curved rods, helical, or filamentous; some of these forms may be sheathed or encapsulated. Reproduction is by binary fission, but some groups reproduce by budding. Fruiting bodies and myxo spores may be formed by the myxobacteria. Motility, if present, occurs by means of flagella or by gliding motility. Members of this category may be phototrophic or nonphototrophic  bacteria and include aerobic, anaerobic, facultatively anaerobic, and microaerophilic species.

Fig1. The cell shapes that occur among unicellular true bacteria. A: Coccus. B: Rod. C: Spiral. (Phase contrast, 1500×.) (Reproduced with permission from Stanier RY, Doudoroff M, Adelberg EA: The Microbial World, 3rd ed. Copyright © 1970. Reprinted by permission of Pearson Education, Inc., New York, New York.)

B. Gram-Positive Eubacteria

These bacteria have a cell wall profile of the Gram-positive type; cells generally, but not always, stain Gram-positive. The cell envelope of Gram-positive organisms consists of a thick cell wall that determines cellular shape and a cytoplasmic membrane. These cells may be encapsulated and can exhibit flagellar-mediated motility. Cells may be spherical, rods, or filaments; the rods and filaments may be nonbranching or may show true branching. Reproduction is generally by binary fission. Some bacteria in this category produce spores (eg, Bacillus and Clostridium spp.) as resting forms that are highly resistant to disinfection. The Gram-positive eubacteria are generally chemosynthetic heterotrophs  and include aerobic, anaerobic, and facultatively anaerobic species. The groups within this category include simple asporogenous and sporogenous bacteria as well as the structurally complex actinomycetes and their relatives.

C. Eubacteria Lacking Cell Walls

These are microorganisms that lack cell walls (commonly called mycoplasmas and making up the class Mollicutes) and do not synthesize the precursors of peptidoglycan. They are enclosed by a unit membrane, the plasma membrane (Figure 2). They resemble the L-forms that can be generated by breaking down the cell wall of notably Gram-positive eubacteria; unlike L-forms, however, mycoplasmas never revert to the walled state.

Fig2. Electron micrograph of cells of a member of the mycoplasma group, the agent of bronchopneumonia in the rat (1960×). (Reproduced with permission from Klieneberger-Nobel E, Cuckow FW: A study of organisms of the pleuropneumonia group by electron microscopy. J Gen Microbiol 1955;12:99.)

Six genera have been designated as mycoplasmas based on their habitat; however, only two genera contain animal pathogens. Mycoplasmas are highly pleomorphic organisms and range in size from vesicle-like forms to very small (0.2 µm), filterable forms (meaning that they are too small to be captured on filters that routinely trap most bacteria). Reproduction may be by budding, fragmentation, or binary fission, singly or in combination. Most species require a complex medium for growth and tend to form characteristic “fried egg” colonies on a solid medium. A unique characteristic of the Mollicutes is that some genera require cholesterol for growth; unesterified cholesterol is a unique component of the membranes of both sterol-requiring and non–sterol-requiring species.

The Archaebacteria

These organisms are predominantly inhabitants of extreme terrestrial and aquatic environments (high salt, high temperature, anaerobic) and are often referred to as extremophiles; some are symbionts in the digestive tract of humans and animals. The archaebacteria consist of aerobic, anaerobic, and facultatively anaerobic organisms that are chemolithotrophs, heterotrophs, or facultative heterotrophs. Some species are mesophiles, but others can grow at temperatures above 100°C. These hyperthermophilic archaebacteria are uniquely adapted for growth at high temperatures. With few exceptions, enzymes isolated from these organisms are intrinsically more thermostable than their counterparts from mesophilic organisms. Some of these thermostable enzymes, such as the DNA polymerase from Thermus aquaticus (Taq polymerase), are important components of DNA amplification methods such as the PCR.

Archaebacteria can be distinguished from eubacteria in part by their lack of a peptidoglycan cell wall, possession of isoprenoid diether or diglycerol tetraether lipids, and characteristic rRNA sequences. Archaebacteria also share some molecular features with eubacteria (Table2). Cells may have a diversity of shapes, including spherical, spiral, and plate or rod shaped; unicellular and multicellular forms in filaments or aggregates also occur. Multiplication occurs by binary fission, budding, constriction, fragmentation, or other unknown mechanisms.

Table2. Key Characteristics Shared by Archaebacteria and Eukaryotic Cells That Are Absent From Eubacteria

مواضيع ذات صلة

Classification Systems of bacteria

الخصائص التصنيفية للبكتيريا

مقدمة عن تصنيف الاحياء المجهرية وعلاقاتها مع بعضها

المجاميع الانتقالية لوحدات التصنيف للاحياء المجهرية

التداخل بين الاحياء المجهرية

الأسس الجزئية المستعملة في تصنيف الاحياء المجهرية

التصنيف المقترح للأحياء المجهرية