CHAPTER V.
CLASSIFICATION.

The arrangement of living organisms in groups according to their resemblances and the adoption of fixed names is of the greatest advantage in their scientific study. For animal forms and for the higher plants this classification is gradually becoming standardized through the International Congress of Zoölogists and of Botanists respectively. Unfortunately, the naming of the bacteria has not as yet been taken up by the latter body, though announced as one of the subjects for the Congress of 1916 (postponed on account of the war). Hence there is at present no system which can be regarded as either fixed or official.


Fig. 49.—Illustrates the genus Streptococcus. Typical chains, no staphylococcus grouping, no sarcina grouping, no flagella.

Fig. 50.—Illustrates the genus Micrococcus. Diplococcus, tetrads short chains and staphylococcus; no sarcina, no flagella.

Fig. 51.—Illustrates the genus Sarcina. Sarcina grouping, no flagella.

Fig. 52.—Illustrates the genus Bacillus. A bacillus with peritrichic flagella. (Student preparation.)

Since Müller’s first classification of “animalcules” in 1786 numerous attempts have been made to solve the problem. Only those beginning with Ferdinand Cohn (1872–75) are of any real value. As long as bacteria are regarded as plants it appears that the logical method is to follow the well-established botanical principles in any system for naming them. Botanists depend on morphological features almost entirely in making their distinctions. The preceding chapters have shown that the minute plants which are discussed have very few such features. They are, to recapitulate, cell wall, protoplasm, vacuoles, metachromatic granules, capsules, flagella, spores, cell forms and cell groupings. Most bacteria show not more than three or four of these features, so that it is impossible by the aid of morphology alone to distinguish from each other the large number of different kinds which certainly exist. In the various systems which are conceded to be the best these characteristics do serve to classify them down to genera, leaving the “species” to be determined from their physiological activities. One of these systems was adopted by the laboratory section of the American Public Health Association and by the Society of American Bacteriologists and was practically the standard in this country until superseded by the Society’s own classification. It is that of the German Bacteriologist Migula and is given below for comparison. Since practically the entire discussion in this book is concerned with the first three families the generic characteristics in these only will be given. The full classification as well as a thorough discussion of this subject is given in Lafar’s Handbuch, whence the following is adopted:


Fig. 53.—Illustrates the genus Pseudomonas. A bacillus with flagella at the end only.

Fig. 54.—Illustrates the genus Microspira. It is (though the photograph does not prove it) a short spiral with one flagellum at the end.

Fig. 55.—Illustrates the genus Spirillum. Spiral bacteria with more than three, in this case four, flagella at the end.

Fig. 56.—Illustrates the genus Spirochæta.

Fig. 57.—Illustrates the genus Chlamydothrix. Fine threads with a delicate sheath.

Fig. 58.—Illustrates the genus Crenothrix. The thickness of the cell walls is due to deposits of iron hydroxide. (After Lafar.)

Fig. 59.—Illustrates the genus Beggiatoa. The filament A is so full of sulphur granules that the individual cells are not visible. B has fewer sulphur granules. In C the granules are nearly absent and the separate cells of the filament are seen. (After Winogradsky, from Lafar.)

ORDER I. Eubacteria.

Cells without nuclei, free from sulphur granules and from bacteriopurpurin (p. 112); colorless, or slightly colored.

1. Family: Coccaceæ (Zopf) Migula, all cocci.
Non-flagellated, Non-motile  
  • Genus 1. Streptococcus Billroth: division in one plane only (Fig. 49).
  • Genus 2. Micrococcus (Hallier) Cohn: division in two planes only (Fig. 50).
  • Genus 3. Sarcina Goodsir: division in three planes only (Fig. 51).
Flagellated, motile  
  • Genus 4. Planococcus Migula: division in two planes only.
  • Genus 5. Planosarcina Migula: division in three planes only.
2. Family: Bacteriaceæ Migula, all bacilli.
 
  • Genus 1. Bacterium (Ehrenberg) Migula: no flagella; non-motile.
  • Genus 2. Bacillus (Cohn) Migula: flagella peritrichic (Fig. 52).
  • Genus 3. Pseudomonas Migula: flagella at the end: monotrichic, lophotrichic, amphitrichic (Fig. 53).
3. Family: Spirillaceæ Migula, all spirilla.
Cells stiff  
  • Genus 1. Spirosoma Migula: non flagellated; non-motile.
  • Genus 2. Microspira (Schrœter) Migula: flagella one to three at the end (Fig. 54).
  • Genus 3. Spirillum (Ehrenberg) Migula: flagella more than three at the end (Fig. 55).
Cell flexible  
  • Genus 4. Spirochæta Ehrenberg: motile; no flagella (Fig. 56).
4. Family: Chlamydobacteriaceæ.
Cells cylindrical in long threads and surrounded by a sheath. Reproduction also by gonidia formed from an entire cell.
 
  • Genus 1. Chlamydothrix Migula (Fig. 57).
  • Genus 2. Crenothrix Colin (Fig. 58).
  • Genus 3. Pragmidiothrix Engler.
  • Genus 4. Spherotilus (including Cladothrix).

ORDER II. THIOBACTERIA: SULPHUR BACTERIA.

Cells without a nucleus, but containing sulphur granules, may be colorless or contain bacteriopurpurin and be colored reddish or violet.

1. Family Beggiatoaceæ.
2. Family Rhodobacteriaceæ.

This has five subfamilies and twelve genera, most of which are due to the Russian bacteriologist Winogradsky who did more work than anyone else with the sulphur bacteria.

THE CLASSIFICATION OF THE SOCIETY OF AMERICAN BACTERIOLOGISTS.

The Committee on Classification of the Society of American Bacteriologists at the meeting held in December, 1919, submitted its final report. This report has not been formally adopted as a whole, but in all probability will be substantially as outlined below. This outline does not attempt to give the detailed characterizations of the different groups as defined by the committee, but does show the names to be applied to the commoner organisms. These organisms are included in the 4th and 5th orders. Details of the first three orders have not been worked out. They are listed merely for completeness.

CLASS SCHIZOMYCETES.

Unicellular, chlorophyl-free plants, reproducing by transverse division (some forms by gonidia also).

Orders:
Genera of Orders D and E.

As compared with Migula’s classification it is to be noted that there are 38 genera listed by the Committee instead of 13 in the same general groups.

The following list of Genera conservanda submitted by the Committee was formally adopted by the Society and these are therefore its official names for the organisms included in these genera.

It is greatly to be desired that the Society’s Classification when finally completed shall become the standard in the United States at least.

Such names as have been adopted by the Society are used throughout this work.

The Committee also submitted the following artificial key for determining the genera in the two orders ACTINOMYCETALES AND EUBACTERIALES: