A compend on bacteriology : including pathogenic protozoa

Front matter

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Preface

This little book was designed by the writer to serve the needs of the medical student preparing for examination, and for the practitioner of medicine who desires to acquaint himself with the principal facts of the rapidly growing science of bacteriology. An effort has been made to reduce the subject matter to as concrete a form as possible.

While the literature of the subject of immunity is as vast almost as the rest of bacteriology, yet it is hoped that the chapter in this book on immunity gives in outline the essential accepted teachings on the subject. |

Minute details of cultures and technic are not given. They must be sought for in books on descriptive bacteriology.

The author has drawn very freely from many’ standard text-books. Many illustrations are from Kolle & Wassermann’s Atlas, Williams, McFarland, Tyson’s Practice and Abbott.

The writer’s best thanks are tendered to Dr. Herbert Fox of the University of Pennsylvania (Pepper Laboratory) to whom entire credit is due for the chapters on filterable viruses; the rearrangement of chapters, and the new matter that has been added throughout the book.

ROBERT L. PITFIELD.

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Table Of Contents

CHAPTER I PAGE THE CLASSIFICATION, MORPHOLOGY, AND THE BIOLOGY OF BACTERIA... I CHAPTER II Propucrs OF Bacemezal BMRRGT: os id ee hs ecacte toss See 21 CHAPTER III MMIPROTION 3 56. 5. 30S e oR ees ok La es ee ORE eee ee 27 CHAPTER IV PUBMED TELM eae sy «8 0's) quchs a egcm tan Beater Soca ag tebe ty oh ag Ue Se Tae ce eae rn eo 42 CHAPTER V STUDY -OF BACTERGA. Yio iii nauk ies TE St ae ese ae 85 CHAPTER VI BACTERIOLOGICAL LABORATORY. TECHNIC 2.5 Vitae so 1. oo ee ee 97 CHAPTER VII ANPISEPTICS AND DISTHPECRANUGsc...3 56 suis 2s w elem at steers peceeno neat ee 122 CHAPTER VIII BIR UEIATA,, « sips. 2 hie itech «ic Benes axe x srgierooe ec as ohana x oe eae ce ies ee eee oa 129 CHAPTER IX POUMAT: PARASTIUG ga¢ 55's 0.08% Pate ME Alar es Sah ng Pe EE he Foal ee 215 . “CHAPTER = Tan Crean Veins, f. 22. beet esas: kp ee ee ae 237 CHAPTER XI BACTERIOLOGY OF WatTrEr, Som, Am AND MIKE... ........ 6.050.005: 258 WE oO aa eee hike 05 4 oa Noe Rent rok ge Ce cet eer than eee one 269 vii

COMPEND OF BACTERIOLOGY

Chapter I The Classification, Morphology, And The Biology Of Bacteria

BACTERIA (fission fungi or schizomycetes) may be defined as very minute unicellular vegetable organisms, almost always devoid of chlorophyll, and generally unbranched, that reproduce themselves asexually by means of direct division or fission, spores, or gonidia. They are allied closely on the one hand to the higher fungi, such as the moulds, and on the other to the alge. Many forms in one phase of development closely resemble members of other groups, and it has always been difficult to classify them. Various botanical classifications have been employed by different bacteriologists. The following one is based somewhat upon Migula’s, and that adopted by Lehmann and Neumann, which was compiled from the systems of Fliigge, Fischer, Léffler, and Migula.

CLASSIFICATION .— Bacteria may be conveniently divided into six families, according to their morphology or shape.

I. COCCACEA!.—Spherical or spheroidal bacteria. Globular in free state but usually seen with one axis slightly larger. They do not have parallel sides like the bacilli. To multiply, the cell divides into halves, quarters, or eighths, each of which grow again into perfect spheres. Endospores and flagella are very rare (Lehmann and Neumann). If mobile they are called Planococcus or Planosarcina.

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(a) Streptococcus.—Cells that divide in one direction only and grow in chains.

(b) Micrococcus.—Cells that divide in two directions, or irregularly; with this group staphylococcus may be classed. Also tetrads, which form into fours by division in two directions.

(c) Sarcina.—Cells that divide in three directions so that bale-like packages, or blocks of eight are formed. At least one variety (Sarcina agilis) is motile, having flagella. Plates of cocci, one thick in the plane, are called ““merismo pedia.”

Il. BACTERIACEZ.—ROD bacteria are straight or slightly curved. Each cell is from two to six times as long as broad. Division takes place in one direction only, and at right angles to the long axis. Spores may be produced or may not. They may have flagella, or may not.

(a) Bacterium.—Neumann—Have no endospores. Migula —no flagella.

(b) Bacillus.—Neumann—Have endospores, and often grow in long threads. Migula—Flagella present at any part of cell, peritrichic in arrangement.

(c) Pseudomonas.—Have endospores very rarely. Flagella only at ends.

III. SPIRILLACEA.—Spiral bacteria. Unicellular, more or less elongated. Twisted more or less like a corkscrew. Cells are sometimes united in short chains. Generally very motile. Spores are known in two varieties only.

(a) Spirosoma.—Rigidly bent. No flagella.

(0) Vibrio or Microspira.—Cells that are rigidly bent like a comma, and have always one, occasionally two polar flagella.

(c) Spirillum.—Are long and spiral, like a corkscrew, are rigid, and have a bunch of polar flagella.

V.

CLASSIFICATION 3

(d) Spirocheta.—Cells with long flexible spiral threads, without flagella. Some move by means ofan undulating membrane. These have been thought to belong to the bacteria but those that move by an undulating membrane should be classified with the protozoa.

. MYCOBACTERIACEZ.— Cells as short or long filaments, which are often cylindrical, clavate (club-shaped), cuneate

or irregular in outline, and display true or false branching.

Spores are not formed, but gonidia are. They have no fla-

gella, and division takes place at right angles to the long

axis. There is no surrounding sheath as in the next

family (V).

(a) Mycobacterium.—Cells are short cylindrical rods, sometimes wedge-like, bent, or Y-shaped: long and filamentous. They exhibit true branching, and perhaps produce coccoid elements and gonidia, but no flagella. The Corynebacterium of Lehmann and Neumann belongs to this group. Many are acid-fast.

(6) Streptothrix or Actinomyces (ray fungus) are long mycelial threads, that radiate in indian-club, or loop-like forms, with true branching and delicate sheaths, devoid of gonidia and flagella. Growth coherent, mould-like and dry. Often powdery on the surface in culture media, frequently emitting a musty odor. A few species are weakly acid-fast.

CHLAMYDOBACTERIACE.—Sheathed bacteria. Cells

are characterized by an enveloping sheath about branched

and unbranched threads. Division takes place at right

angles to the long axis of the cells.

(a) Cladothrix are distinguished by false dichotomous branching. Multiplication is affected by separation of whole branches, and by swarm spores or motile gonidia having flagella.

(b) Crenothrix—Filaments are fixed to a nutrient base. Are usually thinner at the base than at the apex, formed of unbranched threads that divide in three directions of space, and produce in the end two kinds of gonidia, probably of bisexual nature.. .

(c) Phragmidiothrix.—Cells are first united into unbranched threads by means of delicate sheaths, branching threads are then formed. Division takes place in three directions of space, producing sarcina-like groups of gonidia, which, when free, are spherical.

(d) Thiothrix.—Are unbranched cells, sheathed, without flagella, divided only in one direction, and contain sulphur granules.

VI. BEGGIATOACEZ.—Cells united to form threads that are not sheathed: have scarcely visible septa; divide in one direction, and motile only by an undulating membrane, not by flagella.

(a) Beggiatoa.—Cells containing sulphur mh om

Bacteria may furthermore be classified according to their biological characteristics, which may be wonderfully different. The ultimate differentiation of one species from another depends not only on the morphology, which may be precisely similar, but on its biological behavior in culture media and in the tissues of animals under identical conditions. Again, different individuals of a given species may vary extraordinarily one from another in form and size, yet the chemical behavior is invariably the same. Hence it is only by observation of the development of bacteria in culture media, and the reactions produced in it, and in the bodies of experiment animals, that we canidentify them positively from others of a foreign species. No bacteriologist is able by a simple microscopical examination of a given bacterium, to identify it absolutely at all times.

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CLASSIFICATION 5

Bacteria that are globular in form are called cocci.

Cocci that divide in one direction of space and grow in chains are called streptococci (Fig. 1).

Cocci that divide irregularly and form pairs or paves or irregular groups, are called micrococci. Those of this class that form pairs

Fic. 1.—Large and very large streptococci. (Kolle and Wassermann.)

are frequently called diplococci.1_ When they form fours by division in two directions, they are called tetrads. But when they divide irregularly and form masses resembling bunches of grapes, they are spoken of as staphylococci (Fig. 2).

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Fic. agar eee Streptococci. ‘ighilieichh Tedrads. Sarcine. Williams.)

Cocci that divide in three directions are called sarcine. One single coccus, by division in three directions, forms cubes of eight

1 This word is frequently used as if it were a biological term indicating some species identity, e.g., Diplococcus pneumonie. There is no_ biological group called Diplococci and the term should be used in a descriptive sense. The cause of pneumonia is now called Streptococcus pneumonie.

or more, each of which becomes globular and equal in size to the parent.

Motile micrococci are those that divide in two directions of space, and have flagella. They are known as planococci.

Micrococci that divide in three directions, and are motile, are called planosarcine (Fig. 3).

Bacteria that resemble straight rods are called bacilli. These may be short and thick, or long and thread-like; are never curved, but may be slightly bent.

Fic. 3.—Planosarcina ure, showing very long flagella. (Kolle and Wassermann.)

Bacilli may grow singly or in chains; may be flagellated; contain spores and gonidia; or, may be devoid of flagella.

Members of the spirillacee that resemble a curved rod, or are comma-shaped, are known as vibrios (Fig. 4). Those of the same family that resemble a corkscrew, are called spirilla (Fig. 5). When they are like long spiral threads they are called spirochaete.

Any of these different members of the family of Spirillacee may grow in chains.

In clinical medicine it is common to speak of the streptococcus pneumonie asthepneumococcus. As the organism appears in the diseased lung, or in the sputum, one diameter of the coccus is inva-

CLASSIFICATION 7

.

riably longer than another, and the rule of equal diameters cannot be applied to it. But in culture media, the organism resembles a true coccus, being globular and growing in chains. It is then called the Streptococcus pneumonia. It is common also to speak of members of the family of M ycobacteriacee as bacilli, as they are more commonly met with in this form in clinical examinations, and incultures. Hence, we frequently hear of the bacillus of tuberculosis, and not the M ycobacierium tuberculosis.

Among the higher bacteria, the differentiation of those belonging to the sheathed group, or Chlamydobacteriacee, is difficult, as it depends largely upon the formation of the false branching and the gonidia. When bacteria exhibit many, or various forms, in the same culture, as does the typhoid bacillus, we speak of them as pleo-

Fic. 4.—Cholera vibrios. Fic. 5.—Spirocheta of relapsing fever. (Greene’s Medical Diagnosis.) (Greene’s Medical Diagnosis.)

morphic, or as showing pleomorphism. To elucidate: Man is pleomorphic, because among adult individuals some are tall or short, fat or thin.

Involution or Degeneration Forms.—When the best or optimum conditions for bacterial life (see page 17) are not found, bacteria present appearances quite different from those of the young, active or perfect adult type. These changes are called involutionary if temporary, or degenerative if permanent. For example: the diphtheria bacillus under good conditions for lifeis a straight or slightly bent rod staining in a granular manner. If living under unsuitable conditions it becomes quite short, and stains solidly. Again, bacilli that are accustomed to appear as short elements may grow to long threads without dividing, or swell into unrecognizable form. Branching is sometimes seen in rods and spirals, a condition due in certain cases to involution, in others naturally among the higher bacteria.

To measure bacteria, we use the thousandth part of a millimeter, called the micromillimeter, or micron,as the unit. The Greek letter w is the symbol for this unit. A micron is about 145,900 of an inch, yet a bacterium 1 yu long, and 1% yu in width, is very large in comparison to some things that scientists measure, such as the thickness of oil films, soap bubbles, or light-wave lengths, in which the unit is a micromicron, and is symbolized by uu. The shortest light-wave lengths are about 400 uu, or .4u, while chromatic threads in cells of bacteria are often 100 wy in width. Then again there are many things smaller than these threads. The thinnest part of a bursting soap bubble is but 7uy in thickness. There are certain infectious agents that are submicroscopic; that is, invisible even by the aid of Siedentopf’s ultraviolet microscope, which shows objects smaller by half a light-wave length (.2 uy).

The structure of the bacterial cell is very simple, consisting of a delicate poorly staining limiting membrane or wall enclosing a mass of substance with strong affinity for basic dyes like methylene blue. Just what part of the bacterial interior is cytoplasm and what is nucleus is not definitely known. Some observers believe that all that is stained is chromatin, or nuclear matter diffusely distributed through the bacterial cell, while others think that a delicate cytoplasm exits under the wall and that it is overshadowed by relatively great proportional bulk of the nucleus.

In the course of the rod we often see metachromatic bodies, called the Babes-Ernst granules, and unstained spaces called vacuoles, both of which are common to many bacteria. They may be ingested substances but some are lipoidal or carbohydrate in nature. These bodies are demonstrated by staining with basic dyes and may be of importance in determining the

CLASSIFICATION 9 mycobacteria. It is thought that they play a réle in reproduction (Fig. 65).

The food of the bacterium passes through the cell wall by osmosis. The cell wall of certain organisms, for example the pneumococcus, undergoes a change whereby a mucilaginous or gelatin - ous capsule is formed outside the cell wall. Its use is not known. The cell wall is generally the first portion of the cell to be attacked by certain specific substances (ferment) found in the blood of immunized animals, called bacteriolysins and agglutinins. Where

Fic. 6.—Zodéglea formation. (Leuconostoc.) (Kolle and Wassermann.)

great masses of bacteria are clumped in excessive mucilaginous material we speak of this condition as zodglea (Fig. 6).

We sometimes find, as a prolongation of the cell wall, filamentous organs of locomotion known as flagella. Not all bacteria possess these, but those that do, are called trichobacteria. Those that have not flagella are called gymnobacteria. Trichobacteria are classified according to the number and location of the flagella. When they have one flagellum we call them monozrichous bacteria, and am phitrichous when there are two flagella, one at each pole (Fig. 7). When the cell is surrounded by flagella, it is known asa peritrichous

Ate) BACTERIA bacterium, and lophotrichous when the flagella are arranged in tufts of two or more. These are simple adjectives and not now used as terms of classification. The tetanus bacillus is an example of a peritrichous organism, while the bacillus of green pus is called monotrichous, because of its single flagellum (Fig. 8).

Flagella are not pseudopods, but distinct organs of locomotion.

In certain bacteria of the Beggiatoa, loco- 3 motion is accomplished by a peculiar amce- | boid motion, or by an undulating membrane. On looking at bacteria known to

Fic. 7.—Spirillum undula with polar Fic. 8.—Bacillus proteus vulflagella. (Kolle and Wassermann.) garis, showing peritrichous flagella. (Kolleand Wassermann.)

have no powers of voluntary motion, they are seen to oscillate, tremble or move slightly. Suspensions of india-ink in water are seen to do the same thing, as are other inanimate suspensions. This molecular movement is known as the Brownian motion. By ordinary staining methods, and in preparations of living bacteria known to be flagellated, these organs of locomotion cannot be seen, as arule. Occasionally, however, one may be seen under either condition. Generally, strong solutions of aniline dyes, to

REPRODUCTION II which powerful mordants have been added, are necessary to stain the capsule of bacteria and the attached flagella. -The motion of bacteria varies from a simple rotatory, on one axis, to a swinging, shaking, boring or serpentine action. The location of the flagella has some influence upon the motion they impart. Flagella may be broken off from the cell body by agitation, but when separated may still be clumped by agglutinating sera. Flagella may have other functions than locomotion. It is possible that they may serve as organs for the absorption of nourishment from the surrounding media. The presence of very long or very numerous flagella does not necessarily presage very active motion. At times, under certain conditions, an organism ordinarily motile and flagellated will appear immobile and non-flagellated (Lehmann and Zierler), but this israre. Certain flagella have in their continuity little round granules, or bodies, which apparently have nothing to do with the functions of locomotion

_ but may have something to do with the nutrition of the cell. The test of motility of a bacterium is to see it progress by itself completely across the field of the microscope. REPRODUCTION.—The process of direct cell division is the commonest way by which bacteria multiply; hence comes the name of fission fungi. The ways of reproduction of the bacteria high in the scale are by direct division, branching,and by means of

_ Spores, and by other granules called gonidia. ‘The spores appear-

_ ing in the lower bacteria, bacilli for example, are not reproduction forms but states of high resistance.

The process of direct or binary division is very simple, and

_ may be a matter of twenty minutes, or as long as six hours. Divi-

, 7 ¢ , i sion is almost always across the cell in the direction of the short axis, though it may in some bacteria be in a direction parallel to

_ the long axis, but this is uncommon.

By means of the hanging-drop or the block-culture method, on

_ an inverted cover-glass the process may be observed easily. The

_ phenomena of division begin by an elongation of the cell, soon fol-

E2 BACTERIA lowed by a constriction or pinching in of the cel] on both sides, at an equatorial point. The process begins to be apparent in the cell wall and extends inward.

Division may occur in one, two, or three directions, or planes.

By cell division bacteria multiply by geometrical progression. One cell at the end of a period becomes two, and at the end of a second period these two become four; at the end of another period these four become eight; after twenty-four hours they may number many millions.

It is well that the food supply soon gives out and that the products of bacterial metabolism, such as acids and ferments, inhibit their growth. By this rapid bacterial multiplication, carcasses of animals are disintegrated and the higher nitrogenous compounds are reduced to simple gases that are quickly dissipated in the air.

SPORULATION.—Sporulation is of two kinds: the first and most important for hygiene is that into which some pathogenic

Fic. 9.—The formation of Fic. 10.—Spores and their location in bacspores. (After Fischer from terialcells. (After Frost and McCampbell.) Frost and McCampbell.)

bacteria go when they meet unfavorable conditions and it affords protection against all but the most vigorous disinfection; the second kind is a specialized function of the higher bacteria and moulds by which reproduction occurs (vegetative). In the latter r

SPORULATION 13 case it is not impossible that some sexual specialization occurs. The first mentioned are called Endospores.

Vegetative sporulation corresponds to the flowering of the higher plants, and is observed.under the most favorable vital conditions. Endospores are produced under stress of circumstances, when certain agencies or conditions, such as absence of food, drying, and heat, threaten the extinguishment of the organism. Spores are bright, shining, oval, or round bodies, which do not take aniline dyes readily, and which, when they are stained, retain the color more tenaciously than the adult cells. They resist heat, often withstanding a temperature of 150°C. dry heat for an hour. Steam under pressure at a temperature of 150°C. will invariably kill them after a short exposure.

Spores are situated either in the ends of the adult organism (polar) or in the middle (equatorial), and the spore is discharged (sporulation) either from the end or through the side.

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; Fic. 11.—Spore germination. a, direct conversion of a spore into a bacillus without the shedding of a spore-wall (B. leptosporus); b, polar germination of

_ Bact. anthracis; c, equatorial germination of B. subtilis; d, same of B. megaterium; e, same with ‘‘horse-shoe”’ presentation. (After Novy.)

The spore is developed in the bacterial cell as follows: If the organism is a mobile one it becomes quiet before sporulation, during which the flagella are retained. The position of the spore is early marked by a granularity of the bacterial body at one point, an area soon assuming a clear glistening character, often with a double contour, which may or may not increase the thickness of the cell. If unfavorable conditions continue the cell

body disintegrates and disappears leaving the spore bare.

Certain spore bearing bacteria grown for a week at 42°C. lose the power to form spores; likewise their progeny. As a rule the anthrax bacillus does not form spores in the bodies of animals. Free oxygen is required for sporulation by some bacteria. One spore only is produced by an adult cell. Some forms of bacteria can be differentiated from each other only by the way in which they sporulate, whether from the poles or the equator.

Fic. 12.—Capsules. Bact. pneumonie (Friedlander). (After Weichselbaum from Frost and McCampbell.)

Spores.are formed chiefly by the rod-shaped bacteria especially the anaerobic and saprophytic organisms and these varieties always have a high thermal death-point. Certain round bodies found in bacteria of low thermal death-point, are called by Heuppe arthrospores. It is believed that they are without significance. Arthrospores are common among the micrococci and may be associated with capsule formation and cell enlargement. The whole cell may stain more intensely. They are also to be sought among the Streptothrix genus.

Spores resist chemicals for a long period, and withstand drying, even in lime plaster, for years. It is believed that the thick capsule enables them to resist these deleterious agents.

SPORULATION 15

Sporulation is more apt to occur under poor nutritive conditions.

The anthrax bacillus thrives at 13°C. but cannot sporulate below 18°C. Anthrax spores have been known to resist the germicidal action of a 5 percent carbolic acid solution for forty days.

Capsules.—Certain well-known pathogenic bacteria have thick well-marked capsules. The pneumococcus, pneumobacillus, and Bacillus aerogenes capsulatus, are well-known examples of such capsulated organisms. The capsule is not always constant. It

Fic. 13.—Pest bacilli showing capsules. (Kolle and Wassermann.)

often disappears when the organism is grown in culture media (Figs. 12 and 13).

The higher bacteria are those from the Mycobacteriacee up to the yeasts and moulds. They are higher than the Bacteriacee because they tend to form truly or falsely branching filaments and specialized segments, gonidia, which may behave as sex organs. Few of them are pathogenic, except in the genera Mycobacterium and Streptothrix. To the former belongs the diphtheria and tubercle bacillus, both of which are said to have branching involution forms, while to the latter belong the organisms of actinomycosis and Madura foot. The Chlamydobacteriacee and Beggiatoa areSaprophytes. These require special technique for their laboratory culture.

The Yeasts or Blastomycetes or budding fungi are next in order. They consist of sharply and doubly outlined, refractive, oval bodies which may grow out into short stalks called mycelia. They grow well in the laboratory and may produce pigments. They are much larger than the bacteria (10-25 uw Jong). They multiply by budding with a separation and removed growth of the young form. They may produce a local or general infection in man, Blastomycosis. They are used in beer making. The commonest genus is Saccharomyces.

The Moulds or Hyphomycetes represent the next highest group of the plant alge. They are characterized by a greater prominence of the mycelium over simple segments or bodies. They are widespread in nature and many are pathogenic. They multiply by segmentation of the mycelia into gonidia or by the development ot special spore masses called sporangia. Further refinements of the spores into sexual elements is known. They are chiefly of interest to the physician on account of the skin diseases that they occasion.

THE CHEMICAL COMPOSITION OF BACTERIA

Bodies of bacteria contain water, salts, certain albumins, and bodies that may be extracted with ether. Among the latter are lecithin, cholesterin, and triolein. In acid-fast organisms, fatty acids and wax have been found. In others, xanthin bases, cellulose, starch, chitin, iron salts, and sulphur grains have been discovered. The essential protein of the cell body is highly nitrogenous and is usually combined with some carbohydrate as a glyco-nucleo-protein. The salts in the ash are mostly composed of various phosphates. Intracellular toxins in combination with the cytoplasm are found in certain groups of bacteria, e.g., B. typhosus.

BIOLOGICAL CONDITIONS 17

BIOLOGICAL CONDITIONS

Bacteria are arbitrarily classed either as parasites, or saprophytes. They may be so dependent upon the tissues of the infected organism as to be a strict parasite and incapable of growth under any other condition (Mycobact. lepre), or they may be capable of life on artificial culture media (tubercle bacillus), or of life in the body, on culture media containing organic matter (influenza bacillus), or in the soil (B. tetani).

Saprophytes are bacteria capable of living upon dead organic matter, in soil, in water, in air; they are not parasitic and do not resist the defenses of the living body.

Certain biological conditions are essential for the growth of bacteria: water, oxygen, carbon, nitrogen, and salts are necessary. For certain parasitic bacteria, highly complex substances are indispensable: meat albumins, peptones, milk, egg albumin, blood serum, and sugars are the ingredients of various culture media.

The chemical reaction of such media is important: it should either be faintly acid or faintly alkaline. The greatest number of water bacteria grow in media that are slightly acid, while diphtheria produces its strongest toxins and grows best in alkaline media. Saltfree media is required for a number of pathogenic bacteria, e.g., the gonococcus, B. lepre.

All bacteria require for their growth either free oxygen, as in air, or combined oxygen, as in albumin, water, etc. Those that only grow when deprived of free oxygen are known as obligate anaerobes, while those that require the presence of oxygen are called obligate aerobes. Those that grow under either conditions are named facultative anaerobes. Free oxygen is needed for spore formation by certain bacteria. Anaerobes obtain oxygen as they need it by breaking up their foodstuffs. !

Nutriment is most important for the growth of bacteria, nitrogenous compounds (albumins) particularly being required. Simple aquatic forms of bacteria can live and grow in distilled water. The addition of the various sugars is of advantage in the cultivation of many bacteria, and glycerine for the growth of some members of the Mycobacteriacee. Blood serum or whole blood is required by some pathogenic organisms. ‘The foodstuffs must be in a form that can diffuse through the cell wall.

The temperature of the medium in which various bacteria grow is most important. Bacterial growth is possible between o°C. and 70°C., some varieties thrive at the one extreme, and others at the other.

Psychrophilic bacteria, are those that grow at 15°C., with a maximum of 30°C. and a minimum of o°C. Water bacteria of the polar seas belong to this group.

Mesophilic grow best at 37°C.—the temperature of the body— and thrive from 10°C. (minimum) to 45°C. (maximum). All pathogenic bacteria belong to this group.

Thermophilic (min. temp. 40°C., max. 60-70°C.) are most prolific at 50-55°C. To this class belong bacteria of the soil. All of this class are spore-bearing.

Darkness favors bacterial growth.

Association of different kinds of bacteria is of some importance in their growth and welfare and when thus associated, they sometimes benefit each other. Such combination is called symbiosis. Antibiosis is the condition when one or more of a mixture of organisms suffers by the presence of others, e.g., the destruction of putrefactive germs in the intestinal tract by lactic acid bacilli.

Certain anaerobic bacteria grow in the presence of oxygen if other particular varieties of aerobic bacteria are present.

Attenuated tetanus bacilli become virulent if cultivated with Bacterium vulgae. Again, complicated chemical changes, as the decomposition of nitrites with the evolution of nitrogen cannot _be accomplished by certain bacteria severally, but jointly, this is quickly brought about.

Pfeiffer has shown that certain chemical substances (foods, albumins, etc.), attract bacteria (positive chemotaxis), while other substances, as turpentine, repel them (negative chemotaxis).

AGENTS PREJUDICIAL TO BACTERIAL LIFE 19

Oxygen repels anaerobes and is particularly attractive to aerobes.

FREE AGENTS PREJUDICIAL TO THE LIFE OF BACTERIA

High temperatures are surely germicidal: 60°C. coagulates mycoprotein of bacteria andother commonalbumins. The degree of temperature at which bacteria are killed is called the thermal death-point. Most vegetative forms die after a short exposure at 60°C., though some require a higher temperature, e.g., tubercle bacillus.

Spores resist boiling, often for hours. Spore-bearing bacilli from the soil often survive a temperature of 115°C. moist heat (steam), from thirty to sixty minutes. Bacteria resist dry heat of 175°C. from five to ten minutes.