Outlines of Dairy Bacteriology Part 1

Outlines of Dairy Bacteriology, 8th edition.

by H. L. Russell.

PREFACE.

Knowledge in dairying, like all other technical industries, has grown mainly out of experience. Many facts have been learned by observation, but the _why_ of each is frequently shrouded in mystery.

Modern dairying is attempting to build its more accurate knowledge upon a broader and surer foundation, and in doing this is seeking to ascertain the cause of well-established processes. In this, bacteriology is playing an important role. Indeed, it may be safely predicted that future progress in dairying will, to a large extent, depend upon bacteriological research. As Fleischmann, the eminent German dairy scientist, says: "The gradual abolition of uncertainty surrounding dairy manufacture is the present important duty which lies before us, and its solution can only be effected by bacteriology."

It is therefore natural that the subject of Dairy Bacteriology has come to occupy an important place in the curriculum of almost every Dairy School. An exposition of its principles is now recognized as an integral part of dairy science, for modern dairy practice is rapidly adopting the methods that have been developed as the result of bacteriological study.

The rapid development of the subject has necessitated a frequent revision of this work, and it is gratifying to the writer that the attempt which has been made to keep these Outlines abreast of bacteriological advance has been appreciated by students of dairying.

While the text is prepared more especially for the practical dairy operator who wishes to understand the principles and reasons underlying his art, numerous references to original investigations have been added to aid the dairy investigator who wishes to work up the subject more thoroughly.

My acknowledgments are due to the following for the loan of ill.u.s.trations: Wisconsin Agricultural Experiment Station; Creamery Package Mfg. Co., Chicago, Ill.; and A. H. Reid, Philadelphia, Pa.

H. L. Russell.

University of Wisconsin.

CHAPTER I.

STRUCTURE OF THE BACTERIA AND CONDITIONS GOVERNING THEIR DEVELOPMENT AND DISTRIBUTION.

Before one can gain any intelligent conception of the manner in which bacteria affect dairying, it is first necessary to know something of the life history of these organisms in general, how they live, move and react toward their environment.

~Nature of Bacteria.~ Toadstools, s.m.u.ts, rusts and mildews are known to even the casual observer, because they are of evident size. Their plant-like nature can be more readily understood from their general structure and habits of life. The bacteria, however, are so small, that under ordinary conditions, they only become evident to our unaided senses by the by-products of their activity.

When Leeuwenhoek (p.r.o.nounced Lave-en-hake) in 1675 first discovered these tiny, rapidly-moving organisms he thought they were animals.

Indeed, under a microscope, many of them bear a close resemblance to those minute worms found in vinegar that are known as "vinegar-eels."

The idea that they belonged to the animal kingdom continued to hold ground until after the middle of the nineteenth century; but with the improvement in microscopes, a more thorough study of these tiny structures was made possible, and their vegetable nature demonstrated.

The bacteria as a cla.s.s are separated from the fungi mainly by their method of growth; from the lower algae by the absence of chlorophyll, the green coloring matter of vegetable organisms.

~Structure of bacteria.~ So far as structure is concerned the bacteria stand on the lowest plane of vegetable life. The single individual is composed of but a single cell, the structure of which does not differ essentially from that of many of the higher types of plant life. It is composed of a protoplasmic body which is surrounded by a thin membrane that separates it from neighboring cells that are alike in form and size.

~Form and size.~ When a plant is composed of a single cell but little difference in form is to be expected. While there are intermediate stages that grade insensibly into each other, the bacteria may be grouped into three main types, so far as form is concerned. These are spherical, elongated, and spiral, and to these different types are given the names, respectively, _coccus_, _bacillus_ and _spirillum_ (plural, _cocci_, _bacilli_, _spirilla_) (fig. 1). A ball, a short rod, and a corkscrew serve as convenient models to ill.u.s.trate these different forms.

[Ill.u.s.tration: FIG. 1. Different forms of bacteria. _a_, _b_, _c_, represent different types as to form: _a_, coccus, _b_, bacillus, _c_, spirillum; _d_, diplococcus or twin coccus; _e_, staphylococcus or cl.u.s.ter coccus; _f_ and _g_, different forms of bacilli, _g_ shows internal endospores within cell; _h_ and _i_, bacilli with motile organs (cilia).]

In size, the bacteria are the smallest organisms that are known to exist. Relatively there is considerable difference in size between the different species, yet in absolute amount this is so slight as to require the highest powers of the microscope to detect it. As an average diameter, one thirty-thousandth of an inch may be taken. It is difficult to comprehend such minute measurements, but if a hundred individual germs could be placed side by side, their total thickness would not equal that of a single sheet of paper upon which this page is printed.

~Manner of Growth.~ As the cell increases in size as a result of growth, it elongates in one direction, and finally a new cell wall is formed, dividing the so-called mother-cell into two, equal-sized daughter-cells.

This process of cell division, known as _fission_, is continued until growth ceases and is especially characteristic of bacteria.

~Cell Arrangement.~ If fission goes on in the same plane continually, it results in the formation of a cell-row. A coccus forming such a chain of cells is called _strepto-coccus_ (chain-coccus). If only two cells cohere, it is called a _diplo-coccus_ (twin-coccus). If the second cell division plane is formed at right angles to the first, a _cell surface_ or _tetrad_ is formed. If growth takes place in three dimensions of s.p.a.ce, a _cell ma.s.s_ or _sarcina_ is produced. Frequently, these cell aggregates cohere so tenaciously that this arrangement is of value in distinguis.h.i.+ng different species.

~Spores.~ Some bacteria possess the property of forming _spores_ within the mother cell (called _endospores_, fig. 1g) that are a.n.a.logous in function to the seeds of higher plants and spores of fungi. By means of these structures which are endowed with greater powers of resistance than the vegetating cell, the organism is able to protect itself from the effect of an unfavorable environment. Many of the bacilli form endospores but the cocci do not. It is these spore forms that make it so difficult to thoroughly sterilize milk.

~Movement.~ Many bacteria are unable to move from place to place. They have, however, a vibrating movement known as the _Brownian_ motion that is purely physical. Many other kinds are endowed with powers of locomotion. Motion is produced by means of fine thread-like processes of protoplasm known as _cilia_ (sing. _cilium_) that are developed on the outer surface of the cell. By means of the rapid vibration of these organs, the cell is propelled through the medium. Nearly all cocci are immotile, while the bacilli may or may not be. These cilia are so delicate that it requires special treatment to demonstrate their presence.

~Cla.s.sification.~ In cla.s.sifying or arranging the different members of any group of living objects, certain similarities and dissimilarities must be considered. These are usually those that pertain to the structure and form, as such are regarded as most constant. With the bacteria these differences are so slight that they alone do not suffice to distinguish distinctly one species from another. As far as these characters can be used, they are taken, but in addition, many characteristics of a physiological nature are added. The way that the organism grows in different kinds of cultures, the by-products produced in different media, and effect on the animal body when injected into the same are also used as data in distinguis.h.i.+ng one species from another.

~Conditions favoring bacterial growth.~ The bacteria, in common with all other living organisms are affected by external conditions, either favorably or unfavorably. Certain conditions must prevail before development can occur. Thus, the organism must be supplied with an adequate and suitable food supply and with moisture. The temperature must also range between certain limits, and finally, the oxygen requirements of the organism must be considered.

~Food supply.~ Most bacteria are capable of living on dead, inert, organic matter, such as meats, milk and vegetable material, in which case, they are known as _saprophytes_. In contradistinction to this cla.s.s is a smaller group known as _parasites_, which derive their nourishment from the living tissues of animals or plants. The first group comprise by far the larger number of known organisms which are concerned for the most part in the decomposition of organic matter. The parasitic group includes those which are the cause of various communicable diseases.

Between these two groups there is no sharp line of division, and in some cases, certain species possess the faculty of growing either as parasites or saprophytes, in which case they are known as _facultative_ parasites or saprophytes.

The great majority of bacteria of interest in dairying belong to the saprophytic cla.s.s; only those species capable of infecting milk through the development of disease in the animal are parasites in the strict sense of the term. Most disease-producing species, as diphtheria or typhoid fever, while parasitic in man lead a saprophytic method of life so far as their relation to milk is concerned.

Bacteria require for their growth, nitrogen, hydrogen, carbon, oxygen, together with a limited amount of mineral matter. The nitrogen and carbon are most available in the form of organic compounds, such as alb.u.minous material. Carbon in the form of carbohydrates, as sugar or starch, is most readily attacked by bacteria.

Inasmuch as the bacteria are plant-cells, they must imbibe their food from material in solution. They are capable of living on solid substances, but in such cases, the food elements must be rendered soluble, before they can be appropriated. If nutritive liquids are too highly concentrated, as in the case of syrups and condensed milk, bacteria cannot grow therein, although all the necessary ingredients may be present. Generally, bacteria prefer a neutral or slightly alkaline medium, rather than one of acid reaction; but there are numerous exceptions to this general rule, especially among the bacteria found in milk.

~Temperature.~ Growth of bacteria can only occur within certain temperature limits, the extremes of which are designated as the _minimum_ and _maximum_. Below and above these respective limits, life may be retained in the cell for a time, but actual cell-multiplication is stopped. Somewhere between these two cardinal temperature points, and generally nearer the maximum limit is the most favorable temperature for growth, known as the _optimum_. The temperature zone of most dairy bacteria in which growth occurs ranges from 40-45 F. to somewhat above blood-heat, 105-110 F., the optimum being from 80-95 F. Many parasitic species, because of their adaptation to the bodies of warm-blooded animals, generally have a narrower range, and a higher optimum, usually approximating the blood heat (98-99 F). The broader growth limits of bacteria in comparison with other kinds of life explain why these organisms are so widely distributed in nature.

~Air supply.~ Most bacteria require as do the green plants and animal life, the free oxygen of the air for their respiration. These are called _aerobic_. Some species, however, and some yeasts as well possess the peculiar property of taking the oxygen which they need from organic compounds such as sugar, etc., and are therefore able to live and grow under conditions where the atmospheric air is excluded. These are known as _anaerobic_. While some species grow strictly under one condition or the other, and hence are _obligate_ aerobes or anaerobes, others possess the ability of growing under either condition and are known as _facultative_ or optional forms. The great majority of milk bacteria are either obligate or facultative aerobes.

~Rate of growth.~ The rate of bacterial development is naturally very much affected by external conditions, food supply and temperature exerting the most influence. In the neighborhood of the freezing point but little growth occurs. The rate increases with a rise in temperature until at the _optimum_ point, which is generally near the blood heat or slightly below (90-98 F.), a single cell will form two cells in 20 to 30 minutes. If temperature rises much above blood heat rate of growth is lessened and finally ceases. Under ideal conditions, rapidity of growth is astounding, but this initially rapid rate of development cannot be maintained indefinitely, for growth is soon limited by the acc.u.mulation of by-products of cell activity. Thus, milk sours rapidly at ordinary temperatures until the acc.u.mulation of acid checks its development.

~Detrimental effect of external conditions.~ Environmental influences of a detrimental character are constantly at work on bacteria, tending to repress their development or destroy them. These act much more readily on the vegetating cell than on the more resistant spore. A thorough knowledge of the effect of these antagonistic forces is essential, for it is often by their means that undesirable bacteria may be killed out.

~Effect of cold.~ While it is true that chilling largely prevents fermentative action, and actual freezing stops all growth processes, still it does not follow that exposure to low temperatures will effectually destroy the vitality of bacteria, even in the vegetative condition. Numerous non-spore-bearing species remain alive in ice for a prolonged period, and recent experiments with liquid air show that even a temperature of -310 F. for hours does not effectually kill all exposed cells.

~Effect of heat.~ High temperatures, on the other hand, will destroy any form of life, whether in the vegetative or latent stage. The temperature at which the vitality of the cell is lost is known as the _thermal death point_. This limit is not only dependent upon the nature of the organism, but varies with the time of exposure and the condition in which the heat is applied. In a moist atmosphere the penetrating power of heat is great; consequently cell-death occurs at a lower temperature than in a dry atmosphere. An increase in time of exposure lowers the temperature point at which death occurs.

For vegetating forms the thermal death point of most bacteria ranges from 130-140 F. where the exposure is made for ten minutes which is the standard arbitrarily selected. In the spore stage resistance is greatly increased, some forms being able to withstand steam at 210-212 F. from one to three hours. If dry heat is employed, 260-300 F. for an hour is necessary to kill spores. Where steam is confined under pressure, a temperature of 230-240 F. for 15-20 minutes suffices to kill all spores.

~Drying.~ Spore-bearing bacteria like anthrax withstand drying with impunity; even tuberculous material, although not possessing spores retains its infectious properties for many months. Most of the dairy bacteria do not produce spores, and yet in a dry condition, they retain their vitality unimpaired for considerable periods, if they are not subjected to other detrimental influences.

~Light.~ Bright sunlight exerts on many species a powerful disinfecting action, a few hours being sufficient to destroy all cells that are reached by the sun's rays. Even diffused light has a similar effect, although naturally less marked. The active rays in this disinfecting action are those of the chemical or violet end of the spectrum, and not the heat or red rays.

~Influence of chemical substances.~ A great many chemical substances exert a more or less powerful toxic action of various kinds of life. Many of these are of great service in destroying or holding bacterial growth in check. Those that are toxic and result in the death of the cell are known as _disinfectants_; those that merely inhibit, or r.e.t.a.r.d growth are known as _antiseptics_. All disinfectants must of necessity be antiseptic in their action, but not all antiseptics are disinfectants even when used in strong doses. Disinfectants have no place in dairy work, except to destroy disease bacteria, or preserve milk for a.n.a.lytical purposes. Corrosive sublimate or pota.s.sium b.i.+.c.hromate are most frequently used for these purposes. The so-called chemical preservatives used to "keep" milk depend for their effect on the inhibition of bacterial growth. With a substance so violently toxic as formaldehyde (known as formalin, freezene) antiseptic doses are likely to be exceeded. In this country most states prohibit the use of these substances in milk. Their only function in the dairy should be to check fermentative or putrefactive processes outside of milk and so keep the air free from taints.

~Products of growth.~ All bacteria in their development form certain more or less characteristic by-products. With most dairy bacteria, these products are formed from the decomposition of the medium in which the bacteria may happen to live. Such changes are known, collectively, as fermentations, and are characterised by the production of a large amount of by-products, as a result of the development of a relatively small amount of cell-life. The souring of milk, the formation of butyric acid, the making of vinegar from cider, are all examples of fermentative changes.

With many bacteria, especially those that affect proteid matter, foul-smelling gases are formed. These are known as putrefactive changes.

All organic matter, under the action of various organisms, sooner or later undergoes decay, and in different stages of these processes, acids, alkalies, gases and numerous other products are formed. Many of these changes in organic matter occur only when such material is brought in direct contact with the living bacterial cell.

In other instances, soluble, non-vital ferments known as _enzyms_ are produced by the living cell, which are able to act on organic matter, in a medium free from live cells, or under conditions where the activity of the cell is wholly suspended. These enzyms are not confined to bacteria but are found throughout the animal and plant world, especially in those processes that are concerned in digestion. Among the better known of these non-vital ferments are rennet, the milk-curdling enzym; diastase or ptyalin of the saliva, the starch-converting enzym; pepsin and trypsin, the digestive ferments of the animal body.

Enzyms of these types are frequently found among the bacteria and yeasts and it is by virtue of this characteristic that these organisms are able to break down such enormous quant.i.ties of organic matter. Most of these enzyms react toward heat, cold and chemical poisons in a manner quite similar to the living cells. In one respect they are readily differentiated, and that is, that practically all of them are capable of producing their characteristic chemical transformations under anaesthetic conditions, as in a saturated ether or chloroform atmosphere.

~Distribution of bacteria.~ As bacteria possess greater powers of resistance than most other forms of life, they are to be found more widely distributed than any other type. At the surface of the earth, where conditions permit of their growth, they are found everywhere, except in the healthy tissues of animals and plants. In the superficial soil layers, they exist in myriads, as here they have abundance of nourishment. At the depth of several feet however, they diminish rapidly in numbers, and in the deeper soil layers, from six to ten feet or more, they are not present, because of the unsuitable growth conditions.