Читать книгу The Fundamentals of Bacteriology - Charles Bradfield Morrey - Страница 12

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Though there is apparently a wide variation in the shapes of different bacterial cells, these may all be reduced to three typical cell forms. These are: first and simplest, the round or spherical, typified by a ball and called the coccus form, or coccus, plural cocci4 (Fig. 33). The coccus may be large, that is, from 1.5µ to 2µ in diameter. The term macrococcus is sometimes applied to these large cocci. If the coccus is less than 1µ in diameter, it is sometimes spoken of as a micrococcus; in fact, this term is very commonly applied to any coccus. When cocci are growing together, many of the cells do not appear as true spheres but are more or less distorted from pressure of their neighbors or from failure to grow to full size after recent division. Most cocci divide into hemispheres and then each half grows to full size. A few cocci elongate before division and then appear oval or elliptical.

The second cell form is that of a cylinder or rod typified by a section of a lead-pencil. The name bacillus, plural bacilli, is applied to this type (Fig. 34). The bacillus may be short (Fig. 35), 1µ or less in length, or long, up to 40µ in rare cases. Most bacilli are from 2µ to 5µ or 6µ long. The ends of the rod are usually rounded, occasionally square and very rarely pointed. It is evident that a very short rod with rounded ends approaches a coccus in form and it is not always easy to differentiate in such cases. Most bacilli are straight, but some are slightly curved (Fig. 36).

The third cell form is the spiral, typified by a section of a cork-screw and named spirillum, plural spirilla (Fig. 37). A very short spiral consisting of only a portion of a turn is sometimes called vibrio (Fig. 38). Vibrios when seen under the microscope look like short curved rods. The distinction between the two can be made only by examining the organism alive and moving in a liquid. The vibrio shows a characteristic spiral twisting motion. Very long, flexible spirals are usually named spirochetes (Fig. 39). The spirochetes are motile but flagella have not been shown to be present.

Fig. 33.—Cocci.

Fig. 34.—Bacilli.

Fig. 35.—Short bacilli.

Fig. 36.—Curved bacilli. Only the one in the center of the field is in focus. The others curve out of focus.

Besides the three typical cell forms bacteria frequently show very great irregularities in shape. They may be pointed, bulged, club-shaped or even slightly branched. These peculiar and bizarre forms practically always occur when some of the necessary conditions for normal growth, discussed in Chapters VI and VII, are not fulfilled. They are best regarded as involution or degeneration forms for this reason (Fig. 40). In a very few cases it is not possible to obtain the organism without these forms (the diphtheria group). It is probable that these cell forms are normal in such cases, or else conditions suitable for the normal growth have not been obtained.

Fig. 37.—Spirilla.

Fig. 38.—Vibrio forms of spirilla. Compare with Fig. 36.

Fig. 39.—Spirochetes.

Fig. 40.—Involution forms. The organisms are tapering and branched at one end.