Micrococci

Micrococci, cocci that occur in pairs or groups. M. is a collective concept uniting various cocci [see separate table (pp. 83-84), Figure 1]. Of great practical importance are the M. found in the nasal or pharyngeal mucus of healthy and diseased individuals; due to their similarity to meningococci (see), they can cause diagnostic difficulties. The most important M. are as follows. The catarrhal M. (Micrococcus catarrhalis) is found almost constantly in the nasopharyngeal mucus in catarrhs of the upper respiratory tract; it is also found in healthy people; a paired coccus (in the shape of a coffee bean), also occurring in groups of 3-4 individuals; it decolorizes by Gram's method. Unlike the meningococcus, it grows on ordinary nutrient media from the first generations, forming round colonies of grayish or yellowish-white color with steep edges on agar. On ascitic agar, whitish, rather dense colonies with an uneven, dry surface are obtained; at low magnification they are granular; dryness is characteristic of the colonies. Gelatin is not liquefied; broth becomes uniformly turbid; does not decompose sugars. Not agglutinated by meningococcal sera. For humans, the micrococcus is generally non-pathogenic, although sometimes (in children) it causes pneumonia; cases of purulent meningitis caused by it have also been described. Often M. is the microbe of mixed infection in pulmonary tuberculosis. The yellow micrococcus, or diplococcus (Diplococcus, s. Micrococcus flavus) forms a yellow or golden-yellow pigment (two varieties). Both varieties grow well on ordinary agar; they decompose maltose, dextrose, and levulose from sugars. The first variety (Micrococcus pharyngis flavus I) forms round, transparent, slightly yellowish colonies on agar; the second (Micrococcus pharyngis flavus II) gives small colonies on agar, 1-1₂ mm in diameter. Occasionally a third variety (Micrococcus pharyngis flavus III) is also found, giving colonies very similar to meningococcal colonies; the main difference is the yellow coloring of the colonies; this micrococcus does not decompose levulose. -Micrococcus pharyngis siccus- a small coccus (smaller than the meningococcus), growing only on media containing ascitic fluid or blood serum. On ascitic agar it forms coarse, dry, wrinkled colonies, 2-3 mm in diameter; it decomposes glucose, maltose, and levulose from sugars. - Micrococcus pharyngis cinereus grows on ordinary media; on ascitic agar it gives dry colonies similar to those formed by the dry micrococcus. Does not decompose sugars. M. pharyngis cinereus is also characterized by size: these micrococci are essentially large cocci. Micrococcus crassus (Diplococcus crassus) is of interest in that, while often found in the nasopharyngeal mucus of healthy people, it is also sometimes observed as an accompanying microbe in the cerebrospinal fluid in epidemic cerebrospinal meningitis. It occurs in diplo- or tetra-forms, and unlike other M. of the group under discussion, it does not decolorize by Gram's method; however, it should be borne in mind that when stained by Gram's method, individual individuals of M. decolorize. Unlike the meningococcus, Diplococcus crassus grows at a temperature of 20°. Colonies on ascitic agar are smaller and more compact than meningococcal colonies, grayish-white in color, brown in transmitted light. It also grows on ordinary nutrient media, but worse than on ascitic agar. It decomposes glucose, maltose, levulose, sucrose, and lactose. Of other M., it is necessary to mention M. tetragenes, widely distributed on the skin and in the air and found in sputum in pulmonary tuberculosis, in saliva, in nasal mucus, etc. Its pathogenicity is small, but under certain conditions it can cause suppuration. Micrococcus tetragenes-a coccus the size of a staphylococcus (see); most often forms tetrads, i.e., groups of 4 cocci; also occurs in groups of 2 or 3 individuals. Does not decolorize by Gram's method; in preparations from sputum, the tetrad is often surrounded by a capsule that does not stain by ordinary methods. It gives a precipitate in broth; does not liquefy gelatin; on agar it gives small, grayish-yellow colonies. Lit.-see lit. to art. Bacteriology and Microbiology.