S. aureus is a facultative anaerobe but also grows well under aerobic conditions (Figure 5.1(C), (D), and (E)). After incubation on semisolid thioglycollate medium, colonies grow rapidly and are uniform and dense. The final pH of glucose broth incubated with S. aureus is always between pH 4.3 and 4.6.

S. aureus is associated with human infections, such as facial furuncle and carbuncle, loose connective tissue inflammation, and tumor postoperative wound infections.

S. aureus is able to produce golden pigments on common agar plate. The colonies are round, raised, shiny opaque, and have a smooth surface and a whole edge. The diameter is 2–3 mm. A β-transparent hemolytic zone surrounds the colony when S. aureus is incubated on blood BHI agar.

Streptococcus mitis cells grow variably at 45 °C, and the final pH value of a culture in glucose broth is around pH 4.2–5.8. Cells in broth culture change from rough to smooth following subculture, and cells that are grown under aerobic conditions can be smooth or rough. TPY agar is the common medium on which S. mitis is grown (Figure 5.2(B)). Significant α-hemolytic reaction can be observed when S. mitis is grown on blood agar plates (Figure 5.2(C)). S. mitis forms small broken-glass-like colonies on sucrose agar plates, and very few strains can form the typical sticky colonies. Colonies observed by stereomicroscope are shown in Figure 5.2(D).

Biochemical reactions. S. mitis can ferment glucose, maltose, sucrose, lactose, and salicin to produce acid, and some strains can also ferment raffinose and trehalose to produce acid. However, inulin, mannitol, sorbitol, glycerol, arabinose, and xylose cannot be fermented by S. mitis.

Colonization characteristics. S. mitis can be detected in human saliva, oral mucosa, sputum, or excrement. It is a common oral Streptococcus and a member of the oral microflora.

S. pyogenes is a facultative anaerobe, and the optimal temperature for growth is 37 °C. A nutrient-rich medium is required for S. pyogenes growth, and both blood and serum can promote its growth. Colonies growing on MS agar are shown in Figure 5.3(C). Cells from overnight blood agar plate culture have three different colony types: (1) colonies with β-hemolytic zone; (2) mucoid colonies; (3) lackluster colonies. In fact, the type of colony is mainly related to the growth condition and production of hyaluronidases.

S. pyogenes can produce several pathogenic virulence factors, including bacteriocins, hemolysin O, streptokinase, NADH enzyme, hyaluronidase, and M protein. The main sites of colonization for S. pyogenes are the dental plaque, hypopharynx, and the upper respiratory tract. Clinical samples can be isolated from skin lesions, inflammatory secretions, or blood. S. pyogenes can also be found in loose connective tissue inflammation in the maxillofacial region, pulpitis, or infection after exelcymosis.

S. pneumoniae is a kind of facultative anaerobe, but could generate a substantial amount of H₂O₂ under aerobic conditions. Growth of S. pneumoniae requires a complex medium rich in nutrients, including blood, serum, or ascites, as well as vitamin B. Cells of this species grow well on BHI-blood agar plates, and form roof-like, reflective and smooth colonies or rugose, mycelium-like, rough colonies (Figure 5.4(D)). Cells of S. pneumoniae can generate a great deal of capsular polysaccharides and therefore often form sticky colonies (Figure 5.4(E)). In fact, this capsular polysaccharide is the species-specific antigen and virulence factor of S. pneumoniae. S. pneumoniae cells observed by stereomicroscope are shown in Figure 5.4(F). The final pH value of a culture in glucose broth is pH 5.0. Compared to other streptococci, the addition of bile or cholate is required for the isolation and identification of S. pneumoniae.

S. pneumoniae carries out metabolic reactions through fermentation. Cells of this species can ferment glucose, galactose, fructose, sucrose, maltose, raffinose, and inulin to produce acid, and some strains can also ferment mannitol but not dulcite and sorbitol. Bacteriolysis of S. pneumoniae cells by bile is positive, and this characteristic is helpful in distinguishing S. pneumoniae from other streptococci.

S. pneumoniae mainly inhabits the upper respiratory tracts of healthy individuals or livestock and can be isolated from clinical samples of amygdalitis, pneumonia, meningitis, and otitis media. However, the colonization, distribution, and pathogenicity of this species in oral cavity are still unknown.

S. vestibularis is a facultative anaerobe, and the optimal temperature for growth is 37 C, but cells of this species can also still grow at 10–45 C. Except the type strain MM, the majority of strains belonging to this species can grow in media containing 10% bile. However, the species cannot grow in medium containing 4% NaCl, 0.004% crystal violet, or 40% bile. Alpha-hemolysis can be detected for all strains of this species growing on horse blood agar plates. In addition, black blue, lackluster, and umbilicate colonies (about 2–3 μm in diameter) with wavy edges can be observed when cells are grown on MS agar plates under anaerobic conditions (37 

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