Medically Important Beta-Hemolytic Streptococci
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Medically Important Beta-Hemolytic Streptococci PATRICK CLEARY AND QI CHENG
Introduction Streptococci are a diverse collection of species, which are Gram-positive cocci that grow in chains or pairs. Although most are normal flora of mammalian mucous membranes, all can cause disease, and some are primary pathogens. With the exception of Streptococcus pneumoniae, the most medically relevant species are β hemolytic when grown on sheep blood agar plates. Streptococcus pyogenes and S. agalactiae are by far the most common, most dangerous and best studied human streptococcal pathogens, and are therefore the primary focus of this chapter. Streptococci are fastidious chemoorganotrophs, which are catalase negative. All grow aerobically, but growth is more luxurious in an atmosphere that contains elevated carbon dioxide (CO2) levels. Although unable to utilize oxygen (O2) as an electron sink, it can be metabolized to reactive oxygen radicals and hydrogen peroxide (H2O2). Strains within a species vary in their sensitivity to O2 and excretion of H2O2. Streptococcus pyogenes and others produce manganese (Mn)-dependent superoxide dismutases (Gerlach et al., 1998). Streptococci are unable to synthesize hem, the basis for their negative response to the benzidine and the porphyrin tests (Ruoff, 1991). A hallmark of most pathogenic species of streptococci is β hemolysis. Five percent washed sheep red blood cells in agar medium are most often used to evaluate hemolysis. Both the diameter and transparency of the zone of hemolysis can vary from species to species, within a species, even among colonies of a single culture (Fig. 1). Panels A and B (Fig. 1) show the morphology and hemolysis of a typical S. pyogenes culture. Agar plates are often stabbed in the first streak quadrant to reveal more intense hemolysis. Both the zone of hemolysis and colony size varies from culture to culture. Panel C shows an S. pyogenes culture that primarily makes α hemolytic colonies but occasionally segregates a brightly β hemolytic colony that is more typical. Streptococcus agalactiae usually forms smaller colonies with a smaller zone of β hemolysis (Fig. 1, Panel
D). Human group C and G clinical isolates are also β hemolytic (Fig. 1, Panels E and F, respectively). The group G culture shown in Panel F (Fig. 1) also demonstrates variable β hemolysis. Therefore, β hemolysis is not a particularly useful characteristic for speciation or subspeciation of cultures. Neither the source of this variability nor the genetic regulation of hemolysins has been studied. Because pathogenic species are usually β hemolytic, it has been assumed that hemolysins contribute to the pathogenesis of streptococci. Convincing evidence for this assumption is still, however, lacking. Most hemolysins are poorly defined. The one exception is streptolysin O, and it will be discussed in more depth in another section (Adderson et al., 1998). Genetic variability is an important, often overlooked characteristic of pathogenic
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