Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Groups C and G Streptococci
The β-hemolytic streptococci can be subdivided on the basis of whether they form large colonies or minute colonies on solid agar. Most notable among those β-hemolytic streptococci that form large colonies are Streptococcus pyogenes (group A) and Streptococcus agalactiae (group B). Among the remaining Lancefield-reactive β-hemolytic streptococci, groups C and G organisms are most commonly associated with human disease. In this chapter, groups C and G streptococci refer exclusively to the large-colony–forming “ S. pyogenes -like” organisms. Minute-colony–forming species belonging to groups C and G are placed in the S. anginosus group (formerly Streptococcus milleri) (see Chapter 121 ). In some healthcare organizations, group G streptococci cause more cases of invasive disease than does S. pyogenes , perhaps because these organisms are more commonly associated with infection in immunocompromised hosts.
Characteristics of the Pathogens
Groups C and G organisms, like all streptococci, are catalase-negative, gram-positive cocci. Human isolates are almost always β hemolytic on sheep blood agar culture, although rare isolates are α hemolytic or nonhemolytic. Groups C and G organisms resemble S. pyogenes on blood agar, thus necessitating biochemical testing and serologic analysis for definitive identification. Latex agglutination typically is used in clinical microbiology laboratories to determine the Lancefield grouping of large-colony β-hemolytic streptococci. Most groups C and G isolates are resistant to bacitracin, but significant fractions are susceptible, thereby making bacitracin susceptibility testing alone unreliable in distinguishing these organisms from S. pyogenes. Hydrolysis of L-pyrrolidonyl-P-naphthylamide (PYR reaction) is a useful adjunctive test because the groups C and G organisms are PYR negative and S. pyogenes is PYR positive. Colony morphology and lack of acetoin production (negative results of the Voges-Proskauer [VP] test) differentiate large-colony groups C and G streptococci from the S. anginosus group ( Table 122.1 ).
TABLE 122.1
Differentiation of β-Hemolytic Group C and G Streptococcus Isolates From Streptococcus pyogenes and Streptococcus anginosus
| Organism | Lancefield Group | Bacitracin | PYR Test Result | VP Test Result |
|---|---|---|---|---|
| Streptococcus pyogenes–like groups C and G, large-colony–forming ( Streptococcus dysgalactiae subspecies equisimilis ) | C or G | Resistant a | − | − |
| S. pyogenes | A | Sensitive | + | − |
| Minute-colony–forming groups C and G ( Streptococcus anginosus group) | C or G | NA | NA | + |
NA, data not available; PYR, L-pyrrolidonyl-β-naphthylamide; VP, Voges-Proskauer; +, most isolates produce a positive reaction; −, most isolates produce a negative reaction.
Speciation
Most large-colony group C and G human isolates now are grouped together as Streptococcus dysgalactiae subsp. equisimilis . The remaining large-colony group C streptococci, predominantly animal pathogens, are grouped as S. dysgalactiae subsp. dysgalactiae . Nonhuman group G isolates are often considered part of a single species designated as Streptococcus canis and are genetically distinct from the S. dysgalactiae subsp . equisimilis group G organisms .
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here