Chapter 8 : Antimicrobial Resistance in Streptococcus spp.

Abstract:

More than 60 Streptococcus species have been recognized so far. Some of these, such as S. pyogenes, S. agalactiae, S. equi, S. canis, and S. iniae, produce hemolytic factors and, when cultivated on solid media containing blood, can be classified as beta-hemolytic. However, nonhemolytic variants can also be observed ( 1 ). Isolates belonging to other species, such as S. dysgalactiae subsp. dysgalactiae, S. pneumoniae, S. mutans, S. salivarius, S. sanguinis, S. gordonii, S. mitis, and S. oralis, produce hydrogen peroxide that partially lyses the erythrocytes, with the subsequent oxidation of the heme group resulting in a greenish pigment in the medium that is often interpreted as alpha-hemolysis. This oxidation process is influenced by several cultivation conditions and is variably evident. For this reason, it is preferable to consider those latter-mentioned species as nonhemolytic. The truly nonhemolytic species, mainly encompassing S. gallolyticus (formerly S. bovis), were also named gamma-hemolytic. A classification of Streptococcus species proposed by Rebecca Lancefield in the 1930s was based on the antigenic reaction of the cell wall-associated carbohydrates and remains classically used ( 2 ). On the basis of this approach, streptococci are distributed into groups ranging from A to W, depending on the antibodies recognizing the specific carbohydrates of a definite streptococcal species. Nevertheless, the whole picture is sometimes complicated by the fact that several antibodies can react with isolates belonging to the same species. For instance, depending on the isolates, S. dysgalactiae subsp. equisimilis may be classified as belonging to the C or G group, while it may also be classified, even though less commonly, as group A or L ( 3 ); isolates from S. phocae may belong to either the C or G group; isolates from S. infantarius are sporadically considered as group D; isolates from S. anginosus are indifferently classified as group A, C, G, F, or N; isolates from S. constellatus subsp. constellatus belong to either group F or N; sporadic isolates belonging to S. constellatus subsp. pharyngis can be considered as group C; isolates from the S. intermedius species can be considered as group N; and finally, isolates belonging to S. porcinus are classified in either group P, U, or V.