DNA Macrorestriction Analysis of Nontypeable Group B Streptococcal Isolates: Clonal Evolution of Nontypeable and Type V Isolates

Abstract

Group B streptococci (GBS) are serotyped according to capsular polysaccharide (CPS) type (Ia to VIII); an isolate is classified as nontypeable (NT) if no detectable CPS is found. Surface-localized protein antigens (␣,␤, R1, and R4) serve as additional markers to classify GBS isolates, which is particularly useful since NT isolates often express one or more of these proteins. To compare genetic resemblance among isolates with similar protein profiles, we studied 58 NT isolates digested with the SmaI macrorestriction enzyme prior to pulsed-field gel electrophoresis (PFGE). Of these 58, 15.5% expressed ␣ only, 20.7% expressed ␣؉␤, 15.5% expressed R4, and 25.8% expressed R1,R4, while 22.4% of the isolates expressed no detectable proteins. The largest PFGE profile group, with 48% of the isolates, was group 4, composed primarily of isolates that expressed R1,R4 or no proteins. The second most common profiles were 3 and 32, each with 13.8% of the isolates. Since NT isolates in profile group 4 were highly related to type V isolates, as demonstrated by PFGE profiles, we investigated 45 type V isolates. Two-thirds of the type V isolates within profile group 4 were classified into subgroup 4a, compared to 28.2% of 39 NT isolates. Only 11% of the V/R1,R4 isolates were identical to the prototype group 4 profile, in contrast to 75% of the NT/R1,R4 isolates. A shift of type V isolates into profile 4 subgroups may be indicative of a genetic change over time. PFGE is a valuable approach for comparison of GBS isolate relatedness and for monitoring of NT and typeable GBS isolates for potential clonal divergence.Group B streptococcal (GBS; Streptococcus agalactiae) isolates are classified according to their capsular polysaccharide (CPS) into one of nine types: Ia, Ib, and II to VIII (13,19). However, when tested by routine typing methods, approximately 2.9% of colonizing isolates and 1.4% of invasive isolates lack a detectable CPS type and consequently are categorized as nontypeable (NT) (4, 9, 10).In addition to the CPS type, protein markers can be useful for classification since most GBS isolates express either the c or the R surface-localized protein(s) (9, 10). The c protein is made of two components that are distinguished on the basis of their reaction with trypsin; ␣ is trypsin resistant, while ␤ is trypsin sensitive and binds immunoglobulin A. Isolates may possess only one or both of these components (14). The R proteins (R1, R2, R3, and R4) are the second group of surface proteins and are trypsin resistant; isolates may express one or more of them (10, 18).Identification of surface-localized proteins is an important aspect of the classification of GBS isolates since some proteins are highly associated with specific CPS types. For example, ␣ protein is commonly expressed by the majority of serotype Ia isolates, while more than half of serotype V isolates possess the R1 and R4 proteins (10). Thus, these CPS type-protein profile associations may permit one to compare similarities among NT and typeable isolates on the basis of ...

Molecular Characterization of Nontypeable Group B Streptococcus

“…Previous studies characterizing GBS isolates have found a relationship between DNA macrorestriction band patterns and serotype/protein profile, especially for type V/R1,R4 and NT/R1,R4 isolates (1). The same study also found remarkable heterogeneity within the NT/R4 group compared with typeable isolates (1). However, the distribution of molecular serotypes and surface protein antigen genes associated with NT isolates is not known.…”

“…Previous studies have shown that certain surface antigenic proteins are frequently associated with a given serotype, and such associations are utilized in classifying GBS isolates (1,16,23). Associations between molecular serotypes and protein antigens may represent specific evolutionary lineages and could lack functional significance, but an interesting hypothesis predicts that immune selection has favored strains with specific combinations of protein antigens because it improves their fitness (18).…”

“…Some of these proteins are highly associated with specific serotypes. For example, serotype Ia isolates express C alpha protein, serotype III isolates express Rib, and serotype V isolates are associated with R1 and R4 proteins (1,14,23).…”

“…Subgroup letter "a" was assigned in addition to the profile number if there was a one-or two-band difference from the prototype strain. An isolate with a three-band difference was assigned letter "b," one with a four-band difference a "c," and one with a five-band difference a "d." Isolates with more than a five-band difference were considered to belong to a different profile group (1).…”

Molecular Epidemiology and Distribution of Serotypes, Surface Proteins, and Antibiotic Resistance among Group B Streptococci in Italy

“…Based on their capsular polysaccharides, GBS isolates can be classified into nine different serotypes (Ia, Ib, and II to VIII), with serotypes Ia, II, III, and V being the predominant causes of human GBS diseases (11,29). In particular, the serotypes most commonly causing neonatal infections are Ia, III, and V (32), with the latter being the main serotype recovered from patients of all age groups, including nonpregnant adults (1,19).Some of the best-characterized GBS protein antigens belong to the alpha-like protein (Alp) family, a class of surface proteins characterized by internal long identical tandem repeats. These proteins are named alpha, Alp1, Alp2, Alp3, Alp4, and Rib (28).…”