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 (alpha, beta, 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 alpha only, 20.7% expressed alpha+beta, 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.

FIG. 1.

PFGE profile group distribution among 58 nontypeable GBS isolates. PFGE profile groups with less than 5% were consolidated into a miscellaneous group. This group included two isolates each from PFGE profile groups 2, 15, 16, and 28 and one isolate each from profile groups 11, 13, 22, 33, 34, and 35.

FIG. 2.

SmaI macrorestriction analysis by PFGE of NT/R1,R4 and V/R1,R4 GBS isolates. Numbers 1 to 10 at the top of each gel are lane designations; the numbers at the bottom of the gel represent PFGE profile groups (−, NT). (A) Lanes: 1, lambda molecular size standard with sizes in kilobases on the left; 2 to 8, NT/R1,R4 isolates; 9, V/R1R4 profile 4 control; 10, internal control Ib/α+β isolate. (B) Lanes: 1, lambda molecular size standard; 2 to 8, V/R1,R4 isolates; 9, profile 4 control; 10, internal control. (C) Lanes: 1, lambda molecular size standard; 2 to 7, V/R1,R4 isolates; 8, VII/R1,R4; 9, profile 4 control; 10, internal control.