Increased Sensitivity of a Latex Agglutination Method for Serotyping Group B Streptococcus

Group B streptococcus (GBS) has been recognized as a significant cause of neonatal disease since the middle 1970s (1). There are presently nine major serotypes of GBS, with a few serotypes accounting for the majority of disease (3). It has been shown by experimental infections in animals that protection from GBS disease can be accomplished by using serotype-specific antigens (8). Vaccines using these serotype antigens are being developed and have been shown to be safe and immunogenic in humans (4). The goal is to immunize at-risk women before they deliver, which would protect their newborns without the use of antibiotics. Because the serotypes that cause the majority of disease in different countries vary, population-based serotype surveys will need to be instituted to ensure that the proper mix of serotypes is used in the vaccine. For example, serotypes VI, VII, and VIII are only rarely seen in the United States and would not be included in the vaccine for the United States, but serotypes VI and VIII would need to be included in a vaccine for Japan because they are common serotypes found in that country (5).

This scheme should work as long as the GBS are producing the serotype antigen. In the past, only 1 to 2% of the GBS we have received from invasive disease would not serotype (NT-GBS) using serotype I to VIII antisera. In the last few years, however, this number was approaching 10%, making this group one of the most significant causes of invasive GBS disease in our survey. If mothers are carrying these NT-GBS, vaccination with serotype antigens might not protect their newborns against early-onset GBS disease.

There are several possibilities for GBS to be NT. They may not be producing enough of the serotype antigen for our test to detect (7). We use the classic Lancefield extraction procedure followed by capillary precipitation (6). This test may not be sensitive enough to detect very small quantities of antigen. Another possibility is that the GBS have some genetic defect in the production of the serotype antigen and may not be producing any serotype antigens. A third possibility is that a new serotype has been introduced and our observed increase in NT-GBS may simply be due to a serotype that we are not able to detect with our antisera or our serotyping methods.

Recently a new method for serotyping GBS, using latex agglutination, was reported and was shown to be serotype specific (9). We determined if this method would be more sensitive for serotyping NT-GBS that might be producing reduced quantities of antigen.

NT-GBS received from 1995 to 2001 were recovered from our freezers. All of these bacteria were isolated from cases of invasive disease. All GBS were retested for serotype antigens by using the hot HCl antigen extraction procedure described by Lancefield (6). One-hundred fifty-three remained NT-GBS.

By using latex agglutination we were able to serotype 109 (71.2%) of the 153 NT-GBS. Four (2.6%) of the NT-GBS were serotype Ia, 5 (3.4%) were serotype Ib, 6 (3.9%) were serotype II, 2 (1.3%) were serotype III, 19 (12.4%) were serotype IV, 71 (46.4%) were serotype V, 2 (1.3%) were serotype VI, and 44 (28.8%) remained NT. We did not find any of serotype VII or VIII. The latex agglutination procedure reduced our percentage of NT-GBS to less than 3% of the total number of GBS we have received during this time period.

Our latex serotyping results indicate that the majority of the previously NT-GBS were producing serotype antigens. A few GBS remain that cannot be serotyped, which may indicate that new serotypes are present, the bacteria are not producing enough antigen to be detected even by this more sensitive procedure, or they are not producing any antigen. Because of the apparent increased sensitivity, ease of use compared to that of other serotyping methods (2, 6), and the previously reported specificity of the latex agglutination serotyping procedure, we are using this procedure to serotype the GBS we receive for our survey studies. One purpose of the surveillance study is to monitor the serotype distribution of GBS so that the most common serotypes will be included in the potential vaccine. These previously NT-GBS can now be included in the data set and the percentages of each serotype can be more accurately determined.