Group B Streptococcus in retail food– beyond ST283 and raw fish

Abstract

An unprecedented Group B Streptococcus (GBS) outbreak, associated with freshwater fish consumption, occurred in Singapore in 2015. We hypothesized that GBS might be present in food other than freshwater fish, and that there could be potential foodborne transmission of GBS other than the ST283 outbreak strain. As GBS has not been generally recognised as a foodborne pathogen, its presence in food is less commonly tested for. As such, there is a limited number of food isolates available for genomic comparison with animal and human isolates. A study was thus conducted to determine the frequency and distribution of GBS sequence types (STs) in local retail food and to compare their genomic sequences with those from a regional fish (animal) isolate collection and published STs of human isolates. Between October 2016 and June 2018, a total of 238 prepared/ready-to-eat (RTE) seafood samples and 400 unprepared food samples sold at retail were tested for the presence of GBS using molecular and culture methods. Isolates were characterized using whole genome sequencing. GBS was detected in 3.8% (9/238) of the prepared seafood samples and 18% (72/400) of the unprepared food samples by PCR. Among the unprepared food sampled, GBS was most frequently detected in fish (32%, 32/100), followed by meat (22%, 22/100), non-fish seafood (14%, 14/100), and vegetables (4%, 4/100). GBS ST283, which was responsible for the Singapore 2015 outbreak, was detected in an uncooked prepared oyster sample. Seven other known STs (ST1, 3, 7, 10, 23, 24, and 103) and three newly assigned STs (ST 1647, 1648 and 1649) were isolated from food samples. The 146 regional fish (animal) isolates yielded 11 known STs (ST1, 7, 10, 23, 28, 103, 261, 283, 314, 500, and 651) and one newly assigned ST (ST 1650). The prevalence of GBS in non-fish seafood and meat were observed to be significantly higher among wet market samples (28.6–36.5%) than those of the supermarket samples (3.4–6.3%) (p < 0.05). Higher contamination rate is expected in wet market samples as fresh produce in wet markets are generally sold and displayed with little or no primary packaging and are subjected to higher risks of human handling and temperature abuse. Our findings revealed that (i) non-ST283 GBS strains associated with human carriers and invasive disease can be found in food types such as seafood and meat, and (ii) that GBS ST283 can indeed be found in food types beyond freshwater fish. A larger study involving the genomic analysis of human, animal and food isolates collected within the same geographical region and temporal period will be beneficial to understand whether non-ST283 GBS may also cause foodborne infections.