July 15, 2020

What makes GBS so deadly? A new study reveals the genetics behind the bacteria’s armoury

According to the Royal College of Obstetricians and Gynaecologists, Group B Streptococcal (GBS) bacteria is the most common cause of life-threatening infection in newborn babies. GBS can result in still birth, preterm birth, sepsis and meningitis.

Many pregnant women carry GBS bacteria on the surface of their vaginas without it causing any problems to their babies. However, some babies can become infected by GBS.

Some GBS bacteria are more deadly than others. Whilst some strains can be carried around harmlessly on our bodies, others are much more likely to cause problems. A few days ago, a new study has been published revealing the genetic characteristics of the most deadly strains.

A closer look at Group B Streptococcus

Like most bacteria, GBS bacteria have a number of virulence factors which form part of their armoury. The bacteria are surrounded by a thick capsule which is an important virulence factor. The capsule can interact with the host’s environment (that can be us!) to cause disease, and even protect the bacteria from being engulfed by our immune system. It can also protect the bacteria from being killed by viruses and chemicals.

GBS also produces pore-forming toxins that can create pores in host cell membranes, making it easier for the bacteria to invade host cells causing the host cells to die.

The study

The study, funded by the Meningitis Research Foundation, the Wellcome Trust, the UK Medical Research Council and the UK Department for International Development, has the catchy title: Pan-GWAS of Streptococcus agalactiae Highlights Lineage-Specific Genes Associated with Virulence and Niche Adaptation.

The study analysed the DNA of almost 2000 GBS samples (1988 to be precise) from both animals and humans using a technique called multilocus sequence typing (MLST). MLST can look for similarities and variations in the DNA that allows the DNA to be grouped into families which are otherwise known as ‘clonal complexes’.

MLST has identified six major clonal complexes of GBS in humans. More specifically these are CC1, CC10, CC17, CC19, CC23 and CC26.

By looking closely at the DNA of each of these clonal complexes, the study sheds light on why some GBS bacteria are more deadly than others.

The study looked in particular at the genetic characteristics of the harmful CC17, a clonal complex associated with neonatal sepsis and meningitis. The results showed, for example, that CC17 has genes that enhance the bacteria’s ability to take up nickel: essential for survival in the human host. It also has genes that strengthen the capsule of the bacteria, assist in its pore-forming ability, and ones which help deactivate the immune system of the host.

Fortunately it is CC1, CC19 and CC23 that are the most likely to colonise pregnant women as they are well adapted to vaginal mucosa, all of which have limited ability to infect babies.

Looking to the future

The more we know about GBS, the better chance we have to fight it. With a deeper knowledge of the genetics of the most virulent strains of GBS, we have a greater understanding of how bacteria can survive in humans and how they are transmitted between us, enabling more specific targeting of medicines.

If you have a baby who has been affected by GBS infection, you may have questions that require answers. As lawyers working with families affected by GBS in young babies, we're keen to raise awareness Group B Strep Support's campaign for improved screening of the bacteria as part of Group B Strep Awareness Month.

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