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Streptococcus agalactiae (group B strep)- causes, symptoms, diagnosis, treatment, pathology

September 20, 2019


Learning medicine is hard work! Osmosis makes it easy. It takes your lectures and notes to create
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much more. Try it free today! With Streptococcus agalactiae sometimes called
Strep agalactiae, strepto means a chain, coccus means round shape, and agalactiae literally
means “no milk”. So, Strep agalactiae refers to the round bacteria
that grow in chains and that was previously known to infect cattle, resulting in reduced
milk production. Later on, Strep agalactiae was found to also
be a human potential pathogen responsible for a number of infections that most commonly
affect pregnant women and newborns. Strep agalactiae are also called Group B Strep
– GBS – in Lancefield classification developed by an American microbiologist Rebecca Lancefield. Ok now, a little bit of microbe anatomy and
physiology. Strep agalactiae has a thick peptidoglycan
cell wall, which takes in purple dye when Gram stained – so this is a gram-positive
bacteria. It’s non-motile and doesn’t form spores,
and also, it’s a facultative anaerobe, meaning that it can survive in both aerobic and anaerobic
environments. Now, a particular trait of Streptococcus species
is that they are catalase negative, meaning they do not produce an enzyme called catalase. This is unlike other common gram positive
cocci, like Staphylococcus, which are catalase positive. When cultivated on a medium called blood agar,
Strep agalactiae colonies cause beta hemolysis, also called complete hemolysis. That’s because Strep agalactiae makes a
toxin called beta-hemolysin, that causes complete lysis of the hemoglobin in the red blood cells,
making them blood agar change color from red to transparent yellow around the colonies. However, other Streptococcus species, like
Strep pyogenes, are also beta hemolytic. So to identify Strep agalactiae specifically,
the bacitracin test, the hippurate test, or the CAMP test can be done. With the bacitracin test, a disk of bacitracin
is added to the blood agar. Strep agalactiae is bacitracin resistant,
so the colonies remain intact, whereas Strep pyogenes is bacitracin sensitive, so the colonies
die off. With the CAMP test, Strep agalactiae is grown
with Staphylococcus aureus on the same blood agar. Both these bacteria are beta-hemolytic, but
Strep agalactiae makes a substance called CAMP factor, which enhances the action of
staphylococcal beta-hemolysin. This results in greater areas of beta-hemolysis
where these colonies cross each other on the blood agar plate. Finally, the hippurate test is based on the
fact that Strep agalactiae is the only hippurate positive Strep, because it produces an enzyme
called hippuricase or hippurate hydrolase. To test for this, a colony of bacteria is
transferred from the culture to a test tube containing a special hippurate medium, and
incubated for two hours. Then, a few drops of a substance called ninhydrin
are added to the mix. With Strep agalactiae, hippuricase converts
hippurate to glycine and benzoic acid – and glycine reacts with ninhydrin, making the
mix turn deep blue – or positive. Ok, now, Strep agalactiae has a number of
virulence factors, that are like assault weaponry that help it attack and destroy the host cells,
and evade the immune system. First, Strep agalactiae is encapsulated, meaning
it’s covered by a polysaccharide layer called a capsule. The capsule also has pilli, which are hair-like
extensions that help the bacteria attach to a host cell. Additionally, the capsule is rich in sialic
acid, which is a substance that can also be found in human cells. So inexperienced immune cells, like that of
a newborn, may confuse Strep agalactiae with self cells, allowing it to survive inside
the body. And finally, Strep agalactiae makes beta-hemolysin,
which destroys the host’s tissues and causes hemolysis, meaning it destroys red blood cells. Surprisingly, Strep agalactiae is very common
to a level that up to thirty percent of healthy adults are colonized by it, without causing
any disease at all. They mostly colonize the gastrointestinal
tract and the vagina. Problems arise in people with a weakened immune
system, like newborns, pregnant women and adults with underlying diseases like diabetes
mellitus, malignancy, or a preexisting infection like HIV. Additionally, since the spleen plays an important
role in immunity against encapsulated bacteria, Strep agalactiae infections are more common
in people who’ve had a splenectomy, meaning their spleen was surgically removed, or in
those with sickle cell disease who have functional asplenia. Strep agalactiae can cause a number of infections. In pregnant women who are colonized with Strep
agalactiae, bacteria can ascend from the vagina, through the cervix and get right into the
uterus. From here they can infect the fetal membranes,
namely the chorion and amnion, causing chorioamnionitis. When this happens, these membranes can get
damaged and then rupture before delivery time, resulting in a miscarriage or a premature
delivery. Sometimes the infection can even spread to
the fetus itself, causing intrauterine death. Alternatively, Strep agalactiae can get from
the vagina on the perineum, and make their way up the urethra, causing cystitis – which
is the inflammation of the urinary bladder. In pregnant females who are colonized with
Strep agalactiae, there’s also a risk that they’ll pass the bacteria to the newborn
during natural childbirth. In newborns, Strep agalactiae is most likely
to go down their airways, descend down the respiratory tracts right into the lungs. This can cause inflammation of the lung tissue,
or pneumonia. Occasionally, Strep agalactiae can destroy
the baby’s alveolar lining with their beta-hemolysin, and break through the alveoli-capillary wall
to get straight into the bloodstream, what’s known as bacteremia. Bacteremia can cause neonatal sepsis, which
is when bacteria trigger a widespread immune reaction that causes vasodilation, hypotension,
and hypoperfusion of the baby’s vital organs. Alternatively, bacteria from the blood can
cross the blood-brain barrier, and migrate into the CSF, causing neonatal meningitis. Less likely, but possible: the bacteria can
also travel down the bloodstream to the joints, causing septic arthritis. Treatment for Strep agalactiae requires antibiotics
like Penicillin G and Ampicillin. For penicillin-resistant strains, or in individuals
who are allergic to penicillins, Cefazolin or Vancomycin. No vaccine for Strep agalactiae is currently
available, so the only way to prevent infection in the newborn is to give colonized mothers
intrapartum antibiotic prophylaxis, meaning antibiotics are given a couple of hours before
delivery. Alright, as a quick recap, Strep agalactiae,
is a gram-positive round bacteria that tend to live in chains. It is beta-hemolytic, non-motile, non-spore
forming, catalase negative, hippurate positive, bacitracin-resistant and CAMP positive. This bacteria can colonize the gastrointestinal
tract and vagina asymptomatically. Most frequently, it causes neonatal infections
like pneumonia, sepsis, meningitis, and septic arthritis. But it can also cause chorioamnionitis or
cystitis in pregnant females. Treatment relies on penicillin G or ampicillin,
but Cefazolin and Vancomycin may be used as alternatives. To prevent Strep agalactiae infections in
neonates, intrapartum antibiotic prophylaxis can be administered to vaginally colonized
pregnant females.

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