Bacterial infections tend to more commonly infect the meninges rather than the brain

Pic: http://sites.psu.edu/rclzico/files/2013/09/12668156-bacteria-and-bacterium-cells-floating-in-microscopic-space.jpg

itself: however, that doesn’t mean that the brain is completely protected from bacterial infections: there are a couple of bacteria which can still harm brain tissue and they are both spirochetes: one of which is a sexually-transmitted disease that you may have heard of. I will be discussing both types here, however I’ll try to keep my focus on those bacteria that attack the brain rather than the surrounding structures and keep the other stuff brief. The bacteria that I will be talking about here are: Escherichia coli, Streptococcus pneumoniae, Listeria monocytogenes, Neisseria meningitides, Mycobacterium tuberculosis, Treponema pallidum  and Borrelia burgdorferi. If you want to know more about how what bacteria are and how they infect the body, you can look here at Introduction to Bacteria.

There are two specific types of meningitis which bacteria are responsible for: acute and chronic. The first four bacteria are responsible for the acute type, and Mycobacterium tuberculosis is responsible for the chronic type. I’ll talk more about meningitis in the page with the same name: you can find it here.

Acute (Pyogenic) Meningitis: Symptoms:

The symptoms of acute meningitis are common with all the bacteria, and they include the following:

  • Headaches
  • Photophobia (sensitivity to bright light)
  • Neck stiffness
  • Irritability
  • Clouding of consciousness (are they there or not there?)

It can be fatal if left untreated, however it can often be cured by use of the correct antibiotics i.e. those that can cross the blood-brain barrier. When I refer to age groups most affected, I mean in terms of how often they contract meningitis or encephalitis from this bacterium in comparison to other age groups, and not necessarily how often they get infected with the bacteria itself.

Escherichia coli

Pic: http://media-3.web.britannica.com/eb-media//87/141087-050-24850517.jpg

  • Shape of bacteria: rod
  • Gram stain +/-: negative
  • Age group/s most affected: Infants

Escherichia coli is probably the most famous bacteria in the world. It is known more commonly as E.coli: the genus name is abbreviated. It normally resides in the human gut: when it lurks there, it is actually beneficial to our health, as it helps us produce some of our vitamins (particularly vitamin B12) that our body alone cannot. However, when outside of that area, it can wreak havoc.



Listeria monocytogenes

Pic: http://www.safeicehealthcare.com/images2/listeria%20monocytogenes.jpg

  • Shape of bacteria: rod
  • Gram stain +/-: positive
  • Age group most affected: elderly

Cases of listeria are most often seen in pregnant women who have eaten products contaminated with this bacterium, however in terms of contracting meningitis, it is more often than not the elderly who get it because their immune system is weakened. Listeria can cause both meningitis (inflammation of the meninges) and encephalitis (inflammation of the brain) at the same time: it is more commonly referred to as meningoencephalitis.




Neisseria meningitides

Pic: http://textbookofbacteriology.net/N.meningitidis.jpeg

  • Shape of bacteria: diplococci (two circles)
  • Gram stain +/-: negative
  • Age group/s most affected: young adults

N. meningitides is the cause of meningococcal disease and it can also cause meningitis. In around a tenth of the population, it can exist normally within the nasal passages. Unlike many other bacteria, which can exist in both humans and other animals, N. meningitides can only exist within humans because we are the only species capable of producing the iron-rich compounds (transferrin and lactoferrin) necessary for its survival.



Streptococcus pneumoniae

Pic: http://bioultra.com/wp-content/uploads/2011/12/streptococcus_pneumoniae.jpg

  • Shape of bacteria: rod
  • Gram stain +/-: positive
  • Age group/s most affected: elderly

Streptococcus pneumoniae is the main cause of pneumonia. Under normal circumstances, it can hang around both the nasal passages and also within the upper lung area, but with people with weakened or immature immune systems, this bacterium can turn on its host. There are many other diseases besides pneumonia that S. pneumoniae can cause, however, and they include meningitis.

Chronic Meningitis: Symptoms

Symptoms of chronic meningitis (usually caused by M. tuberculosis) include the following:

  • Headaches
  • Malaise (feeling ‘off’ or ‘not 100%’, often happens before an illness takes hold)
  • Mental condition
  • Vomiting



Mycobacterium tuberculosis

Pic: http://globalvoicesonline.org/wp-content/uploads/2012/01/tuberculosis.jpg

  • Shape of bacteria: undefined (due to waxy coat)
  • Gram stain +/-: ineffective (due to reason above: special staining required to detect it under the microscope)
  • Age group most affected: can affect any age group

Mycobacterium tuberculosis is more commonly known as TB. In the microbiology lab, the normal method of detection (a Gram stain) does not work, so therefore a Ziehl-Nielson stain is used to pick up TB under the microscope. TB is well known for infecting the lungs, but in some cases, it can produce either a tuberculoma (a solid tumour within the meninges) or arachnoid fibrosis, which is hardening of the arachnoid mater. Arachnoid fibrosis can lead to water in the brain, which is also known as hydrocephalus: this is very dangerous as it can put increased pressure on the brain.

Bacterial Infections of the Brain Tissue

There are two main types of bacteria that can infect the brain tissue: Treponema pallidum and Borrelia bugdorferi.

Borrelia burgdorferi

Pic: http://healthcareadministration1.com/wp-content/uploads/2013/10/lyme-disease-bacteria-under-microscope.jpg

  • Type of bacteria: spirochete
  • Gram stain +/-: unknown
  • Age group most affected: can affect any age group

Borrelia burgdorferi is the bacteria responsible for Lyme disease. It is often transferred to humans by tick bites, and it usually does not affect the nervous system unless it has been in the human body for a while and has not been treated. In around about one in ten cases, neurological symptoms such as facial palsy (droopiness on one side of the face) as well as headaches, sensitivity to light and neck stiffness can occur. There are other cases where there are shooting pains during the night. In late-stage Lyme disease, neurological symptoms can be very serious, and they include psychosis, panic attacks, anxiety and detachment from reality.

Treponema pallidum

Pic: http://www.epsomandewellhistoryexplorer.org.uk/images/TreponemaPallidum.jpg

  • Type of bacteria: spirochete
  • Gram stain +/-: unknown
  • Age group most affected: immunosuppressed

Treponema pallidum is the agent responsible for the development of neurosyphillis: that may sound familiar to you, as you might have heard of syphilis, which is a sexually-transmitted disease. Neurosyphilis is actually the tertiary (third and final) stage of syphilis. It usually presents itself in patients who have had syphilis for at least a decade, and by this time, it can appear on the face as necrotic tissue, as the picture below shows (Warning: not for the faint of heart!).



Pic: http://rantchick.com/wp-content/uploads/2009/08/Tertiary-Syphilis-150×150.jpg

There is a huge list of symptoms associate with neurosyphilis. I won’t list all of them, since the list is extensive, but the most notable ones are: blindness, dementia, headaches, confusion, psychosis and seizures. There is more than one type of neurosyphilis: meningovascular, paretic, and tabes dorsalis. These types of neurosyphilis affect the base of the brain, the frontal lobe and the spinal cord, respectively.