Bacterial meningitis is caused by several species of bacteria that infect the protective membranes surrounding the brain and spinal cord. The specific bacterium responsible depends largely on the patient’s age, with different organisms posing the greatest threat to newborns, children, and older adults. About 1 in 6 people who develop bacterial meningitis die from it, and these bacteria account for more than half of all meningitis deaths worldwide.
The Main Bacteria by Age Group
In adults over 16, the pneumococcus bacterium (Streptococcus pneumoniae) dominates, causing roughly 72% of bacterial meningitis cases. It tends to hit older adults and people with weakened immune systems hardest, often following a respiratory tract infection like pneumonia. The bacterium first colonizes the back of the throat, then enters the bloodstream and eventually crosses into the brain.
The meningococcus bacterium (Neisseria meningitidis) is the primary concern for older children, teenagers, and young adults. It spreads through respiratory droplets and saliva, which is why outbreaks cluster in college dormitories and military barracks. Meningococcal disease can progress from first symptoms to life-threatening illness in under 24 hours.
Newborns face a different set of threats entirely. Group B streptococcus, E. coli, and Listeria are the most common causes of meningitis in the first months of life. Babies typically acquire Group B strep by inhaling contaminated amniotic or vaginal secretions during labor. In early-onset disease, symptoms usually appear within the first 12 to 48 hours after birth. Late-onset disease can develop anytime from one week to about three months of age.
Older adults are vulnerable to pneumococcus again, but also to Listeria and various other bacteria that rarely cause meningitis in younger, healthy people. A weakened immune system, whether from age, medications, or chronic illness, is a common thread linking most adult cases.
How Bacteria Reach the Brain
The brain is protected by the blood-brain barrier, a tightly sealed layer of cells lining the blood vessels that supply the brain. Bacteria have evolved several strategies to breach it. Some pass directly through the cells of this barrier in a process called transcytosis. Others squeeze between cells by loosening the junctions that hold them together. A third method, sometimes called the “Trojan horse” mechanism, involves bacteria hitching a ride inside immune cells that are allowed to cross the barrier freely.
Once bacteria begin multiplying in the brain, the resulting inflammation actually makes the barrier more permeable, allowing even more bacteria to cross. This creates a dangerous feedback loop where infection fuels further infection. Bacteria can also enter through a separate barrier at the choroid plexus, the tissue that produces cerebrospinal fluid, or through the membranes surrounding the brain itself.
Infections That Spread to the Brain
Not all cases begin in the bloodstream. Meningitis is the most common brain complication of sinus infections. Bacteria from a severe sinus infection can spread through thin bone or along blood vessels directly into the membranes covering the brain. Ear infections, particularly chronic or untreated ones, pose a similar risk because of the ear’s proximity to the brain’s protective layers.
Penetrating head wounds create a direct path for bacteria. Staphylococcus aureus, which lives on the skin, is the most common cause of meningitis following a head wound or neurosurgery. Skull fractures that breach the sinuses or inner ear can also allow bacteria in, sometimes causing meningitis days or even weeks after the injury.
Listeria: A Food-Borne Route
Listeria meningitis stands apart because the bacterium enters the body through contaminated food rather than person-to-person spread. The highest-risk foods include unpasteurized milk, soft cheeses, cold deli meats, cold hot dogs, raw sprouts, and smoked seafood. The bacterium spreads through the fecal-oral route and can survive refrigeration, which is why ready-to-eat foods are a particular concern.
Pregnant women, newborns, adults over 65, and anyone with a compromised immune system face the greatest risk. In pregnant women, a Listeria infection can cross the placenta and cause meningitis in the newborn. For healthy younger adults, the same exposure typically causes nothing more than a brief gastrointestinal illness.
Why Vaccines Changed the Picture
The landscape of bacterial meningitis has shifted dramatically because of vaccination. Haemophilus influenzae type b (Hib) was once a leading cause of meningitis in young children, with an estimated 20,000 cases per year in the United States during the early 1980s. Since the Hib vaccine became widely available, invasive Hib disease has dropped by more than 99%. It remains a threat in countries where the vaccine is not widely used.
Vaccines against pneumococcus and meningococcus have similarly reduced cases in their target age groups, though neither bacterium has been eliminated. Pneumococcal meningitis still occurs regularly in older adults who haven’t been vaccinated, and meningococcal outbreaks still emerge in communities with gaps in coverage.
Risk Factors That Increase Vulnerability
Certain conditions make meningitis more likely regardless of the specific bacterium involved. A weakened immune system is the most significant, whether caused by HIV, cancer treatment, organ transplant medications, or the natural decline of immune function with age. People without a functioning spleen are at particularly high risk because the spleen filters bacteria from the bloodstream.
Living in close quarters increases exposure to the bacteria that spread through respiratory droplets, especially meningococcus. Recent respiratory infections can damage the mucous membranes of the throat and sinuses, giving bacteria easier access to the bloodstream. Chronic ear or sinus infections that go untreated create an ongoing risk of direct bacterial spread to the brain.
How It’s Identified
Doctors confirm bacterial meningitis by analyzing cerebrospinal fluid drawn through a lumbar puncture. In bacterial meningitis, the fluid shows elevated protein levels and glucose levels that drop below 50% of the patient’s blood sugar, sometimes becoming extremely low. Viral meningitis, by contrast, typically shows normal glucose levels. This glucose distinction is one of the quickest ways to differentiate the two, which matters because bacterial meningitis requires immediate treatment while viral meningitis usually resolves on its own.