Bacteria are single-celled micro-organisms. They possess a cytoplasmic membrane for protection, and some bacteria also carry another barrier called a capsule. Generally this is a polysaccharide capsule, though some bacteria types possess protein-based capsules. Encapsulated bacteria are some of the most virulent. Microbiologists strive to understand the mechanisms of encapsulated bacteria to seek better treatments and preventatives for disease.

A list of virulent encapsulated bacteria with a polysaccharide capsule includes Streptococcus pnemoniae, Klebsiella pneumonia, group B streptococci, Escherichia coli, Neisseria meningitides and Haemophilus influenzae. This is not a comprehensive list of bacteria with capsules that cause diseases, but rather it contains the most common examples. The capsules of these bacteria lead to their virulence, as mutant versions of them without a polysaccharide capsule do not cause disease. Disease-causing bacteria with protein rather than polysaccharide capsules include Bacillus anthracis and Yersinia pestis. People infected with encapsulated bacteria display blood serum containing anticapsular antibodies.

The polysaccharide capsule of encapsulated bacteria is made of complex carbohydrates with high molecular weight. Sometimes this capsule carries the name of "slime layer" due to its polysaccharide content. Such encapsulated bacteria appear shiny via microscopy. This slimy capsule serves to protect the bacteria from drying out, and its slippery quality prevents attack by white blood cells of hosts or protects against being consumed by macrophages. Some bacteria even secrete capsules when triggered by changing conditions, such as increased sugar in their surroundings. The polysaccharide capsule can serve as a disguise mechanism by mimicking qualities of its host. Evidence suggests the bacterial capsule contributes to toxicity in a host, enabling the spread of disease. Some of the most virulent bacteria have these capsule membranes. The capsule itself indicates the level of virulence. For example, some bacteria can produce polysaccharide capsules with substances such as acid that hinder disease-fighting leukocytes. Polysaccharide capsules provide adherence to hosts and protection of the bacteria from the surrounding environment. The capsule also contributes to antibiotic resistance.

Diseases caused by bacteria with capsules comprise some of the most dangerous and potentially lethal diseases. These include pneumonia, otitis media and meningitis. Meningitis is defined as inflammation of the meninges (protective membranes) and fluid surrounding the brain and spinal cord. Meningitis caused by encapsulated bacteria is the most dangerous form of meningitis and needs immediate treatment as it can lead to paralysis, limb loss, hearing loss or death. Vaccines for pneumonia and meningitis exist and can help control these diseases, even in an environment of increasing antibiotic resistance. Polysaccharide vaccines work via removing and purifying polysaccharides from bacteria, so that when this is injected, the immune system creates antibodies to fight the bacteria. While polysaccharide vaccines have been historically used, newer, polysaccharide-protein conjugate vaccines may provide greater protection.

Some people either are born with no spleen, as in asplenia, or possess spleens with poor function. Certain conditions additionally entail the removal of the spleen, or a splenectomy. Spleens rarely rupture, but may need to be removed due to traumatic injury. Some examples of conditions leading to splenectomy include benign hematological diseases, immune thrombocytopenic purpura, sickle cell anemia, autoimmune hemolytic anemia, hemolytic spherocytosis, Thalassemia, various non-malignant lymphoid disorders and some cancers such as non-Hodgkin’s lymphoma.

Asplenic patients face great risk of infection by diseases caused by bacteria with capsules, which can lead to death. Asplenic children particularly are at higher risk of developing overwhelming sepsis than adults. Sepsis can occur due to encapsulated bacteria, most commonly Streptococcus pneumonia. Sepsis is a medical emergency of widespread bodily infection requiring immediate assistance and treatment, without which death can rapidly occur. In asplenic patients, the severity of infection from encapsulated bacteria is far greater because of the absence of the spleen’s disease-fighting and blood-cleaning capability. Spleens produce lymphocytes and monocytes that provide immunological responses and defense against polysaccharide capsule bacteria. Asplenic patients therefore may be given antibiotics as a preventative (prophylaxis) measure to decrease the likelihood of infection. Additionally, preventative vaccines may be required such as pneumococcal conjugate vaccine, haemophilus influenzae type B vaccine, meningococcal conjugate vaccine and annual influenza vaccine. While vaccines and prophylactic antibiotics provide the best current, needed protection against encapsulated bacterial infection, they are not completely guaranteed to prevent sepsis. Asplenic patients must take special care in travel to avoid malaria-prone areas and to avoid dog and tick bites, which can transmit diseases and lead to infection.

Microbiologists work to create better medicines to target encapsulated bacteria. This may entail determining surface protein characteristics for various strains or other methods for research.