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Introduction
Bacterial Meningitis is an inflammation of the meninges and the brain. The meninges is a membrane that encloses the brain and spine. It consists of three layers within which flows the cerebrospinal fluid which acts as a "shock absorber" for the brain and central nervous system. The three layers of the meninges are the outer Dura mater, the Arachnoid membrane and the innermost Pia mater. The meninges surrounding the brain are more complex than the spinal meninges but the two structures are continuos and serve the same basic functions, providing protection, a blood supply, drainage and channels for the flow of cerebrospinal fluid. Bacterial meningitis is caused by the presence of bacteria in the cerebrospinal fluid. Bacterial meningitis is one of two types of meningitis, the other being viral meningitis. Bacterial meningitis, given sufficient time without treatment, will cause damage to the meninges and indwelling nervous system resulting in such conditions as partial or complete deafness (due to a sub-infection of the cochlea) and, particularly in younger victims, epilepsy or retardation. If bacterial meningitis remains untreated, it will also result in death due to excessive damage to the brain or nervous system.
Symptoms of Bacterial Meningitis include fever, headaches, seizures, vomiting, impairment of consciousness and stiff neck and back. The most important symptom of bacterial meningitis for early recognition is that of stiffness of the neck on bending forward.
Overview of Bacterial Meningitis
Bacterial Meningitis is an infection of the meninges, a membrane surrounding the brain, spinal cord and optic nerves, and it's fluid contents, the cerebrospinal fluid. Due to the continuos nature of the meninges and the cerebrospinal fluid, an infection of any area of the system can rapidly extend to the rest of the system. The presence of bacteria and their toxins within these areas results in an inflammation of the cranial or spinal structures. This inflammation will result in damage to the nervous system and brain structures unless treated. The damage caused may be transient, in which case the victim will recover from the effects of the inflammation if treated quickly, or permanent if treatment is not forthcoming or unsuccessful. The damage culminates in death due to damage to the brain or nervous system. Bacterial Meningitis is caused by a number of different bacteria as detailed in the table 1 below,
| Table 1 - Typical pathogens involved in Bacterial Meningitis | ||
| Neisseria meningitidis
Streptococcus pneumoniae Hemophilus influenzae | Most common pathogens | |
| Staphlyococcus aureus
Staphlyococcus epidermis (Various other Streptococci) Escherichia coli Klebsiella enterobacter Proteus Citrobacter Pseudomonas |
Pathogens associated with complications in Medical procedures on the nervous system such as neurosurgery, lumbar punctures, spinal anaesthesia and cranial trauma | |
| Salmonella
Shigella Clostridium perfringens Neisseria gonorrheae | Rare meningeal pathogens | |
| Listeria monocytogenes | A relatively newly recognised pathogen in bacterial meningitis which mainly occurs in the elderly | |
As can be seen, the bacteria responsible for bacterial meningitis are many and varied. One thing that most of these bacteria have in common however is their method of invasion of the human body. Many of them are present on or in healthy humans at one time or another, either on the skin or in the respiratory tract and as a result of trauma or weakness in the immune system invade the human body via the bloodstream. The bloodstream is their main route to the point of infection, the meninges and cerebrospinal fluid. Bacterial meningitis can result from infections of the respiratory system, medical procedures, trauma to the nervous system or injury to the cranial region.
Common Pathogens in Bacterial Meningitis
The three most common causative agents of Bacterial meningitis are Neisseria meningitidis, Streptococcus pneumonaie and Hemophilus influenzae (type B). These bacteria are found normally as inhabitants of the nasopharynx in a large proportion of the population. It is not certain what causes them to travel from this area, via the bloodstream, to the meninges although it is postulated that they result from infections of the upper respiratory tract or lungs (pneumonia for instance seems to lead to pneumococcal infections of the meninges). The type of bacteria which is responsible for particular cases of meningitis is also dependent on age as detailed in the table below,
| Table 2 - Age dependence on cause of Bacterial Meningitis | ||
| Bacteria | No. of cases | |
|---|---|---|
| Neisseria meningitidis | 10-30% in adults
30-40% in children up to the age of 15 Very rare in infants | |
| Streptococcus pneumonaie | 30-50% in adults
10-20% in children Up to 5% in infants | |
| Hemophilus influenzae | 1-3% in adults
35-45% in children Rare in infants | |
From this it can be seen that N.meningitidis, S.pneumoniae and H.influenzae account for the most significant portion of Bacterial Meningitis cases overall. Most of the remainder of cases are caused by the bacteria listed on the previous page.
Pneumococcus (Steptococcus pneumoniae)
Pneumococci consist of separate, paired or short chains of oval-shaped cocci measuring about 0.05-1.25m. Fresh cultures of pneumococci stain gram positive. The individual or paired cells are enclosed by a polysaccharide envelope. Pneumococci have been found in the upper respiratory tract of various healthy mammals including humans, monkeys, calves, horses and dogs. They are present in most healthy humans at one stage or another and are likely to cause disease only if the lining of the respiratory tract is damaged (by a previous infection for example) in which case the lung tissue may become infected. As well as causing lobar pneumonia and bacterial meningitis in humans, pneumococcus may also cause pleural empyhsema, pericarditis, endocarditis, arthritis, peritonitis and infection of the middle ear (cochlea). Bacterial meningitis can follow on from any of the listed diseases if the bloodstream is invaded by pneumococci.
Antibiotics, particularly Penicillin, have been instrumental in reducing the high mortality rate of pneumococcal infections.
Meningococcus (Neisseria meningitidis)
The Meningococcus is a diplococcus, aerobic bacteria. It provides a gram negative stain. The two main serogroups of meningococcus responsible for previous epidemics have been serogroups A and C. Humans are the only known natural host of the Meningococcus. Meningococcus are frequently present in healthy humans in the nasopharyngeal area. Death occurs in a high proportion of cases of meningitis caused by N.meningitidis, mainly due to the rapid release of large amounts of bacterial endotoxin into the host which results in toxic shock and hemorraging of the affected areas. A large number of strains of meningococci are resistant to treatment by Sulfonamides and so the most suitable treatment available is that of Penicillin (type G) or alternatively, Chloramphenicol for the treatment of individuals who are allergic to Penicillin.
Haemophilus (Hemophilus influenzae)
Haemophilus is an obligate parasite, incapable of surviving in an inanimate environment. Most members of the genus are harmless (although some act as opportunistic pathogens) and live in the upper respiratory tract, lower genital tract, mouth and pharynx of humans and animals. Haemophilus is most inclined to become pathogenic in the very young or those who have suffered from infections of the respiratory system. H.influenzae is thus an agent of Bacterial meningitis, particularly in young infants. The name H.influenzae was given to this bacteria based on the mistaken belief that it was an agent of influenza, later disproven. H.influenzae is also responsible for conjunctivitis, infections of the middle ear and secondary infections of the respiratory tract.
Ampicillin was originally the most prevalent form of treatment for Haemophilus infections but as a result of developing resistance to this drug, Chloramphenicol and Tetracycline are more suitable and effective.
Conclusions
Bacterial meningitis is a potentially fatal disease although modern medicine has brought large decreases to the mortality rate. Most cases of Bacterial meningitis can be treated to full recovery provided the disease is diagnosed rapidly and correctly although conditions such as alcoholism and high-dose steroid use seem to do reduce the chances of a full recovery. Bacterial meningitis has it's highest occurence in the autumn, winter and spring as has been the trend in Ireland in the last few months where the severe winter conditions and subsequent increased incidence of respiratory infection may have been responsible for the surprising number of recent cases. Antibiotics are the most common form of treatment although some vaccines are available.
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