Meningitis Microbiology AAST 2024 PDF

Dr. Marwa A. Meheissen MICROBIOLOGICAL APPROACH TO MENINGITIS ILOs By the end of this lecture, students will be able to: 1. Recognize the microbial causes of meningitis. 2. Explain the pathogenesis of meningitis. 3. Interpret the laboratory results of cerebrospinal fluid mi...

Dr. Marwa A. Meheissen MICROBIOLOGICAL APPROACH TO MENINGITIS ILOs By the end of this lecture, students will be able to: 1. Recognize the microbial causes of meningitis. 2. Explain the pathogenesis of meningitis. 3. Interpret the laboratory results of cerebrospinal fluid microbiological analysis. Cellular barriers such as the blood–brain barrier and blood–cerebrospinal fluid (CSF) barrier protect against pathogen invasion of the central nervous system. The blood–brain barrier consists of tightly joined endothelial cells surrounded by glial processes, whereas the brain–CSF barrier at the choroid plexus consists of endothelium with fenestrations, and tightly joined choroid plexus epithelial cells. Pathogenesis of CNS infection Source of CNS infection: (1) Blood-borne invasion is the most common route of infection, for example, by polioviruses or Neisseria meningitidis. (2) Invasion via peripheral nerves is less common; examples include herpes simplex, varicella-zoster and rabies viruses. (3) Local invasion from infected ears or sinuses, local injury or congenital defects such as spina bifida also occurs. Blood-borne infection: Blood-borne invasion takes place across: the blood–brain barrier to cause encephalitis the blood–CSF barrier to cause meningitis. Pathogens can traverse these barriers by: growing across, infecting the cells that comprise the barrier being passively transported across in intracellular vacuoles being carried across by infected white blood cells. Example: Poliovirus invades the CNS across the blood–brain barrier. After the virus gains entry via oral ingestion, a complex stepwise series of events leads to CNS invasion. Poliovirus also invades the meninges after localizing in vascular endothelial cells, and can cross the blood–CSF barrier. Haemophilus influenzae, meningococci and pneumococci behave in the same way. Once infection has reached the meninges and CSF, the brain substance can in turn be invaded if the infection crosses the pia. In poliomyelitis, for instance, a meningitic phase often precedes encephalitis and paralysis. Neural spread: HSV and VZV present in skin or mucosal lesions, and travel up axons using the normal retrograde transport mechanisms that can move virus particles (as well as foreign molecules such as tetanus toxin) at a rate of about 200 mm/day, to reach the dorsal root ganglia. Rabies virus, introduced into muscle or subcutaneous tissues by the bite of a rabid animal, infects muscle fibres and muscle spindles. It then enters peripheral nerves and travels to the CNS, to reach glial cells and neurones, where it multiplies. Cellular response to infection The response to invading viruses is reflected by an increase in lymphocytes, mostly T cells, and monocytes in the CSF. A slight increase in protein also occurs, the CSF remaining clear. This condition is termed ‘aseptic’ meningitis. The response to pyogenic bacteria shows a more spectacular and more Page 1 of 10  Dr. Marwa A. Meheissen rapid increase in polymorphonuclear leukocytes and proteins, so that the CSF becomes visibly turbid. This condition is termed ‘septic’ meningitis. Certain slower growing or less pyogenic microorganisms induce less dramatic changes, such as in tuberculous meningitis. Figure 1. The mechanism of central nervous system (CNS) invasion. Causes of meningitis Septic meningitis (purulent/bacterial) Aseptic meningitis Meningitis where the organism is cultivable Meningitis for which the cause is not apparent after Refers mainly to pyogenic/bacterial meningitis initial evaluation of routine stain and culture of CSF. Refers mainly to viral meningitis/atypical bacterial, parasitic causes/Non-infectious causes Bacterial causes Viral causes Neisseria meningitidis Enteroviruses Streptococcus pneumoniae Herpes viruses (HSV-2, HSV-1, EBV, CMV, HHV-6) Haemophilus influenzae Measles Neonatal meningitis Mumps Streptococcus agalactiae Arboviruses E. coli Non-cultivable bacteria/difficult to culture Listeria monocytogenes Treponema Leptospira Brucella Mycobacterium tuberculosis BACTERIAL MENINGITIS Bacterial meningitis is more severe, but less common, than viral meningitis and may be caused by a variety of agents. Prior to the 1990s, Haemophilus influenzae type b (Hib) was responsible for most cases of bacterial meningitis. However, the introduction of the Hib vaccine into childhood immunization regimens has lowered overall Hib incidence in favour of Neisseria meningitidis and Streptococcus pneumoniae, which are now responsible for most bacterial meningitis. These three pathogens have several virulence factors in common, including possession of a polysaccharide capsule. Page 2 of 10  Dr. Marwa A. Meheissen Table 1. Virulence factors of bacterial pathogens causing meningitis. Bacterial pathogens Virulence factor Neisseria meningitidis Streptococcus Haemophilus influenzae pneumoniae Capsule + + + IgA protease + + + Pili + - + Endotoxin + - + Outer membrane proteins + - + Table2. Polysaccharide capsules as important virulence factors in the pathogenesis of bacterial meningitis Pathogens Important type Vaccines Streptococcus pneumoniae Many A polysaccharide vaccine containing 23 capsular types (PPSV23). A conjugate vaccine contains capsular polysaccharides (13-valent) conjugated to diphtheria CRM197 protein, recommended for children as a four-dose series at 2, 4, 6, and 12–15 months of age. Hemophilus influenzae B Hemophilus influenzae type B (Hib) polysaccharide and conjugate vaccines Neisseria meningitidis A, B, C, Y, W-135 A, C, Y, W quadrivalent vaccine (polysaccharide and conjugate vaccine) B vaccine Group B streptococci Ia, Ib, II, III in neonatal meningitis E. coli K1 meningitis 1. Meningococcal meningitis Morphology Neisseria meningitidis is a Gram-negative diplococcus which closely resembles N. gonorrhoeae in structure, but with an additional polysaccharide capsule that is antigenic and by which the serotype of N. meningitidis can be recognized. Pathogenesis The bacteria are carried asymptomatically in the population, up to 20% depending on geographical location, and are attached by their pili to the epithelial cells in the nasopharynx. Invasion of the blood and meninges is a rare and poorly understood event. People possessing specific complement-dependent bacterial antibodies to capsular antigens are protected against invasion. Those with C5–C9 complement deficiencies show increased susceptibility to bacteremia. Those most often infected include young children who have lost the antibodies passively acquired from their mother and adolescents who have not previously encountered the infecting serotype and therefore have no type-specific immunity. Person-to-person spread takes place by droplet infection and is facilitated by other respiratory infections, often viral, that cause increased respiratory secretions. Thus, conditions of overcrowding and confinement such as prisons, military barracks and college dormitories contribute to the frequency of infection in populations. Page 3 of 10  Dr. Marwa A. Meheissen During outbreaks of meningococcal meningitis, which most frequently occur in late winter and early spring, the carrier rate may reach 60–80%. Specific serotypes associated with infection exhibit some geographical variation. serotypes B, W, Y and C, in that order, tend to predominate in more resource-rich countries, whereas serotypes A and W-135 are more common in less-developed regions. Available vaccines target serotypes A, C, Y and W-135 or target serotype B. From the nasopharynx, organisms may reach the bloodstream, producing meningococcal bacteremia; the initial symptoms during this stage of the actual infection may be similar to those of an upper respiratory tract, “flu-like” infection, but invasive meningococcal disease (IMD) quickly ensues. IMD typically presents as meningitis, sepsis (ie, meningococcemia), or as a combination of both. Meningitis is the most common. Fulminant meningococcemia is more severe, presenting with a high fever and a hemorrhagic rash; the patient may also develop disseminated intravascular coagulation and ultimate circulatory collapse with bilateral hemorrhagic necrosis of the adrenal glands with subsequent adrenal failure (Waterhouse-Friderichsen syndrome). After an incubation period of 1–3 days, the onset of meningococcal meningitis is sudden with a sore throat, headache, drowsiness and signs of meningitis which include fever, irritability, neck stiffness and photophobia. There is often a hemorrhagic skin rash with petechiae, reflecting the associated septicemia. In about 35% of patients, this septicemia is fulminating, with complications due to disseminated intravascular coagulation, endotoxemia and shock, and renal failure. In the most severe cases there is an acute Addisonian crisis, with bleeding into the brain and adrenal glands referred to as Waterhouse–Friedrichsen syndrome. Mortality from meningococcal meningitis reaches 100% if untreated, but remains around 10% even if treated. In addition, serious sequelae such as permanent hearing loss may occur in some survivors. Microbiological Diagnosis A diagnosis of acute meningitis is usually suspected on clinical examination. Laboratory identification of the bacterial cause of acute meningitis is essential so that appropriate antibiotic therapy can be given and prophylaxis of contacts initiated. Specimen: The typical specimens for isolation of N. meningitides include blood for culture and cerebrospinal fluid (CSF) for smear and culture. Puncture material or biopsies from petechiae may be taken for smear and culture. Nasopharyngeal swab cultures are suitable for carrier surveys. 1. Macroscopic examination: Normal CSF is clear and colorless like water. CSF is examined for the presence of turbidity, ‘spider-web’ clot (in extreme cases of TB meningitis) may be present. 2. Direct microscopic examination: Preliminary microscopy results involving white cell counts and Gram staining for bacteria (Performed from the CSF deposits after centrifugation) should be available within an hour of receipt of the CSF sample in the laboratory and should at once be reported to the physician to guide his initial choice of antibiotics. Gram-stained smears shows typical Gram-negative diplococci within polymorphonuclear leukocytes or extracellularly. 3. Chemical examination: The CSF/serum glucose ratio is also useful as bacteria breakdown glucose and so a low CSF sugar compared with serum glucose indicates a bacterial infection Page 4 of 10  Dr. Marwa A. Meheissen in the CSF. A high protein level indicates bacterial infection as well. (Performed from CSF supernatant after centrifugation) 4. Culture: CSF deposits and blood culture specimens are plated on blood agar and chocolate agar and then incubated at 37°C in an atmosphere of 5% CO2. Selective medium (Modified Thayer Martin agar) favors the growth of neisseriae, inhibits many other bacteria, and is used for nasopharyngeal cultures. Colonies of N. meningitidis are gray, convex, and glistening, with entire edges; a positive oxidase test together with a Gram-stain showing Gram-negative diplococci provides presumptive organism identification. The organism can be further identified by carbohydrate oxidative reactions and subsequent agglutination with type- specific serum. 5. Antigen detection: A rapid indicator of the type of infection may be obtained by agglutination test performed using the CSF to demonstrate the presence of the antigens of meningococci, pneumococci and Haemophilus influenzae type b. 6. Molecular diagnosis: can also be carried out and may be of clinical assistance as early treatment saves lives (either using uniplex or multiplex PCRs; commercially available to detect the most common pathogens responsible for meningitis or encephalitis in CSF samples) 7. Serology: is not helpful in the diagnosis because the infection is too acute for an antibody response to be detectable. Treatment & Prevention Meningococcal meningitis is a medical emergency and antibiotic therapy, such as Penicillin G, or ceftriaxone must be given immediately if the diagnosis is suspected and is the treatment of choice if the diagnosis is confirmed. Close contacts in the family, referred to as ‘kissing contacts’, should be given single-dose ciprofloxacin. Note that penicillin is not used for prophylaxis because it does not eliminate nasopharyngeal carriage of meningococci. Rifampicin used to be recommended but it is associated with rapid induction of resistance, has to be taken for a longer time period. 2. Pneumococcal meningitis Morphology Streptococcus pneumoniae is a capsulate Gram-positive diplococcus, it is a common cause of bacterial meningitis, particularly in children and the elderly. Little is known about its virulence attributes apart from its polysaccharide capsule. Pathogenesis Strep. pneumoniae is carried in the throat and nasopharynx of many healthy individuals. Invasion of the blood and meninges is a rare event, but is more common in the very young (