Belland/Ryan - Fungal/Bacterial Meningitis

Question 1

What are the 4 major agents of bacterial meningitis?

Answer 1

• Strep pneumo: G+ cocci in chains, alpha hemolytic, facultative anaerobe
• Neisseria meningitidis: G- diplococci
• H. flu: G- bacillus, chocolate agar (+ factors X, V)
• Listeria monocytogenes: G+ rod (but doesn’t gram stain well), facultative IC

Question 2

What are the features of bacterial meningitis? Signs and symptoms?

Answer 2

• MEDICAL EMERGENCY: more severe than viral meningitis (which is self-limiting, and rarely tx’d)
  1. Acute onset and high mortality rate with often irreversible damage to the CNS
• SIGNS/SYMPTOMS: headache, fever, vomiting, stiff neck, photophobia, irritability varying degrees of neuro dysfunction

Question 3

How can you ID bacterial agents in the CNS?

Answer 3

• Spinal tap of CSF:
  1. Gram stain
  2. Rapid antigen detection (capsular material in CSF)
  3. PCR testing for specific species in CSF (timely)
  4. Bacterial culture of CSF and blood on non-selective media like BAP or CAP (timely)

Question 4

What will you generally see in the CSF in pts with bacterial meningitis (table)?

Answer 4

• Large # of PMN’s: chemotactic response
• Reduced glucose: host stress response
• Elevated protein: BBB breakdown

Question 5

What are the agents of bacterial meningitis by age group (table)?

Answer 5

• This disease is more rare in older adults, and may be caused by:
  1. S. pneumo (MCC in US)
  2. N. meningitidis
  3. H. flu
  4. L. monocytogenes
  5. G- bacteria: enterics, pseudomonas aeruginosa

Question 6

What is this?

Answer 6

• Streptococci: G+ spherical bacteria that occur in pairs (diplococci) or in long chains
• Grow in chains because they divide in one plane: this differentiates them from Staph (which grow in clusters)

Question 7

What are the 4 major streptococci that cause human disease?

Answer 7

• STREP PNEUMONIAE: aka, pneumococcus -> normal inhabitants of upper respiratory tract (5-40% of ppl), but MCC of bacterial meningitis in the US (about 50%)
• STREP PYOGENES: aka, GAS -> normal inhabitants of upper respiratory tract (5-15% of ppl), but causative agent of numerous local and systemic diseases and post-disease sequelae
• STREP AGALACTIAE: aka, GBS
• VIRIDANS STREP: many species, incl. S. mutans, a dental pathogen

Question 8

How are the streptococci classified? Which is used to type S. pneumo?

Answer 8

• LANCEFIELD CLASSIFICATION (gp spec. substance): antigenic carbohydrate substance found in cell walls that determines lancefield gps A-H, K-U -> serological specificity determined by an amino sugar:
  1. Gp A = rhamnose-N-acetylglucosamine
  2. Gp B = rhamnose-glucosamine
  3. Gp C = rhamnose-N-acetylgalactosamine
  4. Gp D = glycerol teichoic acid
  5. Gp F = glucopyranosyl-N-acetyl-galactosamine
• CAPSULAR POLYSACCHARIDES: antigenic specificity used to type S. pneumo (>90 types)
• BIOCHEM RXNS: used to type streptococci that don’t have specific hemolysis patterns or the antigenic molecules above (e.g., viridans streptococci)

Question 9

What types of disease does S. pneumo cause?

Answer 9

• PNEUMONIA: usually lobar type that involves all of a single (or multiple) lobes (vs. bronchial pneumo which involves the alveoli) -> case fatality rate 5%
• BACTEREMIA: invasion of tissues and multiplication in bloodstream -> case fatality rate 20%
• OTITIS MEDIA: infections of ear, most common reason for AB prescriptions in infants and children
• MENINGITIS: systemic infection that crosses BBB and has case fatality rate of 30%
• OTHER: paranasal sinusitis, osteomyelitis, septic arthritis, endocarditis, peritonitis, cellulitis, brain abscesses
• NOTE: currently the leading cause of invasive bacterial disease in kids, elderly, immunocompromised

Question 10

What is the pathogenesis of S. pneumo?

Answer 10

• COLONIZATION: adhere tightly to nasopharyngeal epithelium by multiple mechs, and may progress into lungs or middle ear
  1. Inflam in middle ear caused by pneumococcal cell wall components resulting in cytotoxicity on ciliated cells of cochlea
  2. If reach lower resp tract by aerosol, it can progress to alveolus & assoc w/specific alveolar cells that produce choline-containing surfactant
• INVASION: alter vascular permeability to allow access to bloodstream -> lung infection and bacteremia
  1. Can directly invade endo cells where bac are transported across cell and expelled into blood
  2. Can cross BBB by binding cerebral caps, trans-migrate and enter CSF -> meningitis

Question 11

What are the 7 key S. pneumo virulence factors?

Answer 11

• CAPSULE: interferes w/phagocytosis by leukocytes by interfering w/binding of complement C3b to cell surface (100,000x more virulent than unencapsulated)
• PILI: initial event in invasive disease is attachment of encapsulated bac to epi cells in upper resp tract
• CELL WALL: isolated cell walls can recreate many symptoms of pneumonia, OM, meningitis -> similar inflam response to live org (teichoic acids, lipoteichoic acids that contain phosphorylcholine/peptidoglycan)
• CHOLINE BINDING PROTEINS: CbpA major adhesin and has 8 choline-binding repeats -> interacts w/carbs on pulm epi surface and important in crossing BBB in meningitis (defective colonization of nasopharynx w/o)
• HEMOLYSINS: pneumolysin protein can cause lysis of host cells and activate complement
• HYDROGEN PEROXIDE (H2O2): damages host cells and has bactericidal effects against competing bac like S. aureus
• NEURAMINIDASE/IgA PROTEASE: invasion of host tissue and destruction of secreted mucosal IgA

Question 12

What are the salient features of Pneumovax?

Answer 12

• Multivalent: 23 capsule types (non-conjugated)
• For older pts (65+) and high-risk groups: HIV pts, cardiopulmonary disorders, transplant pts, splenic disorders
• Immunity lasts 5-7 years
• NOT effective in infants and young
• Protects against invasive pneumococcal disease

Question 13

What are the salient features of Prevnar?

Answer 13

• Heptavalent (13-valent) capsule types (conjugated)
• Safe, but very expensive
• Anamnestic response: adaptive immunity with memory
• Potential to reduce pneumococcal meningitis by 85% in infants

Question 14

What is the basic structure of Neisseria?

Answer 14

• Gram (-), non-motile bacteria in pairs (diplococci)
• Thought to divide as diplococci (different than streptococci)
• May OR may not have capsule

Question 15

Are there neisseria that are part of the normal flora? What are the 2 critical pathogenic variants?

Question 16

How can you distinguish N. meningitidis from N. gonorrhea?

Answer 16

• Meningitidis identical in its staining & morphological characteristics to N. gonorrhea, but at ultrastructural level, has a prominent anti-phagocytic polysaccharide capsule
• Strains are grouped on basis of their capsular poly-saccharides into 12 serogroups:
  1. Most important serogroups associated with disease in humans are A, B, C, Y, and W₁₃₅

Question 17

What is the epi of N. meningitidis?

Answer 17

• 10-15% fatality rate, in spite of tx with AB’s
• Of survivors, 10% lose arms or legs, become deaf, have problems with nervous system, become mentally retarded, or suffer seizures or strokes
• CROWDED CONDITIONS promote transmission: day care centers, jails, military barracks
• Outbreaks tend to be: SPORADIC (small # of related cases: B, C, Y) or EPIDEMIC (lg # of cases, lg areas: A)
  1. Gp A still predominates in Africa: Sub-Saharan “meningitis belt” w/high rate of disease with epidemics in irregular cycles e/5-12 yrs and last 2-3 dry seasons, dying out during rainy seasons

Question 18

Briefly describe the progression and course of N. meningitidis disease.

Answer 18

• Fulminant meningococcemia in 5-15% of pts with meningococcal disease -> high mortality rate
• Begins abruptly with sudden high fever, stiff neck, chills, myalgias, weakness, N/V, and headache
• Apprehension, restlessness, delirium w/in next few hours and widespread petechial and purpuric skin lesions appear suddenly
• Pulm insufficiency devo in a few hours, and many pts die w/in 24 hours of being hospitalized despite appropriate AB therapy and intensive care

Question 19

What are the early symptoms of N. meningitidis?

Answer 19

• Neck/back stiffness
• Mental changes: agitation, confusion, coma
• Petechial/purpuric rash: late sign

Question 20

What are the virulence factors of N. meningitidis?

Answer 20

• Major toxin is LOS (lipooligosaccharide): endotoxin
• Antiphagocytic polysaccharide capsule
• Human nasopharynx is only known reservoir of N. meningitidis -> spread via resp droplets; transmission requires aspiration of infective particles
  1. Attach to nonciliated columnar epi cells of nasopharynx via pili and possibly outer mem components -> invasion of mucosal cells by mech involving phase variation (turn off capsule genes), then turn on again once in bloodstream

Question 21

How can N. meningitidis be treated?

Answer 21

• PENICILLIN is DOC to tx meningococcemia and meningococcal meningitis
• CHLORAMPHENICOL or 3rd-gen CEPHALOSPORIN like CEFOTAXIME or CEFTRIAXONE in ppl allergic to penicillins

Question 22

List and compare the signs and symptoms of meningococcemia vs. meningococcal meningitis (image).

Answer 22

Question 23

What are the 2 N. meningitidis vaccines currently available in the US?

Answer 23

• MENOMUNE: serogroups A, C, Y, and W₁₃₅
  1. Unconjugated vaccine (carbohydrate only; no memory component) given to high-risk pts and military personnel (ineffective in young kids)
  2. To control epidemics and for travelers in Sub-Saharan Africa
  3. Cochlear transplant pts
• MENACTRA: serogroups A, C, Y, and W₁₃₅
  1. Conjugate to toxoid (anamnestic response) licensed for use in 11-55-y/o’s and recommended for all children 11-12-y/o
• Neither vaccine covers serogroup B: capsular poly-saccharide a homopolymer of sialic acid and NOT imunogenic in humans (vax of outer mem protein Ag’s recently developed, but not licensed in US)

Question 24

Why is N. meningitidis so evasive?

Answer 24

• High frequency genetic variation:
  1. Phase variation: changes in expression states of gene, leading to on-off control
  a. Direct DNA repeats, slipped strand mis-pairing, many systems (attachment proteins, capsule, invasion proteins, LOS, etc.)
  2. Antigenic variation: changes in genes leading to expression of different forms of similar genes
  a. Example of pilin system, where silent pilS loci are incorporated into variable region of pilE expression locus (RecA recombination)

Question 25

What are some key infections caused by H. flu?

Answer 25

• Like S. pneumo, is a major agent of otitis media, resp disease, and (in young children) meningitis
  1. Infection starts nasopharyngeal
  2. Age-specific influence for meningitis

Question 26

How are H. flu isolates classified? Virulence factors?

Answer 26

• Classified according to capsule polysaccharide:
  1. Serotypes a-f, or non-typable
  2. Serotype b is most important pathogen (Hib)
• VIRULENCE FACTORS:
  1. The capsule is chief virulence determinant
  2. Anticapsular Ab’s protective, but polysacc Ag’s are T-cell independent, so short-lived immunity

Question 27

What is the current vaccine for H. flu?

Answer 27

• Hib: type b capsular polysaccharide (polyribosylribitol phosphate, or PRP) conjugated to carrier protein (diphtheria toxoid)
• Given to children b/t 2 and 15 mos
• NOTE: this is important bc Hib was MCC of bacterial meningitis in kids under 5 before 1989 (pretty high morbidity and mortality) -> overall, risk for Hib invasive disease in <5 group was 0.5% (about the same risk for poliomyelitis in 1950’s), but has now declined >98%
  1. First vaccines were T-cell-independent and ineffective in kids <18 mos bc only purified PRP

Question 28

This graph shows mean age of incidence and Ab devo for Hib (in the past). Why is there an early spike in the titer?

Answer 28

Early spike and low incidence due to maternal Ab’s

Question 29

What are the key structural and clinical features of Listeria monocytogenes?

Answer 29

• G+ rod (doesn’t stain well): facultative, IC parasite
• Causes listeriosis: most infections asymptomatic or produce mild influenza-like symptoms; less commonly, diarrhea and abdominal discomfort
• Cell-mediated immunity required to combat infection bc intracellular:
  1. Most serious disease (meningitis, bacteremia) can occur in immune-compromised adults, pregnant women, and infants
  2. Incidence in AIDS pts estimated 300x that in the general population
  3. Neonatal and puerperal (after childbirth) infections associated w/vaginal colonization
  4. Intrauterine infection often kills fetus

Question 30

Where do ppl get Listeria?

Answer 30

• L. monocytogenes is widespread among animals in nature, incl. chickens and sheep
  1. Human reservoir is intestinal colonization (2-12%) and vaginal colonization
• Food-borne transmission also important: epidemics have been assoc w/contaminated dairy, meat, raw veggies
• Killed by pasteurization, but grows at refrigerator temps: concentration in contaminated products can amplify during cold storage

Question 31

What is the pathogenesis (incl. virulence factors) of Listeria?

Answer 31

• Attaches to and invades animal cells, incl. epi cells and macros:
  1. Cytokine activation of macros necessary to kill the bug
  2. Endocytosis into non-phagocytic host cells mediated by bac protein, internalin, which binds E-cadherin expressed on epi cell surface (1) -> released from phagolysosome via listeriolysin O (2) -> F-actin based motility (3) -> invasion of adjacent cell (4) -> bac replication (5) -> cell death
• Org then spreads directly cell-to-cell, avoiding exposure to humoral immune system