Neisseria

Gram-negative diplococci - "kidney bean" appearance

aerobic to facultative anaerobes

nonmotile

nonspore-forming

outer membranes: KDO, core, and short sugar chains - lipooligosaccharide (LOS)

not facultative intracellular parasites

Two pathogenic species: N. meningitidis (meningococcus) and N. gonorrhoeae (gonococcus). These organisms cause pyogenic infections. Closely related spp, but produce very different diseases.

Neisseria grow best in an atmosphere containing 5 - 10% CO2 on enriched media. The gonococcus grows more slowly and is more fastidious than the meningococcus. Both grow best on chocolate agar at 30 to 37C. N. gonorrhoeae and N. meningitidis are exceptionally sensitive to unfavorable environmental conditions. They die out in a few days especially at 4C.

 

Nonpathogenic Neisseria spp. - not fastidious and can grow at temps of 22 to 25C. All species of Neisseria parasitize the mucous membranes of humans. Nonpathogenic Neisseria spp. are almost always present in specimens obtained from the oro- and nasopharynx.

 

 

There are no animal models that closely mimic human infection with pathogenic Neisseria.

Organ cultures have been used to study mechanisms of disease pathogenesis. Pathogenic Neisseriae can invade human mucosal tissue, e.g. fallopian tube organ cultures.

 

Steps in pathogenesis for pathogenic Neisseria:

a. attach via pili to microvilli of nonciliated columnar epithelial cells. Even in vitro, bacterial adherence only occurs to human cells. Results in ciliostasis and damage to ciliated cells; they slough off.

b. extensive membrane to membrane contact (mediated by bacterial opacity proteins - OMPs [outer membrane proteins]).

c. epithelial cells endocytose the bacteria - "parasite-directed endocytosis." This process is initiated by microbial factors (it does not occur unless the organisms are viable, and it involves cells that are not normally phagocytic).

d. bacteria are transported within vesicles to the basement membrane.

e. Organisms are then egested into the submucosal space. In subepithelial mucosa, there is a prompt infiltration of pmns (inflammatory rxn). Mucosa undergoes destruction, and the neisseria and pmns accumulate in subepithelial connective tissue.

f. Bacteria may enter lymphatics or blood vessels to cause disseminated disease.

 

With the meningococcus, mucosal invasion occurs in the nasopharynx, and the infection may be asymptomatic. Suggests some immunity in the host.

With the gonococcus, the most common site of infection is the genitourinary epithelium. Most gonococcal infections are symptomatic. No immunity.

 

 

N. meningitidis

The human nasopharynx is the only known reservoir of N. meningitidis. The organism can exist as an apparently harmless member of the normal flora or produce acute disease.

Carrier rates of meningococci in the upper respiratory tract: 3-30% in healthy adults and children. Higher colonization rates (up to 90%) are found in certain closed populations such as military recruit camps and boarding schools.

The carrier state occurs when the person has specific antibodies and is immune to invasive disease.

Transmission: by respiratory droplets; requires close contact.

If the meningococcus comes in contact with an individual without adequate immunity, disease may ensue.

Clinical disease: divided into 3 clinical entities: nasopharyngitis, bacteremia, and meningitis.

 

Nasopharyngitis (infection of the nasopharynx) - usually short-lived and frequently symptomless. May be a mild febrile infection with pharyngitis.

In a susceptible individual, the organism may cross the mucosal barrier and enter the bloodstream. Bacteremia is characterized by fever and prostration. Often a petechial eruption develops on the skin. The infection may be accompanied by disseminated intravascular coagulation and gram-negative shock. It may be swiftly fatal or it may be followed by acute meningitis. The mortality rate of meningococcal meningitis is approximately 85% without treatment. With antibiotic and supportive therapy, mortality ranges from 1 to 10%. Meningococcal meningitis has the potential of occurring in epidemic form.

Note: 84% of reported cases of bacterial (septic) meningitis in patients over 1 month old are caused by N. meningitidis, S. pneumoniae, and Haemophilus influenzae, all three are encapsulated, do not elaborate exotoxins, and normally colonize the nasopharynx.

 

 

Virulence factors that enable the meningococcus to initiate an infection include:

a. capsule - antiphagocytic; mediates resistance to complement-mediated lysis.

13 capsular serogroups. A, B, C, H, I, K, L, X, Y, Z, 29E, W135.

Most imp: A, B, C, Y, W135

Serogroups B, C, Y. and W135 all contain sialic acid (neuraminic acid) as part of the CP. Approximately 90% of illness is caused by serogroups A, B, and C. There is a vaccine composed of the A, C, Y, and W-135 polysaccharide antigens.

Serogroup B capsule (a-2,8-linked sialic acid homopolymer) is not immunogenic because it resembles sialogangliosides and sialopeptides present within the host. Chemically and serologically identical to E. coli K1 capsule.

Interestingly, N. meningitidis also express increased capsule under low iron and low pH conditions, similar to those that might be encountered in the host.

 

b. invasion - a mechanism by which the meningococcus is transported within host cell vesicles to the submucosa (see above);

c. pili mediate attachment to nonciliated columnar cells of the nasopharynx. Pili present a highly variable adhesin on the surface of N. meningitidis. The ability of these bacteria to switch reversibly between a piliated and nonpiliated state is crucial in allowing the pathogen to adsorb to and desorb from epithelial and endothelial surfaces

However, pili have also been reported for nonpathogenic Neisseria spp.

"Necessary, but not sufficient" to cause disease.

  1. Toxic effects of LOS. No repeating O-antigenic side chain. N. meningitidis produces excess outer membrane fragments that are released into the extracellular space.

e. IgA1 protease that cleaves secretory antibodies; it is specific for human IgA and not found in nonpathogenic Neisseria spp.

e. Iron uptake - N. gonorrhoeae and N. meningitidis produce no known siderophores.

Both species assimilate iron by direct interaction with transferrin or lactoferrin (main mechanism), interaction with heme-containing proteins, and uptake of aerobactin siderophores produced by other microbes.

Transferrin and lactoferrin are mammalian glycoproteins that function in absorption, transport and exchange of iron. Transferrin is found in blood, and lactoferrin in found intracellularly in neutrophils and in secretions.

Iron acquisition from transferrin and lactoferrin requires direct contact between these molecules and the organism. When grown under iron restriction, N. gonorrhoeae and N. meningitidis synthesize specific outer membrane receptors that specifically bind transferrin, lactoferrin, or hemin.

 

 

Treatment: Bacterial meningitis is acute, life threatening and must be treated before the agent is identified. Once Haemophilus is ruled out, the treatment of choice is penicillin administered intravenously. Alternatives are Cm and 3rd generation cephalosporins (ceftaxime, ceftriaxone).

Close household contacts of patients with meningococcal disease are at a much higher risk (500- to 800-fold) than the general population. To prevent secondary cases in household contacts, chemoprophylaxis is often given. Rifampicin is currently the drug of choice.

Prevention

Antibodies, present in most adults, play a major role in preventing meningococcal disease.

Serum antibodies to: capsule, LOS and outer membrane proteins protect against meningococcal disease. Protective antibody is usually acquired through subclinical or overt infection and through the carrier state, which produces immunity within a few weeks.

 

The peak incidence of serious infection is between 6 months and 2 years of age, which corresponds to the time between loss of transplacental antibody and the appearance of naturally acquired antibody (see figure).

 

The incidence of the disease in the US today is very low. Although a vaccine is available, it is used only for epidemic control.

The vaccine consists of the purified group A, C, Y and W135 capsular polysaccharides. It is nontoxic and highly effective in preventing meningococcal disease in military and civilian populations. It is given as a single injection to adults and children >2 years of age. The vaccine does not eliminate nasopharyngeal colonization.

In other parts of the world ("meningitis belt" of sub-Saharan Africa) the disease is more prevalent and a vaccine is needed. Conjugate vaccines in which the capsular polysaccharide is coupled to a carrier protein (such as tetanus toxoid) are being actively explored. Similarly, outer-membrane vaccines are being considered since antibodies directed against outer membrane proteins promotes complement-mediate bactericidal killing of encapsulated strains.

 

Functional abs:

1. opsonic (mediate phagocytosis by PMNs). Meningococci are killed by PMNs.

2. bactericidal. GNís (but not Gps) can be lysed by Ab + Cí. This activity usually restricts the meningococcus to mucosal surfaces (prevents systemic infection). Severe, chronic, or recurrent Neisseria infections occur in persons deficient in C5, C6, C7, C8, or C9.

 

 

 

 

N. gonorrhoeae

 

aerobic

more fastidious than N. meningitidis.

not normal inhabitants of the respiratory or genital flora

High incidence: ~2 million cases a year in the US.

 

Clinical: Gonococcal infections are characterized by an inflammatory response that results in a purulent discharge, the hallmark of the disease.

>90% of infections in men are symptomatic, and the infection is usually restricted to the urethra. Patient experiences burning on urination and a yellow purulent urethral discharge: urethritis.

In women, the primary site of infection is the cervix. The woman experiences burning or frequent urination, vaginal discharge, fever, and abdominal pain. About 30-50% of women with cervical infections are asymptomatic. This is a problem because it represents an important reservoir of the disease because it goes untreated.

As many as 15% of women with uncomplicated cervical infections develop an ascending infection of the uterus and fallopian tubes: causing acute pelvic inflammatory disease. This can cause scarring of the oviducts resulting in infertility or ectopic pregnancies.

In 1 to 3% of infected men or women, the gonococcus invades the bloodstream, leading to disseminated gonococcal infection. Strains of gc that cause cases of DGI differ from other gonococci in possessing a greater resistance to the bactericidal effects of normal human serum. The bacteremia is usually followed by skin lesions and/or arthritis.

 

Gonococcal ophthalmic neonatorum (blindness in the newborn) is acquired by an infant exposed to infected secretions in the birth canal of the mother. This disease was formerly a common cause of blindness, and for this reason eye prophylaxis (silver nitrate solution, or erythromycin or tetracycline ointments) is used routinely at birth, and thus the disease has almost disappeared.

 

 

Virulence factors:

a. pili. Only piliated strains attach to various (nonciliated) cells of susceptible tissues and only piliated strains produce infection in volunteers. Many different pilin types. GC can genetically switch types and also turn pili expression on/off.

b. PII outer membrane protein (OMP) is an adhesin that is subject to phase-(on/off) and antigenic variation (change antigenicity; alter antigen types).

c. parasite-mediated endocytosis - the gonococci attach to the epithelial cells of the cervix, are endocytosed, transported within vacuoles to the base of the cell and released to the subepithelial tissues. Within epithelial cells, the gonococcus is protected from the phagocytic activities of leukocytes.

d. Iron uptake mechanisms - transferrin, lactoferrin binding proteins

e. IgA protease - only found in pathogenic neisseriae. IgA (mostly in mucosal secretions) is important in mucosal defense, blocking bacterial adherence.

f. Gonococcal LOS (no repeating O-antigenic side chain) is responsible for tissue destruction and chronic inflammation and is thought to be responsible for most of the symptoms of gonorrhea.

g. Gonococci do not produce a capsule, but they can sialylate their LOS.

Pathogenic neisseria can put terminal sialic acid residues on their LOS in vivo. However, GC does not make CMP-sialic acid, so this does not occur in vitro. CMP-sialic acid is provided by the host.

Most strains of gonococci isolated from urethral exudates are sensitive to the killing action of normal human serum after growth on laboratory media. However, some gonococci become resistant to serum killing following incubation in vitro with CMP-sialic acid.

GC sialyl transferase

GC + host CMP-sialic acid ---------------------------> GC LOS-sialic acid

N. gonorrhoeae, N. meningitidis and most strains of N. lactamica produce a sialyl transferase with a high affinity for CMP-sialic acid. The CMP-sialic acid donates sialyl groups presumably to a terminal galactose residue on gonococcal LOS. Thus, sialic acid becomes incorporated into its LOS. It is different from the sialyl transferase that is involved in capsule biosynthesis by N. meningitidis. Non-pathogenic neisseria lack this enzyme.

Sialic acid on the LOS makes the microbes more resistant to the killing action of human serum. Experimentally, sialylation of LOS has been shown to occur on gonococci incubated with human PMN's and on gonococci present within PMN's from male urethral exudates. This reflects the availability of sufficient CMP-sialic acid in the environment of the urethral epithelium and associated neutrophils and at this phase of the infectious process to produce sialylated LOS.

Presumably, the sialylated LOS shows antigenic similarity to antigens present on human erythrocytes. This similarity to self may preclude an effective immune response to these LOS antigens.

h. Antigenic variation/Phase variation

Bacteria use surface structures, e.g., pili and certain outer membrane proteins (OMPs), for adherence to and colonization of host mucosal cells. Vaccine strategy: use these surface proteins to elicit a sIgA response to prevent adherence and colonization. However, the gonococcus varies its surface ags (and turn them on/off) so that the original IgA response becomes obsolete.

Alteration of surface structures is an important mechanism used by pathogens for avoiding host defense mechanisms. Change antigens on the bacterial surface - escape neutralizing antibodies produced by the host. Antigenic variation also allows the bacterium to alter its binding specificities to reflect the changing environment experienced by the organism as it invades the mucosa and encounters phagocytes.

Vaccine trials to prevent gonorrhea have met with little success. Development of an effective vaccine has also been hampered by the lack of a suitable animal model for gonococcal disease.

Antigenic variation in gc: pili, OMPs, porin proteins.

2 functions of agic variation:

1. evasion of host ab response

2. alteration of binding specificity to reflect changing environment experienced by the bacteria as they invade mucosa and encounter phagocytes.

Phase variation (on/off): pili, LOS core sugars, opacity proteins (these are outer membrane proteins [opa or PII] that allow the tight adherence of gonococci to mammalian cells)

GC surface mosaic: pili +/-, opa +/-, LOS core variation +/- sialic acid.

 

Spread: The bacteria are fragile and do not survive in the environment; spread occurs only by direct contact.

Factors that predispose to the epidemic proportions of gonorrhea: high degree of transmissibility; short incubation period; high rate of asymptomatic carriers; lack of protective immunity; increasing resistance to antibiotics.

 

Diagnosis: smear of urethral, cervical, or rectal specimens

cultivation on rich selective media

no serologic tests

 

Treatment: Penicillin used to be the drug of choice. Penicillin resistance has become more prevalent now in many parts of the world since plasmids coding for beta-lactamase appeared in gonococci. In the presence of penicillin resistance, the recommended antimicrobial agent is ceftriaxone (third generation cephalosporin). Tetracycline or erythromycin are also effective and may be added to treat frequently concomitant infections with chlamydia.

 

Prevention. The fact that people who are cured of gonorrhea can be reinfected suggests that infection does not elicit a protective host response (tough bug to eradicate by immunization!).

Antibody responses against the gonococcus are directed toward pili, outer membrane proteins and lipooligosaccharide (LOS). However, these antigens shift from one antigenic form to another so frequently that each gonococcal strain appears distinct from others. Immunity to the gonococcus is usually not achieved due to the antigenic variability of the gonococcus.

condoms - prevent transmission