Encapsulated Bacterial Pathogens Flashcards
What are the basic bacteriological features (e.g., Gram positive vs. Gram negative etc) of Haemophilus influenzae, Neisseria meningitidis and Streptococcus pneumoniae?
H. influenza:
- Small Gram-negative bacillus or “coccobacillus”
- Requires NAD = V factor Heme = X factor
N. meningitidis:
- Gram-negative diplococcus,
- Coffee bean shape in contrast to the lancet shaped S. pneumoniae.
- Antisera against the capsules are also available to determine the serogroup of N. meningitidis. No FDA approved vaccine against the serogroup B strains
S. pneumoniae:
- Gram-positive lancet shaped diplococcus
- Grows on blood agar.
- Alpha (a) hemolysis on blood agar indistinguishable from other alpha hemolytic streptococci, which are frequently designated as “viridans” streptococci.
How are the organisms isolated and identified?
Isolation and identification of organism -
- Culture via standard laboratory media (e.g., blood agar, chocolate agar).
- Detection of capsular polysaccharides with antiserum
- Isolation and identification of the serotype or serogroup of the organism is an important aspect of diagnosis, but that may not always be possible.
What are three common problems associated with laboratory diagnosis?
- Treatment with antibiotics prior to a culture being taken
- Difficult to determine the difference between mere colonization and infection. A sufficient laboratory diagnosis is when these organisms are found in a normally sterile site (e.g. blood or spinal fluid) in a patient. However, if these organisms are isolated from a site contaminated by normal flora (e.g. upper respiratory tract) this does NOT provide any laboratory proof that these organisms are causing disease because they can be considered to be “NORMAL FLORA” in these sites.
- Culture difficult to obtain: Also in many cases a culture may be difficult to obtain and is not routinely done because of trauma necessary to obtain a proper culture (e.g., puncture of tympanic membrane). Many times a diagnosis may be made on clinical grounds alone (e.g. otitis media).
What is an immunological method for detecting capsular polysaccharide?
Antiserum:
it may be possible to detect the capsular material in body fluids with antiserum directed at the specific capsular polysaccharide without being able to culture the organism.
What is the most critical virulence factor in encapsulated bacteria and how does it function?
The polysaccharide capsule confers resistance to phagocytosis which allows the organism to cause invasive disease.
The polysaccharide capsule is the most critical virulence factor necessary for these organisms to product invasive disease. If the organism (H. influenzae, S. pneumoniae or N. meningitidis) has a capsule and the host lacks specific antibody against the capsule then the organism would be resistant to phagocytosis and could survive in the lung or bloodstream and produce systemic diseases such as pneumonia, sepsis or meningitis. If these organisms do not produce a capsule then, they are either avirulent or they tend to produce only focal infections (e.g., otitis media, sinusitis) or infections in debilitated patients where trauma has produced a link between sinuses and brain or, in neonates (sepsis).
How many serotypes exist for H. influenza, what are the most virulent and what vaccines exist?
6 serotypes
Designated by the lower case letters a through f.
Serotype b were the ones that most frequently cause invasive disease (more than 20 X more frequently than all other serotypes together).
Wide availability of a vaccine against serotype b
Serotype a is now the most predominant.
How many serotypes exist for S. pneumoniae, what are the most virulent and what vaccines exist?
More than 90 different capsular serotypes.
Only 12 types which cause >80-90% of infections.
Some of the capsules or the teichoic acid of S. pneumoniae will cross react with H. influenzae type b capsule, and cross-reactions may lead to a misdiagnosis of an H. influenzae or S. pneumoniae infection.
No vaccines noted
How many serotypes exist for N. menigitidis, what are the most virulent and what vaccines exist?
- 9 different serotypes (usually called serogroups for historical reasons) designated A,B,D,C,X,Y,Z,W135 and 29E.
- Organisms in groups A, B and C are responsible for the great majority of clinically recognized diseases.
- The type B polysaccharide is composed of a polymer of N-acetyl neuraminic acid. This is medically important because unlike the other capsular groups, group B capsule is non-immunogenic in humans because N-acetyl neuraminic acid, also called sialic acid, is extensively found in humans (e.g. brains, kidneys).
This has prevented development of a polysaccharide capsular vaccine against this group (see below Immunology and Vaccines). - Vaccines against group A and C
Which organisms produce IgA protease, and what does it do?
H. influenza
S. pneumoniae
N. menigitidis
IgA protease cleaves human IgA at the hinge region of the antibody molecule. All these pathogens initiate infection at mucosal surfaces where IgA is the only kind of antibody available to block adherence of these organisms to mucosal surfaces so that they can initiate infection.
What other toxin do H. influenza and N. menigitidis produce and how does it manifest?
LPS
Actually called LOS (liao-oligosaccharides) in these organisms.
Manifestations that are very likely mediated by LOS include: disseminated intravascular coagulation (DIC)
septic shock
What is the most severe disease caused by encapsulated bacteria?
Meningitis
Bacterial meningitis is more severe than viral and may result in brain damage, hearing loss, learning disability or death. Streptococcus pneumoniae and Neisseria meningitidis are the leading causes of bacterial meningitis due to vaccination against H. flu type b.
An increased incidence of disease is found in persons who do not have anti-capsular antibody or who have some predisposing condition (e.g. asplenic-no spleen, as from trauma).
What diseases are caused by H. influenza?
Common diseases caused by H. influenzae type b in decreasing order are: pneumonia, bacteremia, cellulitis, epiglottitis. Most of these diseases were found in children and were relatively uncommon in persons over 7 yrs of age. Systemic disease in adults from unencapsulated strains (e.g., meningitis, pneumonia) has been increasing in recent years. Greatly decreased incidence of meningitis due to vaccine.
What age groups are affected by N. meningitidis?
Most common cause of bacterial meningitis in the 6 mo - 6 yr. age group and in young adults (i.e. high school and college age).
What are the age groups affected by S. pneumoniae and what other diseases does it cause?
- Most common cause of bacterial meningitis in all adults. It is usually preceded by a symptomatic infection of the upper respiratory tract. In young children increased incidence is related to lack of specific capsular antibody.
- Otitis media (middle ear infection) in children (50% of all cases).
- Pneumonia
Why do vaccines only reduce the incidence of serious invasive disease but not the incidence of related but non-invasive diseases?
Vaccines are only effective against the capsule polysaccharides, and unencapsulated organisms are responsible for the highest incidence of disease. Thus, vaccines confer protection only from the encapsulated bacteria which cause the most severe illnesses. Unencapsulated bacteria evade this immunity and continue to cause non-invasive disease (eg. otitis media)
