Flashcards in encapsulated pathogens Deck (34)
Hemophilus influenzae structure
This is a small Gram-negative bacillus, which may appear as a coccus hence, the term coccobacillus is frequently used to describe this organism
H. flu growth requirements
NAD = V factor Heme = X factor. Grows on Chocolate Agar which has these factors added to it. Can also grow near Staph on blood agar b/c staph provides NAD
compare the growth requirements of H. flu, H. ducreyi and H. parainfluenza
H flu: requires factor X and V. H. ducreyi: requires Factor X only. H. parainfluenza: requires factor V only
Strep pneumoniae structure
Gram-positive diplococcus that readily grows on blood agar. Lancet shaped-each side of the diplococcus is pointed. Produces alpha hemolysis. Will see capsule as a ring around the diplococcus
How to distinguish Strep viridians vs strep pneumonia
Both are alpha hemolytic. OVRPS: optochin- viridian are resistant, pneumonia are sensitive
N meningitidis structure
Gram-negative diplococcus, which is very similar morphologically to Neisseria gonorrhoeae.
lab diagnosis of encapsulated bacteria
specific tests frequently involve the detection of capsular polysaccharides with antiserum
Problems with lab diagnosis
1. if pt has been treated with Abx, use immunological methods to diagnose. 2. Must determine 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. 3. Culture may be difficult to obtain
Immunological diagnosis of encapsulated bacterial infection
The organism may shed its capsule in the blood, urine or spinal fluid. , it may be possible to detect the capsular material in these body fluids with antiserum directed at the specific capsular polysaccharide without being able to culture the organism.
Most important virulence factor for encapsulated bacteria
The polysaccharide capsule (composed of polymers of different kinds of sugars) is the most critical virulence factor necessary for these organisms to produce invasive disease. 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).
h flu: total number of serotypes, serotype that causes invasive disease, most predominant serotype
6 serotypes a-f. serotype b is most invasive. Serotype a is most common because of the vaccine against serotype b.
What is unique about h flu serotype b's polysaccharide capsule
The same polysaccharide found in serotype b is found in almost all Gram-positive organisms (in the teichoic acids). these other bacteria probably play a role in the ultimate development a protective immune response against H. influenzae. This can lead to a misdiagnosis of H flu or S pneumonia due to cross reactivity btw their antisera
s. pneumoniae: total number of serotypes
90 serotypes. 12 types cause most infections. Some of the capsules or the teichoic acid of S. pneumoniae will cross react with H. influenzae type b capsule
N. meningitidis: number of serotypes, unique capsules
9 serotypes A,B,D,C,X,Y,Z,W135 and 29E. A-C are most common. The type B polysaccharide is composed of a polymer of N-acetyl neuraminic acid and 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 vaccine against this group. Also, the group B capsule is identical to the K1, capsule of E. coli an organism, which causes meningitis in neonates (0-6 months).
Which bacteria produce IgA protease and what is its function
H. flu, S. pneumoniae, N. meningitidis and N. gonorrhea. This enzyme specifically cleaves human IgA, at the hinge region of the antibody molecule. 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. By cleaving IgA, they are able to initiate infection better
Which encapsulated organisms produce LPS and what does it do
H. flu, N. meningitidis (gram negatives). LPS is a virulence factor which can cause DIC and septic shock
encapsulated pathogens epidemiology
H. flu, N. meningitidis and s. pneumonia can all be found in upper respiratory tract of normal people. With H. influenzae and N. meningitidis most of these strains from normal populations are unencapsulated. spread by droplet contact
compare viral vs bacterial meningitis
Viral meningitis is generally less severe and most of the time resolves without specific treatment, while bacterial meningitis is quite severe and may result in brain damage, hearing loss, learning disability or death
Who is predisposed to encapsulated infections
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).
H. flu diseases
Meningitis, pneumonia, bacteremia, cellulitis, epiglottitis (usually in kids <7 yrs). Systemic dz in adults can occur from unencapsulated strains. Also otitis media.
N. meningitidis diseases
this is the 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)
S. pneumoniae diseases
most common cause of bacterial meningitis in all adults. It is the most serious pneumococcal infection with a case fatality rate of ~40%. It is usually preceded by a symptomatic infection of the upper respiratory tract.Can also cause pneumoniaand otitis media. In young children increased incidence is related to lack of specific capsular antibody.
most common cause of otitis media
The primary site of colonization for H. flu, S. pneumoniae and N. meningitidis
upper respiratory tract
Antibiotics used for H. flu, S. pneumoniae and N. meningitidis
H. flu: third generation cephalosporins. Chloramphenical is alternative (but is toxic). Most are resistant to ampicillin. N. meningitidis: Third and Fourth generation broad, spectrum cephalosporins are now frequently used. S. pneumoniae: Vancomycin only. Resistant to penicillin and multi-drug resistance is emerging.
Administration of rifampin or ciprofloxacin is given to people who have come in contact with other people diagnosed with meningitis caused by H. flu, S. pneumoniae and N. meningitidis
H. flu vaccine
For H flu serotype B: All of them have a polysaccharide linked to a carrier protein (diphtheria toxoid or tetanus toxoid) to make it T-cell dependent. Induces immune response in newborn infants. Is also present in Tetramune vaccine (DPT vaccine). It is recommended that the primary immunizations be administered at 2,4,6 and 15-18 months of age with a booster dose at 4-6 years of age
MCV4 or MenactraT. Includes serogroups C, Y, W-135 and A. A is most common in Africa.
Who should receive MCV4 vaccine
Children aged 2 months through 10 years who are at increased risk for meningococcal disease, Travelers to or residents of countries with high rates of meningococcal dz, kids with asplenia, All children 11-18 yrs, Entry to high school or college, microbiologists, military recruits.