Module 11: Streptococcus (Intro and Group A) Flashcards
3 members of the family Streptococcaceae
Streptococcus
Enterococcus
Aerococcus
Aerococcus viridans
opportunistic organisms from environment
rarely causes infection in humans
Streptococcus and Enterococcus
Gram pos cocci
Form chains
Primarily produce lactic acid from carbohydrate fermentation
Catalase NEG
-Some Enterococcus produce small bubbles in H2O2 (pseudocatalase enzymes)
Many have group specific polysaccharide antigens in the cell wall that for basis for Lancefield Grouping
Antigens now routinely ID’d by other procedures referred to as “Group antigens”
Streptococci ID’d by Group Antigens: Beta Hemolytic
Group A: Most streptococci with cell wall antigen A are Streptococcus pyogenes
Group B: one species, Streptococcus agalactiae
Group C: Streptococcus equisimilis, most common isolate from humans
Group F and G: rarely clinically significant in humans
Group D: two physiological and genetically different groups that possess D antigens (Enterococcus and non Enterococcus)
Enterococcus Group (Group D Enterococcus)
Non hemolytic
Enterococcus faecalis (some strains are VRE) Enterococcus faecium (some strains are VRE) Enterococcus durans
Non Enterococcus group (Group D Non-Enterococcus)
Streptococcus equinus
Streptococcus bovis
Streptococci not identified by group antigens: Alpha hemolytic
Streptococcus pneumoniae
Streptococcus viridans
Streptococcus anginosis-milleri
- may have butterscotch smell
- may type in group A, C, F, G or none
- Appear as pinpoint colonies rather than large colony types of true groups A, C, G
Nutritionally Variant Streptococcus
may be grouped with Streptococcus viridans but differ in that thiol compound (cysteine) or Vit B (pyridoxal) is required for growth
Streptococcus pyogenes (Group A Streptococcus) Cellular morphology
Beta hemolytic
Gram pos cocci
0.5-1um
Tend to form chains (more than 6) in broth (can form singles,pairs,glusters)
May be more oval than spherical
Capsules usually not seen; no spores or flagella
Streptococcus pyogenes (Group A Streptococcus) Growth requirements
Facultative with most strains growing as well or better in anaerobic conditions
Increased CO2 not required
35degC
Medium enriched with blood or serum
Streptococcus pyogenes (Group A Streptococcus) Cultural characteristics
Granular appearance that breaks up on shaking
Overnight colonies: 0.5-1mm, gray to white, opaque to translucent; buttery or hard “matt” that push along agar surface or break apart
Beta hem
Streptococcus pyogenes (Group A Streptococcus) Hemolysis due to
One or both of the following:
Streptolysin S - OXYGEN STABLE but acid labile
May be inactivated by acid (produced when strep interact with carbs in medium)
Streptolysin O- OXYGEN LABILE, may be inactivated by exposure to O2
*Both blood agar base and incubation atmosphere both affect hemolysis
Streptococcus pyogenes (Group A Streptococcus) Genus ID
Gram pos cocci tend to form chains Catalase neg Nitrate neg S. pyogenes recognized by colonial morphology; no genus test required for typical isolates
Presumptive ID of Streptococcus pyogenes (group A)
Bacitracin:
S. pyogenes 99% susceptible to low concentrations of bacitracin while most other beta-hems are resistant
6% of other beta hems are susceptible to bacitracin
PYR:
nearly 100% of S. pyogenes are pos while other beta hem strep (except enterococci) are neg
Must use pure culture
Some coag neg staph are PYR pos
Definitive ID of Streptococcus pyogenes (Group A)
detection of specific cell wall group A antigen
Group A-G antigens of Strep are carbohydrate residues attached to peptidoglycan of cell wall
(Group D antigen is made of teichoic acid attached beneath cell wall)
Direct detection of Group A Antigen from Throat swabs
latex and coagglutination Enzyme immunoassay methods Results in 10 min 65-90% sensitive (bad!) Ideally collect 2 swabs together First = direct detection of antigen Second: if previous test is neg, send to lab for culture
GAS-D test (DNA probe for Group A Streptococcus)
4hr test
Large volume tested at once
Lyse bacteria off swabs, amplify DNA
Patients can be treated within 24hrs of having swab taken
S. pyogenes antimicrobial susceptibility
Penicillin: universally susceptible
Erythromycin: susceptible with rare resistant isolate
Tetracycline: usually susceptible but some resistant straints
Group A Streptococcus Infections (S. pyogenes) 4 categories
Upper respiratory infections
Cutaneous infections
Invasive infections
Post-Stretococcal diseases
Group A Streptococcus Infections (S. pyogenes): Upper respiratory infections
Throat
Acute pharyngitis:
Strep throat: sudden onset of sore throat, swollen lymph glands, fever, headache
Complications: ear/sinus infections, rarely meningitis
When S.pyogenes produces pryogenic exotoxin (fever producing) patient will have scarlet fever unless protected by antibody from previous exposure. Pharyngitis accompanied by rash, Strawberry tongue
Group A Streptococcus Infections (S. pyogenes): Cutaneous Infections
pustular lesions, impetigo
Erysipelas: when strep infection affects underlying dermis
Skin shows reddish patches that enlarge, thicken and swell at the margins
Usually preceded by infection such as pharyngitis
Occasionally result in septicemia
Group A Streptococcus Infections (S. pyogenes): Invasive infections
Numerous cases since 1990 often in young people
Typically, initial wound through which bacteria gain entry to deeper tissue
“flesh eating” disease, necrotizing fasciitis
Symptoms:
Cellulitis with pain, fever, septic shock with decreased blood pressure, respiratory distress, necrotizing fasciitis, renal dysfunction, liver dysfunction
Mortality rate may be 30-50%
Pen G usually given and inclusion of erythromycin is though to help patient recovery
Group A Streptococcus Infections (S. pyogenes): Post-streptococcal diseases
Rheumatic fever: may follow streptococcal pharyngitis 1-5wks. M antigen in cell wall have similar antigenic determinant to those of myosin in heart muscle
Streptococcal antibodies attach to heart causing inflammatory reacting and scarring of heart valves
Acute Glomerulonephritis: may follow strep pharyngitis or cutaneous infection. Antigen-antibody complexes are deposited on the glomerular membranes of kidney; activates complement (proteins in blood) resulting in damage to membrane; membrane allows blood and protein to pass into urine
Antistreptolysin O (ASO) is most frequently tested for; high level indicates recent infection and possible post-streptococcal disease
What percentage of people carry S. pyogenes in respiratory tract
5-15%
Clinical significance of Beta Hem. Group C and F
several species are animal pathogens
Cause pharyngitis, pneumonia, septicemia, meningitis, oral infections, abscesses of CNS, liver and appendix, neonatal sepsis
Clinical significance of beta Hem. Group G
May infect animals, cause pharyngitis, otitis media (middle ear infection), neonatal sepsis, endocarditis, meningitis
