Module 11: Streptococcus (Intro and Group A)

Question 1

3 members of the family Streptococcaceae

Answer 1

Streptococcus
Enterococcus
Aerococcus

Question 2

Aerococcus viridans

Answer 2

opportunistic organisms from environment

rarely causes infection in humans

Question 3

Streptococcus and Enterococcus

Answer 3

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”

Question 4

Streptococci ID’d by Group Antigens: Beta Hemolytic

Answer 4

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)

Question 5

Enterococcus Group (Group D Enterococcus)

Answer 5

Non hemolytic

Enterococcus faecalis (some strains are VRE)
Enterococcus faecium (some strains are VRE)
Enterococcus durans

Question 6

Non Enterococcus group (Group D Non-Enterococcus)

Answer 6

Streptococcus equinus

Streptococcus bovis

Question 7

Streptococci not identified by group antigens: Alpha hemolytic

Answer 7

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

Question 8

Nutritionally Variant Streptococcus

Answer 8

may be grouped with Streptococcus viridans but differ in that thiol compound (cysteine) or Vit B (pyridoxal) is required for growth

Question 9

Streptococcus pyogenes (Group A Streptococcus) Cellular morphology

Answer 9

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

Question 10

Streptococcus pyogenes (Group A Streptococcus) Growth requirements

Answer 10

Facultative with most strains growing as well or better in anaerobic conditions
Increased CO2 not required
35degC
Medium enriched with blood or serum

Question 11

Streptococcus pyogenes (Group A Streptococcus) Cultural characteristics

Answer 11

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

Question 12

Streptococcus pyogenes (Group A Streptococcus) Hemolysis due to

Answer 12

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

Question 13

Streptococcus pyogenes (Group A Streptococcus) Genus ID

Answer 13

Gram pos cocci
tend to form chains
Catalase neg
Nitrate neg
S. pyogenes recognized by colonial morphology; no genus test required for typical isolates

Question 14

Presumptive ID of Streptococcus pyogenes (group A)

Answer 14

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

Question 15

Definitive ID of Streptococcus pyogenes (Group A)

Answer 15

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)

Question 16

Direct detection of Group A Antigen from Throat swabs

Answer 16

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

Question 17

GAS-D test (DNA probe for Group A Streptococcus)

Answer 17

4hr test
Large volume tested at once
Lyse bacteria off swabs, amplify DNA
Patients can be treated within 24hrs of having swab taken

Question 18

S. pyogenes antimicrobial susceptibility

Answer 18

Penicillin: universally susceptible
Erythromycin: susceptible with rare resistant isolate
Tetracycline: usually susceptible but some resistant straints

Question 19

Group A Streptococcus Infections (S. pyogenes) 4 categories

Answer 19

Upper respiratory infections
Cutaneous infections
Invasive infections
Post-Stretococcal diseases

Question 20

Group A Streptococcus Infections (S. pyogenes): Upper respiratory infections

Answer 20

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

Question 21

Group A Streptococcus Infections (S. pyogenes): Cutaneous Infections

Answer 21

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

Question 22

Group A Streptococcus Infections (S. pyogenes): Invasive infections

Answer 22

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

Question 23

Group A Streptococcus Infections (S. pyogenes): Post-streptococcal diseases

Answer 23

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

Question 24

What percentage of people carry S. pyogenes in respiratory tract

Answer 24

5-15%

Question 25

Clinical significance of Beta Hem. Group C and F

Answer 25

several species are animal pathogens
Cause pharyngitis, pneumonia, septicemia, meningitis, oral infections, abscesses of CNS, liver and appendix, neonatal sepsis

Question 26

Clinical significance of beta Hem. Group G

Answer 26

May infect animals, cause pharyngitis, otitis media (middle ear infection), neonatal sepsis, endocarditis, meningitis