Module 11: Streptococcus (Alpha Hemolytic)

Question 1

Streptococcus pneumoniae cellular morphology

Answer 1

alpha hemolytic
gram pos cocci
1um
cells tend to be elongated
typically seen in pairs; singles and chains also seen
Could be mistaken for rods (more evident if pt treated with antibiotic that inhibits cell wall formation)
Capsules usually present in exudate and fresh cultures
No flagella or spores

Gram pos diplococci are very diagnostic for S. pneumoniae

Question 2

S. pneumoniae growth requirements

Answer 2

facultative with some strains requiring increase CO2 for primary isolation
35degC
Medium enriched with blood or seum

Question 3

S. pneumoniae colonial morphology

Answer 3

wet, glistening
surrounded by a narrow zone of alpha hemolysis
1-2mm colonies
mucoid colonies measuring up to 5mm may be seen
Surface of colony may show varying degrees of flattening depending on age of colony
overnight = convex
48hr = life saver
2-3day= flat
Flattening of colony is due to autolysis of cells due to lytic action of H2O2 that accumulates as the bacteria grow in air
Gram stains from old colonies show cell debris and bacteria may stain gram neg (subcultures may fail to grow)

Question 4

bad choice of medium for isolation of S. pneumoniae

Answer 4

chocolate agar
colonies are small and autolyse rapidly
catalase present in blood agar decomposes H2O2 and delays autolysis

Question 5

Blood cultures and S. pneumoniae

Answer 5

grows rapidly in blood culture media
subcultures must be done as soon as growth is detected
If delayed, bacteria autolyse rapidly and growth may not be obtained on subculture plate

Question 6

Genus ID of S. pneumoniae

Answer 6

seldom requires any test procedures
Most alpha hem colonies that are gram pos can be assumed to belong to Streptococcus
Catalase and Nitrate neg but this testing is not routinely necessary

Question 7

Species ID of S. pneumoniae

Answer 7

must be carried out to differentiate S. pneumoniae from S. viridans
Group D strep and Enterococci may also be alpha hemolytic but colonial morphology of S. pneumoniae is very different from them

Bile solubility
Optochin Susceptibility
Antigen ID

Question 8

Bile Solubility

Answer 8

S. pneumoniae is bile soluble (dissolves)
S. viridans, other strep and enterococci are INSOLUBLE

Colonies older than 24 hrs more likely to be insoluble in bile

Question 9

Optochin Susceptibility

Answer 9

S. pneumoniae is susceptible
Other Strep are resistant
Requires overnight incubation
Alpha Hem strep that are susceptible to optochin may be reported as S. pneumoniae

greater than/equal to 14mm = sensitive, S. pneumoniae
Less than 14mm = resistant, S. viridans

Question 10

Antigen ID

Answer 10

S. pneumoniae does not have streptococcal group antigens

Both latex and co-agglutination reagents are available for detection of pneumococcal antigen in body fluids (work best with Spinal fluid or from blood culture broth

S. penumoniae has capsular antigens that may be ID’d by capsular swelling reactions following exposure to homologous antisera

Question 11

S. pneumoniae antimicrobial susceptibility

Answer 11

Penicillin
Some resistant strains out there
Screening for resistant strains by disc diffusion methods (1ug oxacillin disc, MH agar with5% blood)
Incubate in CO2
Zone of 20mm and greater indicate susceptibility to penicillin
Smaller zones indicate resistance

Question 12

Is beta lactamase testing required for S. pneumoniae

Answer 12

no, resistance to penicillin is not related to beta lactamase

Question 13

Clinical significance of S. pneumoniae

Answer 13

found in upper respiratory tract of some healthy individuals
Most common cause of Adult Lobar Pneumonia
Bacteria come from endogenous source (viral respiratory infection and alcoholism contributing to development of pneumonia)
Pneumonia has sudden onset and accompanying septicemia
Some patients have severe diarrhea (indicates bad prognosis)
Otitis media (middle ear infection), sinusitis, conjunctivitis, meningitis are complications

Question 14

Of 3 bacteria responsible for purulent menigitis, which is least common

Answer 14

S. pneumoniae, Haemophilus influenzae, Neisseria meningitidis

S. pneumoniae least common

Question 15

S. pneumoniae infections, WBC count in spinal fluid

Answer 15

2000-6000 x10^6/L
Dominantly neutrophils
Elongated gram pos cocci usually seen in gram stain of sediment of spinal fluid
Direct testing of spinal fluid for Pneumonococcal antigen with latex or co-agglutination reagent helpful to confirm diagnosis

Question 16

Streptococcus viridans

Answer 16

most are alpha hemolytic
“viridans” = green
Includes:
Alpha hem Strep that are not S. pneumoniae, Group D strep or enterococci (bile esculin, groupd D antigen, bile solubility and optochin all neg)

Non hemolytic strep that do not have Group D or B antigens (bile esculin, group B and D antigens all neg)
*The occasional strain of Group B is nonhemolytic

Question 17

Streptococcus viridans cellular morphology

Answer 17

Gram pos cocci
1um
chains formed in broth
smears from solid media often show pleomorphic forms that may be club-shaped resembling diphtheroids
Direct smears from upper respiratory tract often show chains of elongated cocci adhering to epi cells

No spores, flagella or capsules

Question 18

S. viridans growth requirements

Answer 18

facultative organisms that grow best on enriched media such as blood agar
35degC
increased CO2 not required

Question 19

S. viridans colonial morphology

Answer 19

usually seen as normal flora in upper respiratory specimens
overnight colonies 0.5mm
Surrounded by small zone of alpha hem
May be flat or concave
Other strains may be nonhemolytic

Question 20

Differentiating S. pneumonia from S. viridans

Answer 20

Viridans: bile insoluble
Optochin resistant

pneumoniae: Bile soluble
Optochin susceptible

Question 21

S. viridans vs Group D with bile esculin and PYR tests

Answer 21

viridans: Bile esculin neg
PYR neg

Group D: Bile esculin pos
PYR pos

Question 22

Antibiotic susceptibility of S. viridans

Answer 22

should be performed on all viridans isolates causing systemic infections
Majority are susceptible to penicillin

Question 23

Clinical significance of S. viridans

Answer 23

mostly normal flora

Major cause of Subacute bacterial endocarditis
Infections are endogenous and predisposed by damaged or scarred heart valves
Could start from dental work