sreptococci - prac. microbiology

Streptococci

Gram-positive cocci in chains

• Gram positive cocci arranged in chains

Planes of division

Catalase –veStreptococci

Catalase +veStaphylococci

Catalase Test

• Growth requires enriched media containing blood or serum.

Haemolysis on sheep blood agar can be used for

distinguishing between streptococci

Group Antigens

• Cell wall Carbohydrate antigens

• Lancefield group antigens A – W (except I & J)

Streptococcus pyogenes

Gram-positive cocci in chains

Beta hemolytic

Group A Streptococci

Virulence Factors

Pharyngitis

• Sore Throat

• Tonsillitis

Scarlet Fever

Skin & Soft Tissue Infections

• Impetigo– Lesions on

extremities– Commonly on face– Pustular and crusty

Skin & Soft Tissue InfectionsCellulitis & Erysipelas

Invasive streptococcal infections

• Puerperal fever• Acute endocarditis• Necrotizing fasciitis• Toxic shock syndrome

Necrotizing Fasciitis

Laboratory Diagnosis

• Specimen:– Throat swab– Pus– CSF– Blood

• Direct detection

– Gram-stained smear

– Detection of group A streptococcal antigen from throat swab.

S. pyogenes

• Culture:

Blood Agar

Throat swab

Pus

CSF

Culture on

Blood

Blood culture bottle

Culture on

subculture

• Identification :A. Colony morphology

B. Gram-stained smear

C. Catalase test negative

D. Bacitracin sensitivity test

E. Agglutination by group A antibody .

Viridans streptococci

Diseases:

- Subacute BacterialEndocarditis (SBE)

- Dental Caries

Laboratory diagnosis of SBE

Blood culture technique

Streptococcus pneumoniae

Gram positivecapsulated diplococci.

Alpha haemolytic .

Quellung test :

Pneumococcal diseases

Three major diseases:

• Pneumonia

• Meningitis

• Otitis media

Laboratory diagnosis

A-Specimens:– Sputum – CSF – Ear discharge – Blood

B- Direct detection :

1- Gram-stained smear 2- Quellung test 3- Detection of capsular polysaccharide antigen

in CSF by slide agglutination test

• Culture:

Blood Agar

Throat swab

Pus

CSF

Culture on

Blood

Blood culture bottle

Culture on

subculture

Identification :

1-Colony morphology

2-Gram-stained smear

3-Differentiation from viridans streptococci

A. Growth inhibition by optochin

B. Solubility of colonies in bile

C. Capsular polysaccharide antigen detection

D. Quellung reaction

E. DNA probe specific to S. pneumoniae

F. Virulence to mice

Populations at risk of being infected with S. pneumoniae

• Asplenic individuals

• infants

• Immunosuppressed individuals

• Alcoholics

So, how to protect them?

• A vaccine is available.– Capsular polysaccharide vaccine– Antigens from 23 serotypes.

• Under age of TWO years:– Pneumococcal conjugate vaccine

– Capsular polysaccharide + protein carrier

Enterococcus• Gram +ve cocci • singly / in pairs /short chains

Enterococcus

• Catalase negative• Hydrolyze esculin in presence of bile salts• Lancefield group D

Enterococcus Infections

• UTI• Intra-abdominal & pelvic wound infections• Bacteraemia• Endocarditis• Abscesses, meningitis, peritonitis, osteomyelitis &

wound infection

Case 1

A 10-year-old girl presents to herprimary care physician with :

o severe sore throat, o fever, o headache, nausea, and abdominal

pain.

Examination shows:The back of her throat is covered with a purulent exudate with white pus-filled nodules.

How can you reach a diagnosis?

Case (cont.)

A. Specimen

Throat swabs

Specimen

Direct detection:

- Gram-stained smear

Gram-positive cocci arranged in chains

amongst pus cells

B. Direct detection- Detection of group A streptococcal

antigen from throat swab.

Culture:

On blood agar: β- hemolysis

catalase negative Gram positive cocci

in chains

1.Gram-stained smear 2. Catalase test

Identification from colonies:

3.Bacitracin sensitivity test

Bacitracin sensitive

Group A (S. pyogenes)

What treatment would you offer her?

• Penicillin, or erythromycin if allergic to penicillin.

This patient may present afterwards with one of Post - Streptococcal sequelae.

What are they? And what is the underlying mechanism?

1. Rheumatic Fever2. Post-Streptococcal glomerulonephritis

Rheumatic Fever

Immune response against Strept. antigens similar to heart antigens.

Post-streptococcal glomerulonephritis

Deposition of Ag-Ab complexes, activation of complement.

Diagnosis of ARF

No single test is pathognomonic .

Diagnosis is based on modified Jones criteria:

a) Evidence of recent S .pyogenes infection

b) Two of the five major criteria ,or one major and

two minor criteria .

The evidence of recent streptococcal infection

a-History of acute tonsillitis .b-Elevation of ASO titre above 200 units .

N.B. In case of AGN:Anti-DNAse

Review Questions

1. The simplest test to differentiate staphylococci from streptococci is the:

a) Coagulase testb) Bacitracin testc) Gram staind) Catalase teste) Optochin test

2. Group A streptococcus growing on a blood agar plate, shows which of the following:

a) Alpha hemolysis, a clear zone surrounding the colonies

b) Alpha hemolysis, a green zone surrounding the colonies

c) Beta-hemolysis, a clear zone surrounding the colonies

d) Beta-hemolysis, a green zone surrounding the colonies

e) No haemolysis

3.Susceptibility to bacitracin is used to differentiate:

a) Streptococcus pneumoniae from Staphylococcus aureus.

b) Streptococcus pyogenes from Streptococcus pneumoniae.

c) Streptococcus pneumoniae from viridans streptococci. d) Streptococcus pyogenes from non group A beta

haemolytic streptococci. e) Streptococcus pyogenes from Staphylococc aureus

4. Streptococcus pyogenes can not be isolated in case of:

a) Pharyngitis.b) Puerperal fever.c) Acute endocarditis.d) Rheumatic fever.e) Necrotizing fasciitis.

5. The inhibition of growth of alpha-hemolytic colonies by optochin indicates:

a) Streptococcus pyogenesb) Viridans streptococcic) Streptococcus pneumoniaed) Staphylococcus aureuse) Enterococci

6. Blood culture is required for the diagnosis of:

a) Subacute bacterial endocarditisb) Pharyngitisc) Glomerulonephritisd) Rheumatic fevere) Scarlet fever

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