Medical Microbiology I - Lecture9

MEDICAL MICROBIOLOGY I

Lesson 9

Streptococcus and Diseases

Streptococcus

The genus Streptococcus is a diverse collection of Gram positive cocci typically arranged in pairs or chains

Most species are facultative anaerobes, and Most species are facultative anaerobes, and some grow only in an atmosphere enhanced with carbon dioxide (capnophilic growth)

Their nutrient requirements are complex, necessitating the use of blood- or serum-enriched media for isolation

Streptococcus

Streptococcus

Carbohydrates are fermented, resulting in the

production of lactic acid

Catalase negative

Most -haemolytic strains and some - Most -haemolytic strains and some -

haemolytic and non-haemolytic strains

possess group-specific antigens, most of which

are cell wall carbohydrates

Streptococcus

These antigens can be readily detected by immunologic assays and have been useful for the rapid identification of some streptococcal pathogens

Most -haemolytic and non-haemolytic streptococci do not possess the group-specific cell wall antigens

These organisms must be identified from their physiologic properties

Streptococcus pyogenes

2 species of streptococci are classified in group A:

S. pyogenes (flesh-eating bacteria)

S. angionosus

S. pyogenes is the most common pathogen S. pyogenes is the most common pathogen

S. pyogenes is an important cause of a variety of suppurative and non-suppurative diseases

They are the most common cause of bacterial pharyngitis, these organisms have become notorious because they can cause dramatic, life-threatening diseases


1. Physiology and Structure

0.5 - 1.0 m, spherical cocci that form short chains in clinical specimens and longer chains when grown in liquid media

Growth is optimal on enriched blood agar media but it is inhibited if the medium contain high concentration of glucose

After 24 hrs incubation, 1 - 2 mm white

colonies with large zones of -haemolysis are observed


Encapsulated strains may appear mucoid on

freshly prepared media but wrinkled on dry

media

Non-encapsulated colonies are smaller and Non-encapsulated colonies are smaller and

glossy

Cell wall: peptidoglycan (gram positive); within

the cell wall are group-specific and type-

specific antigens


2. Group-Specific Carbohydrates

The group specific carbohydrate, which

constituents approximately 10% of the dry

weight of the cell, is a dimer of N-weight of the cell, is a dimer of N-

acetylglucosamine and rhamnose

This antigen is used to classify group A

streptococci and distinguish them from other

streptococcal groups


3. Type-Specific Proteins

The M protein is a major type-specific protein

associated with virulent streptococci

M proteins are subdivided into class I and class M proteins are subdivided into class I and class

II molecules

The type I M proteins have the constant (C)

exposed, whereas antibodies do not develop

against the C region of class II M proteins


This appears to be important for patients who

develop rheumatic fever, because their disease

is mediated by strains with the class I M

proteinsproteins

A secondary type-specific protein that is useful

epidemiologic marker for bacterial strains that

fail to express the M protein is the t protein

(trypsin-resistant)


4. Other Surface Components

M-like proteins, lipoteichoic acid and F-protein

The M-like proteins are encoded by a complex

of more than 20 genes that comprises the of more than 20 genes that comprises the

emm gene superfamily

These genes are responsible for M proteins, M-

like proteins, and other immunoglobulin-

binding proteins


Lipoteichoic acid and F protein facilitate binding

of host cells by complexing with fibronectin,

which is present on the host cell surface

Capsule-composed of hyaluronic acid Capsule-composed of hyaluronic acid

containing repeating molecules of glucuronic

acid and N-acetylglucosamine; prevent

phagocytosis by providing physical barrier

between the opsonic complement proteins

bound to the bacterial surface and phagocytic

cells


5. Pathogenesis and Immunity

The virulence of group A streptococci is determined by the ability of the bacteria to adhere to the surface of host cells, invade into the epithelial cells, avoid opsonisation and the epithelial cells, avoid opsonisation and phagocytosis, and produce a variety of toxins and enzymes

More than 10 different bacterial antigens have been demonstrated to mediate adherence to host cells, with lipoteichoic acid, M proteins, and F protein the most important, as well as other antigen


The internalisation is believed to be important for maintenance of persistent infections (e.g.recurrent streptococcal pharyngitis) as well as invasion into deep tissues

Streptococcus pyogenes has multiple Streptococcus pyogenes has multiple mechanisms for avoiding opsonisation and phagocytosis

M-related proteins interfere with phagocytosis, peptidase block chemotaxis of neutrophils and mononuclear phagocytes


6. Pyrogenic exotoxins

The streptococcal pyrogenic exotoxins (Spes), originally called erythrogenic toxins are produced by lysogenic strains of streptococci and are similar to the toxin produced in and are similar to the toxin produced in Corynebacterium diphtheriae

3 immunologically distinct heat-labile toxins (SpeA, SpeB and SpeC) have been described in S. pyogenes and in rare strains of groups C and G of streptococci


The toxins act as superantigens, interacting

with both macrophages and helper T cells with

the release of interleukin-1 (IL-1), IL-2, and IL-

6, tumour necrosis factor- (TNF- ) and TNK-6, tumour necrosis factor- (TNF- ) and TNK-

and interferon-

These cytokines mediate a variety of important

effects, including the shock and the organ

failure seen characteristically inpatients with

streptococcal toxic shock syndrome


7. Streptolysin S and O

Streptolysin S is an oxygen-stable, non-

immunogenic, cell-bound haemolysin that can

lyse erythrocytes, leukocytes and plateletslyse erythrocytes, leukocytes and platelets

Streptolysin S is produced in the presence of

serum (serum dependence) and is responsible

for the characteristic -haemolysis seen on

blood agar media


Streptolysin O is oxygen-labile haemolysin

capable of lysing erythrocytes, leukocytes,

platelets, and cultured cells

Antibodies are readily formed against Antibodies are readily formed against

streptolysin O, a feature differentiating it from

streptolysin S, and are useful for documenting

recent group A infection (ASO test)

Streptolysin O is irreversibly inhibited by

cholesterol in skin lipids


8. Streptokinases

2 forms: streptokinase A and B

These enzymes can lyse blood clots and may

be responsible for the rapid spread of be responsible for the rapid spread of

Streptococcus pyogenes in infected tissues

Anti-streptokinase antibodies are also a useful

marker for infection


9. Deoxyribonucleases

4 immunologically distinct DNases: A to D

These enzymes are not cytolytic but can depolymerise free DNA present in pus

This process reduces the viscosity of the This process reduces the viscosity of the abscess material and facilitates spread of the organisms

Antibodies developed against DNase B are an important marker of cutaneous Streptococcus pyogenes infections


10.C5a Peptidase

Complement component C5a mediates

inflammation by recruiting and activating

phagocytic cellsphagocytic cells

C5a peptidase disrupts this process by

degrading C5a

Clinical Diseases

1. Pharyngitis

This develops 2 - 4 days after exposure to the pathogen, with an abrupt onset of sore throat, fever, malaise, and headache

The posterior pharynx can appear The posterior pharynx can appear erythematous with an exudate, and cervical lymphadenopathy can be prominent

50% of patients with strep throat have pharyngeal or tonsilar exudates

Pharyngitis Strep Throat

Clinical Diseases

2. Scarlet Fever

A complication of streptococcal pharyngitis that occurs when the infecting strain is lyosogenised by a temperate bacteriophage that stimulate the production of a pyrogenic that stimulate the production of a pyrogenic exotoxin

Within 1 to 2 days after the initial clinical symptoms of pharyngitis develop, a diffuse erythematous rash initially appears on the upper chest and then spreads to the extremities

Scarlet Fever

Clinical Diseases

The area around the mouth is generally

spared, as are the palms and sole

A yellowish white coating initially covers the

tongue and is later shed, revealing a red, raw tongue and is later shed, revealing a red, raw

surface beneath (strawberry tongue)

The rash disappears over the next 5 - 7 days

and is followed by desquamation

Clinical Diseases

3. Pyoderma (Impetigo)

A confined, purulent (pyo) infection of the skin (derma) that primarily affects exposed areas (e.g. face, arms, legs)

Through direct contact Through direct contact

Vesicles develop and then become pustules (pus-filled vesicles), which then rupture and crust over

The regional lymph nodes can become enlarged, but systemic signs of infection (e.g.fever, sepsis) are uncommon

Pyoderma

Clinical Diseases

Pyoderma is seen primarily in young children

(2 - 5 years old) with poor personal hygiene

and occurs primarily in warm, moist summer

monthsmonths

The strains of streptococci that cause skin

infections are different from those that cause

pharyngitis, although pyoderma serotypes can

colonise the pharynx and establish a persistent

carriage state

Clinical Diseases

4. Erysipelas

An acute infection of the skin

Patients experience local pain and

inflammation (erythema, warmth), lymph inflammation (erythema, warmth), lymph

node enlargement, and systemic signs (chill,

fever, leukocytosis)

The involved skin area is typically raise and

distinctly differentiated from the uninvolved

skin

Erysipelas

Clinical Diseases

5. Cellulitis

Typically involves the skin and deeper

subcutaneous tissue, and the distinction

between the infected and non-infected skin is between the infected and non-infected skin is

not clear

Precise identification of the offending

organism is necessary because many different

microbes can cause cellulitis

Cellulitis

Clinical Diseases

6. Necrotising Fasciitis

An infection that occurs deep in the subcutaneous tissue, spreads along the fascial planes and is characterised by an extensive destruction of muscle and fatdestruction of muscle and fat

The organism is introduced into the tissue through a break in the skin

Initially there is evidence of cellulitis, after which bullae form and gangrene, and other systemic symptoms develop

Necrotising Fasciitis

Clinical Diseases

Systemic toxicity, multi-organ failure and death

(>50%) are the hallmarks of this disease

Fasciitis must be treated aggressively with the

surgical debridement of non-viable tissuesurgical debridement of non-viable tissue

Clinical Diseases

7. Streptococcal Toxic Shock Syndrome

Most patients initially experience soft tissue inflammation at the site of the infection and pain as well as non-specific symptoms, such as fever, chills, malaise, nausea, vomiting and diarrhoeachills, malaise, nausea, vomiting and diarrhoea

The pain intensifies as the disease progresses to shock and organ failure

Patients with streptococcal disease are bacteremic, and most have necrotising fasciitis

Susceptible: HIV, diabetic, cancer, heart or pulmonary disease and varicella-zoster viral infection patients

Clinical Diseases

8. Other Disease

I. Suppurative

Puerperal sepsis

Lymphangitis Lymphangitis

Pneumonia

II. Non-suppurative

Rheumatic fever

Acute glomerulonephritis

Lymphangitis

Laboratory Diagnosis

Microscopic: Gram stain

Antigen detection: immunologic test using

antibodies that react with the group-specific

carbohydrate in the bacterial cell wall, e.g. carbohydrate in the bacterial cell wall, e.g.

enzyme immunoassay (EIA)

Culture: haemolysis on blood agar media

Identification: bacitracin

Antibody detection: ASO test, anti-DNase test

Bacitracin Test

Streptococcus agalactiae

Streptococcus agalactiae is the only species

that carries the group B antigen

This organism was initially recognised as a

cause of puerperal sepsis, later known as an cause of puerperal sepsis, later known as an

important cause of septicaemia, pneumonia,

and meningitis in newborn children, as well as

a cause of serious disease in adults



Gram positive cocci (0.6 - 1.2 m) that form

short chains in clinical specimen and longer

chains in culture

They grow well in nutritionally enriched media

and in contrast with a narrow zone of -

haemolysis

Some strains (1-2%) are non-haemolytic,

although their prevalence may be

underestimated because non-haemolytic strains

are not screened for the group B antigen


Strains of S. agalactiae can be subdivided on

the basis of the 3 serologic markers:

The B antigen or group-specific cell wall

polysaccharide antigen (composed of polysaccharide antigen (composed of

rhamnose, N-acetylglucosamine, and galactose)

Type-specific capsular polysaccharides (Ia, Ib, II

to VIII)

The surface protein, C protein



Antibodies developed against the type-specific

capsular antigens of group B streptococci are

protective, a factor that partly explains the protective, a factor that partly explains the

predilection of this organism for neonates

Genital colonisation with group B streptococci

has been associated with increased risk of

premature delivery

Group B streptococci produce several enzymes,

including DNases, hyaluronidase, neuraminidase,

protease, hippurase, and haemolysins

Clinical Diseases

Early-Onset Neonatal Disease

Bacteremia, pneumonia, or meningitis

Late-Onset Neonatal Disease

Acquired from an exogenous source (e.g. mother, another

infant). Bacteremia with meningitis

Infections in Pregnant Women

Bacteremia; secondary complication: endocarditis,

meningitis, and osteomyelitis, are rare

Infections in Men and Non-Pregnant Women

Bacteremia, pneumonia, bone and joint infections, and skin

and soft tissue infections; mostly among

immunocompromised person

Streptococcus pneumoniae


Encapsulated

Older cells decolorise readily and appear Gram

negativenegative

Colonies of encapsulated strains are generally

large (1 - 3 mm) on blood agar; smaller on

chocolatised or heated blood agar, round and

mucoid; colonies of non-encapsulated strains are

smaller and appear flat


The choline is unique to the cell wall of

Streptococcus pneumoniae and plays an important

regulatory role in cell wall hydrolysis

Choline must be present for activity of the Choline must be present for activity of the

pneumococcal autolysin, amidase, during cell

division

2 forms of teichoic acid: one exposed on the cell

surface and similar form covalently bound to the

plasma membrane lipids


The lipid bound teichoic acid in the bacterial

cytoplasmic membrane is called F antigen because

it can cross-react with the Forssman surface

antigens on mammalian cells



Colonisation and migration

Surface protein adhesions and secretory IgA proteases

and pneumolysin (cytotoxin similar to streptolysin O)and pneumolysin (cytotoxin similar to streptolysin O)

Tissue destruction

Pneumolysin, hydrogen peroxide and

phosphorylcholine in the bacterial cell wall

Phagocytic survival

Capsule


3. Clinical Diseases

Pneumonia

Sinusitis and Otitis media

Meningitis Meningitis

Bacteremia

Otitis Media

Documents

Medical Microbiology I - Lecture9