micro sheet # 12

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micro sheet # 12

Post by Shadi Jarrar on 19/10/2010, 2:44 am

بسم الله الرحمن الرحيم

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Gram positive cocci : the Staphylococci

The stapyhococci are the most important member of the family Micrococcae

1. Planococcus species
2. Micrococcus species
3. Stomatococcus mucilaginous
4. Staphylococcus species
The first 3 cause disease only in immune compromised patients (due to cancer ,surgeries..etc) and they are responsible for oral cavities infections.

Charecteristics :

Staphylococci derive their name due to being in clusters(from the greek work stapyhl)
They can survive even in hostile environment.
The most common staphylococcal infections are skin conditions .
And less commonly they can cause systemic infections like endocarditis and osteomyelitis.

They are non-motile organisms ,lacking flagella
They are facultative anaerobes :they survive with or without the prescence of oxygen.

The basic test to distinguish between Stapyhlococci and Streptococci is catalse production.
The family Micrococcae are strong catalse producers which breaks down hydrogen peroxide into oxygen and water.
They are halophils as they can grow in 10% NaCl which is a very high concentration.
They can grow at low or high temperature ( 18 C to 40) .
Staphylococci can produce an intertoxin which can cause food poisoning within 4 hours of food consumption.
This toxin is resistant to heat ,it can withstand boiling for 30 minutes
They are normally present on human skin and cause no harm except if the continuity of the skin is traumatized or lacerated ,that’s why the continuity of the skin is very important.
Eg.if aspetic techniques are not used during surgery ,staphylococci will enter the body and causes infection.

The species included under the genus satph. There are 32 species that belongs to it. 16 of which have been linked with human diseases, the most important one is Staphyloccocus aureus.
We can also divide the staphyloccoci into 2 major groups according to ability to form the enzyme coagulase, which coagulates plasma,it converts fibrinogen into fibrin which results in forming a clot.
This clot will surround the organism and will protect it (the body won’t attack itself ) ;that’s why Staphycocci can survive in hostile environment because they can form self component to protect themselves.
This protection is temporary,because when they spread , they produce a substance that lyse the clot and they spread.
So coagulase production is taken as a basis to clinically classify the Stapylococci in to coagulase +ve and coagulase –ve .

Stapylococcus aureus is coagulase +ve, while remaining 31 species are coagulase –ve.

Structure
Its outer layer is a capsule (slime, meaning excessive amounts of polysaccharide is produced )these polysaccharide allow the staph. To attach to surfaces especially artificial surfaces like protethis & artificial heart valves.

Teichonic acid is a compenent of gram +ve cell wall, it is utilised to attach to fibronectin ( a basic component of the connective tissue) so Staphylococci establish contact with the connective tissue by binding to fibronectin.


The last structure of the cell wall is the clumping factor which is the bound form of coagulase enzyme
*Coagulase enzyme is of 2 types :
a) Bound (present only in the most staph. Aurus ) it binds fribrinogen converting it to fibrin causing the staph. To clump.
b) Released /free
The clumping factor can be demonstrated on slides ,just add plasma and mix it with Staphylococcus aureus we’ll observe a clot.
But the non-bound form is demonstrated in tubes ,we add Staph. to the tube containing plasma , then the tube is incubated for 4 hours or overnight, we’ll observe a clot.

Now talking about coagulase –ve and +ve staph. :

*Coagulase +ve : Staphylococcus aureus as we’ve mentioned before, the name (Aureus) because it produce golden yellow pigments that are produced around the colonies of Staph. growing in media especially if growing in room temperature which is the optimum temperature to produce these pigments.

*Coagulase –ve : the most important one is Staphylococcus epidermidis (called so because it is abundant in the skin) it was called alba in the past because it forms white colonies.

The other one is Staph.saprophyticus which causes UTI (urinary tract infection) that is common in sexually active young females.
And there are other more than16 pathogenic species under the coagulase –ve category.

Staphylococcus aureus
They usually live on the skin of the nose,anterior nares,and mucous membranes.
among those we have the MRSA (Methicillin Resistant Staphylococcus aureus) they are habitants of the skin and mucous membrane, they can cause minor (skin and soft tissues infections ) or major infections like blood and surgical infections.

The vast majority of skin infections are due to Staph.cocci although some other organisms like gram –ve and others can also cause them.
They can be isolated from towels ,pens ..,and anything used by human carriers.
This can cause infection as a result of poor hygeine ,nutrition ,immune deficiency disease ,diabetes.



Pathogenesis of Staphylococci

It is a result of 2 factors : toxiginicity (the ability to produce toxins) and the 2nd factor is invasiveness (ability to spread)
Both of them measure virulence.

*Toxins include :
1- Cytotoxins: membrane damaging subtances .
Alpha,beta,gamma,delta and leukocidin
(The toxin sometimes is an enzyme)
The alpha toxin destroys the membranes of RBC’s,WBC’s,platelets and hepatocytes. (so it can damage all types of blood cells)

Neutrophils are rich in granules containing hydrolytic enzymes,when these neutrophils are destroyed they pour they contents to the surrounding tissues which cause necrosis.

The beta toxin which is sphingomyelinase C ( a component of membrane phosolipids)
It affects RBC’s,WBC’s,macrophages and fibroblasts.

**Cytotoxins are either chromosomal or plasmid encoded ,it means they can be transferred by transection,conjugation is not an important mechanism in gram +ve but transection is the transfer of charesteristics via a virus ( bacteriophage).
That’s why some organisms that don’t produce these toxins may become producers if they’re infected with a bacteriophage that carried the gene responsible.

2-Exfoliative toxin
Arrises from the epidermis , they can lyse the outermost layer of the skin (epidermis) .
It is a seline protease that has 2 distinct forms A and B.
They act by splitting the intercellular bridges (desmosomes),this will start at the stratum granulosum without cytolosis or inflammation.
This toxin causes scaled skin syndrome usually affecting young children.
The skin is an effective barrier to infection because the outer layer is basically dead cells but if the continuity is broken then the organisms can penetrate the skin.

Anti bodies that are produced in infected individuals are protective ,patients who had scaled skin syndrome become immune to the stray that produces the disease.

3-Toxic shock syndrome Toxin 1 (we have another toxic shock produced by streptococci)

This toxin is common among females, it can be lethal.

4- Enterotoxins
They are 6 distinct types (A-F)
They are heat stable (resist boiling for more than 30 minutes)
Resistance to hydrolysis by gastric and jejunal enzymes.
The action of this toxin is central, it acts on the thermoregulatory center of the brain.
And it also acts on the center that causes vomiting.
Severe vomiting is an important manifstation due to this toxin and to less extent diarrhea and abdominal pain.

Enterotoxins A is most commonly associated with this disease.
Enterotoxins B is associated with pseudomembranais colitis.
Enterotoxins C and D commonly contaminated milk products.

*Coagulase -ve staph may produce enterotoxins so enterotoxins production is not limited to staph. Aureus (coagulase +ve) but all other products are limited to staph. Aureus .

**Staphylococci produce enzymes that enable them to invade into tissues which contributes to their invasiveness ,these enzymes are:

1-Catalase : breaks hydrogen peroxide producing water,they are present in aerobic environment & they can withstand the prescence of oxygen and the toxic metabolites that are produced as a consequence of the prescence of oxygen.

2-Coagulase (bound and free) : which reacts with plasma factor globulin known as the coagulase reacting factor to form staphylothrombin which is a thrombin-like substance that converts fibrinogen to fibrin.

3- hyaluronidase : digest hyaluronic acid which is the
basic component of connective tissue and that’s how connective tissue damage is induced.

4- Fibinolysin (staphylokinase): splits fibrin ,at one point of the organism’s life,it needs fibrin to protect it but once the danger is gone it needs to spread ,therefore needs an enzyme to lyse the clot.

5-Lipases to split lipids
6-Nucleases to split nucleic acids

*Staphylococci movement can be impeded by lipids and nucleic acids so they split both of them to decrease the viscousity and facilitate the spreading .

7-Beta lactamase: the enzyme responsible for the hydrolysis of Penicillin.

**Diseases caused by Staphylococci are divided into 2 major categories : those that are due to the toxin like food poisoning, toxic shock syndrome and scaled skin syndrome
& those that are due to the organism itself like local infections of the skin,deep absecess and cystic infections.

First we’ll talk about the diseases caused by the toxin:

1- Scales skin syndrome
It has an abrubt onset starting with localizes perioral erythema that spreads to cover the entire body.

If you apply pressure on their skin it will be displaced and this is know as nikolsky sign due to the damage to the desmosomes.
Large bullae or cutaneous blisters can form followed by desquamation of the epithelium.
Lesions are culture –ve.
The recovery takes place without scaring within 7-10 days associated with antibody production against the toxin
There are forms of localized scalded skin syndrome known as bollous impetigo is recognizes and unlike systemic form it is culture +ve.

2-Toxic shock syndrome

In the past, it was a disease of menstrauting women associated with the use of highly absorbant tampons (contraceptive devices)
So Staphylococci grow on these tampons .
Nowadays,Toxic shock syndrome follows surgical wonuds infectionc and therefore occurs in men as well as in women.

Symptoms: fever,hypertension, diffuse erythematous rash with multiple organ dysfunction.

Later on desquamation involves the entire the skin including palms and soles.
This disease has a high risk of reoccurence ( 65%) pateints may have a relapse and this is fatal.

3-Food poisoning

Follows the ingestion of enterotoxin in improperly refrigerated food eg Ham,custard,pastries,potato salad and icecream
They don’t involve heating , therefore the organism grow.
The incubation period lasts for 4 hours then symptoms appear which include severe vomiting,diarrhea and abdominal pain.

4-Cotaneous infection

In the form of abscess :collection of pus in the skin as well as the mucous membrane (in dentistry ,oral abscess are very common and represent an emergency)
Abscess in the skin are called boils,medically called furuncles
When they are mutiple and connected they are called carbuncles.
The abscess is a limited collection of pus and is well defined ,has a very high consistancy whereas the spread of infection is called cellulitis where there is no distinction between infected and non-infected skin and it occupies a large area.

Impetigo and stye are associated with poor hygeine , they are localized ,usually itches ( that’s why children can infect themselves by scratching other non-infected areas)

*Stye: infection of the eyelid and because it’s a localizes collection of pus it needs drainage.

Also surgical wounds can be infected.
Hair follicles infections are also very common.

The most common types of HOSPITAL AQUIRED INFECTIONS
are through blood stream(catheters), surgical site infections, UTI
and pneumonia.

Systemic infections with Staphylococci can be the subsequence of oubvious focus which can spread to the blood stream or the bone in osteomyelitis .
In other cases there is no oubvious focal infection like endocarditis or brain abscess (following dental procedures).

Diseases can be associated with predisposing factors like mutiple abscessed ,IV drug users,septicemia and staphylococcal pneumonia

Viral infections like influenza predisposes for Staphylococcal infections.

Osteomyelitis and septic arthritis
Either caused indirectly through the blood stream , or directly through trauma eg .car accident( broken bones and open wounds)
They usually involve the metaphyseal area with sudden localized pain and fever.

Blood culture +ve in 50% of cases.
This could involve the vertebrae and vertebral collapse can take place.
*septic arthritis results from intrarticular injections or infection of abnormal bones.
Eg. Patiens with rhematoid arthritis are given injection and if the aseptic technique is not used ,this can introduce the organims inside the joints.


Bacteremia and Endocarditis
The focus is unkown in 30% of cases.
Accute endocarditis is fatal in about 50 % of cases.
Hospital acquired in more than 50 % of cases.
This is a blood stream infection with organisms that usually results in what’s know as SIRS (severe inflammatory response syndrome) that is the cause of sepsis and death.
The infection can involve the lung in Empyema which is due to aspiration of oral secretions or hematogenous spread from a distant site.


Goodluck 

Sarah Waia
Lecture # 12
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Shadi Jarrar
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عدد المساهمات : 997
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تاريخ التسجيل : 2009-08-28
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الموقع : Amman-Jordan

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