Assisst Proph. Dr.Ilham alsaedi
Staphylococci:
The genus Staphylococcus has at least 35 species.The three main species of clinical importance are Staphylococcus aureus,Staphylococcus epidermidis,and Staphylococcu saprophyticus.
General properties:
1-The Staphylococci are gram positive,spherical cells,usually arranged in grap like irregular clusters.
2-Nonmotile,nonsporeforming facultative anaerobe.
3-Staphylococci are catalase positive,oxidase positive,and coagulase positive.
4-Staphylococci grow readily on most bacteriologic media under aerobic or microaerophilic conditions.
5-They grow most rapidly at 37°C but form pigment best at room temperature (20-25°C).
6-Staphylococci ferment many types of carbohydrates with producing acid without gas.
Staphylococcus aureus forms a large yellow colony on rich medium,S.epidermidis has a relatively small white colony.S. aureus often haemolytic on blood agar; S.epidermidis is non hemolytic.
Staphylococcus aureus:
Cullular morphology:
1-Staphylococc are spherical cells about 1µm in diameter arranged in irregular clusters.
2-The cells usually arranged in grape-like irregular clusters when examind under microscope.
Cultural characteristics:
Colonies on solid media are round,smooth,raised,and glistening.S aureus usually forms gray to deep golden yellow colonies.
Antgenic structure:
1-Antigenic polysaccharide:Staphylococci contain antigenic poly saccharides and proteins as well as other substances important in cell wall structure.
2-Teichoic acids,of cell wall which mediate adherence of microorganism to mucosal cells.
3-Protein A is a cell wall component of many S.aureus.The protein A may interfere with phagocytosis(antiphagocytic factor) then protecting the organism from humoral immune clearance.
4-Clumping factor,S.aureus is produce two types of coagulase;bound and free.Bound coagulase is component of cell wall.It is binds to fibrinogen,thus causing the organism to aggregate in plasma.
Virulence factors:(Enzymes and Toxins)
Staphylococcus aureus can produce disease both through their ability to multiply and spread widely in tissues and through their production of many extracellular substances.Some of these substances are enzymes;others are considered to be toxins.
Enzymes:
1-Catalase:most aerobes and facultative bacteria that utilize oxygen in respiration pathway and produce hydrogen peroxide (H2O2) ,which is toxic to their own enzyme system.Their survival in presence of this anti-metabolite(H2O2) is possible because they produce catalase enzyme,which convert hydrogen peroxide into water and oxygen.
2-Coagulase and Clumping factor:Coagulase (free coagulase); the enzyme causes clot through activating prothrombin to form thrombin by coagulase-reacting factor CRF.
Thrombin then catalyzes the activation of fibrinogen to form fibrin clot around bacteria and prevent the bacteria from phagocytosis (antiphagocytic factor).
Clumping factor:is a surface S.aureus compound that is responsible for adherence of the organisms to fibrinogen and fibrin.When mixed with plasma,S. aureus forms clumps.Clumping factor is distinct from coagulase.
3-Other enzymes: Other enzymes produced by staphylococci include a hyaluronidase, or spreading factor;a staphylokinase resulting in fibrinolysis but acting much more slowly than streptokinase; proteinases;lipases;and ?-lactamase.
Toxins(Exotoxins):
1-Hemolysin: S.aureus produces at least four types of hemolysin known as alpha, beta, gamma and delta.Alpha hemolysin has important role in pathogensis of infection,especially in acute endocarditis .It is lecocidal , cytotoxic, dermonecrotic.
2-Leukocidin: This toxin of S.aureus has two components.It can kill white blood cells of humans such as neutrophils and macrophages.
3-Exofolative toxins: (ET) is extracellular product of many strain of S.aureus.The toxin consists of two proteins types (ET-A and ET-B).Exofolative toxin is epidermic toxin, it cleaves the stratum granulosum granulosum layer (desmosome) of epidermis and is responsible for several syndromes such as Staphylococcal scalded-skin syndrome (SSSS).
4-Enterotoxin:There are multiple (A-E, G-I, K-M) enterotoxins.Approximately 50% of S.aureus strains can produce one or more of them.The toxin is responsible for food poisoning.
5-Toxic shok syndrome toxin:Most S aureus strains isolated from patients with toxic shock syndrome produce a toxic shock produce a toxin called toxic shock syndrome toxin-1 (TSS-1),which is the same as enterotoxin F.The toxin binds to MHC-?? molecules on APC (Antigenic presenting cells) resulting non-specific T-cell stimulation of large number of T-cells of T-cells and production of cytokines that mediate shock and tissue injury.
Pathogenesis:
1-Pyogenic infections:such as
a-Skin infection:abscess,pyoderma (impetigo),boils.
b-Respiratory tract infection:tonsillitis,pharyngitis,pneumonia.
2-Toxigenic diseases:such as:
a-Food poisoning :due to staphylococcal enterotoxin is characterized by a short incubation period (1-8 hours)violent nausea, vomiting, and diarrhea; and rapid convalescence.There is no fever.
b-Toxic shock syndrome:characterized by high fever,vomiting,diarrhea,myalgias,a scarlatiniform rash,and hypotension with cardiac and renal failure in the most sever cases.It occurs within 5 days after the onset of menses in young women who use tampon,but it also occurs in children or in men with staphylococcal wound infections.
c-Exofolation:Staphylococci also cause disease through elaboration of toxins,without apparent invasive infection.Bullous exfoliation,the scalded skin syndrome,is caused by the production of exfoliative toxins.
Transmission:
S.aureus are members of the normal flora of the human skin and upper respiratory and gastrointestinal tracts.Nasal carriage of s.aureus occurs in 20-50% of humans.
Staphylococci are capable of extended survival on environmental surfaces and in air.
Transmission of the bacteria by inhalation of respiratory secretions or by consumption of contaminated food.