normal flora and staphlococci

شNormal microflora
In a healthy human, the internal tissues, e.g. blood, brain, muscle, etc., are normally free of microorganisms. However, the surface tissues, i.e., skin and mucous membranes, are constantly in contact with environmental organisms and become readily colonized by various microbial species which called normal microflora, or normal microbiota these Organisms do not cause disease Most organisms among the normal microflora are
commensals—they obtain nutrients from host secretions, waste substances found on the surfaces of skin and mucous membranes. Two categories of organisms can be distinguished: resident microflora and transient microflora
The resident flora consists of relatively fixed types of microorganisms regularly found in a given area at a given age; if disturbed, it promptly reestablishes itself.
The transient flora consists of nonpathogenic or potentially pathogenic microorganisms that inhabit the skin or mucous membranes for hours, days, or weeks; it is derived from the environment, does not produce disease, and does not establish itself permanently on the surface. if the resident flora is disturbed, transient microorganisms may colonize, proliferate, and produce disease These organisms are called opportunists. There are many condition lead to create opportunistic microorganisms like
1. Failure of the Host’s Normal Defenses. Individuals with weakened immune defenses are said to be immunocompromised. Factors such as advanced malnutrition, the presence of another disease, advanced or very young age, treatment with radiation or immunosuppressive drugs, and physical or mental stress can lead to this state. The failure of host defenses in AIDS patients, for example, allows several different opportunistic infections to develop.
2. Entrance of the Organisms into Unusual Body Sites. The bacterium Escherichia coli is a normal resident of the human large intestine, but it can cause disease if it gains entrance to unusual sites such as the urinary tract, surgical wounds, or burns.

3-Disturbances in the Normal Microflora. The population of normal microflora compete with pathogenic organisms and in some instances actively combat their growth, an effect known as microbial antagonism. When potential pathogens gain a competitive advantage due to diminished of microfloa after antibiotic therapy like C. difficile grow in the gut and cause enterogasteritis
Beneficial Functions of Normal Flora
1.Protect our organs and systems that are in direct contact with the external environment from invading pathogens. Some normal flora produce substances that kills pathogens and others compete for with them for nutrients.
2.In newborns, normal flora stimulates the development of immune system.
3.Normal flora of the gut provides important nutrients such as Vitamin K which aid in digestion and absorption of nutrients.


Staphylococcus
Staphylococci are Gram-positive cocci occurring in clusters. They can be
cultured on normal nutrient mediums both aerobically and anaerobically.
The most important species is S. aureus. A number of extracellular enzymes and exotoxins such as coagulase, alphatoxin, leukocidin, exfoliatins, enterotoxins, and toxic shock toxin are responsible for the clinical symptoms of infections by this pathogen, which are observed in the three types invasive infections, pure toxicoses, and mixed forms. The antibiotics of choice for therapy of these infections are penicillinase-resistant penicillins. Laboratory diagnosis involves identification of the pathogen by means of microscopy and culturing. S. aureus is a frequent pathogen in nosocomial infections and limited outbreaks in hospitals. Hand washing by medical staff is the most important prophylactic measure in hospitals. Coagulase-negative staphylococci are classic opportunists. S. epidermidis and other species are frequent agents in foreign body infections due to their ability to form biofilms on the surfaces of inert objects. S. saprophyticus is responsible for between 10 and 20% of acute urinary tract infections in young women.
General characteristics
Staphylococci are small spherical cells (1 lm) found in grapelike clusters.
Staphylococci are nonmotile, catalase-producing bacteria. The genus Staphylococcus includes over 30 species and subspecies. Table -2 briefly summarizes the characteristics of those most important in the medical context. S. aureus (and E. coli) are among the most frequent causal organisms in human bacterial infections.



species parameter
S. aureus Coagulase-positive; colonies golden yellow. Local purulent
infections: furuncles, carbuncles, bullous impetigo, wound
infections, sinusitis, otitis media, mastitis puerperalis, ostitis,
postinfluenza pneumonia, sepsis. Toxin-caused illnesses: food
poisoning, dermatitis exfoliativa, toxic shock syndrome
S. epidermidis Coagulase-negative; sensitive to novobiocin; most frequent CNS*
pathogen; opportunist; infection requires host predisposition;
foreign body infections with discrete clinical symptoms
S. saprophyticus Coagulase-negative; resistant to novobiocin. Urinary tract
infections in young women (10–20%); occasional nonspecific
urethritis in men

Staphylococcus aureus
1-Morphology and culturing.
Fig. 3a shows appearance of Gram-stained S. aureus, are Gram-positive cocci arranged in irregular cluster single, pair and chain also seen in liquid culture. This is a facultative anaerobe that is readily cultured on normal nutrient mediums at 37 C but form pigment best at room temperature (20-25 C) no pigment is produced anaerobically or in broth. Colonies as in Fig. 3b developed after 24 hours of incubation. Hemolytic zones are frequently observed around the colonies.

Figure-3 a: Gram staining of a pus preparation: Gram-positive cocci, some in grapelike clusters

Figure-3 b-:Culture on blood agar: convex colonies with yellowish pigment

2-Fine structure and antigenic structure .
The cell wall consists of a thick layer of murein. Linear teichoic acids and polysaccharides are covalently coupled to the murein polysaccharide. The lipoteichoic acids permeating the entire murein layer are anchored in the cell membrane. Teichoic and lipoteichoic acids can trigger activation of complement by the alternative pathway and stimulate macrophages to secrete cytokines. Clumping factor, fibronectin- binding protein, and collagen-binding protein bind specifically to fibrinogen, fibronectin, and collagen, respectively, and are instrumental in adhesion to tissues
Protein A is a cell wall component of many S.aureus that binds to the Fc portion of immunoglobulins IgG. some S.aureus have capsule which inhibits phagocytosis by polymorphonuclear cells
3-Enzymes and toxins
A: Catalase: staphylococci produce catalase which prevent hydrogen peroxideinto water and oxygen, it differentiates staphylococci (positive)from streptococci (negative).
B- Plasma coagulase is an enzyme that functions like thrombin to convert
fibrinogen into fibrin. Tissue microcolonies surrounded by fibrin walls are difficult to phagocytose

C –exotoxin: which includes
?-toxin
is a heterogenous protein acts on a eukaryotic cell membrane it is a potent hemolysin.?- and ? toxins: first are degrade red blood cell of human and second toxic for red blood cell of human and animals
Leukocidin
damages microphages and macrophages by degranulation
Toxic shock syndrome toxin-1 (TSST-1)
It is produced by about 1% of Staphylococcus strains. TSST-1 is a superantigen that induces clonal expansion of many T lymphocyte types (about 10%), leading to massive production of cytokines, which then give rise to the clinical symptoms of toxic shock.
Exfoliative toxin or exfoliatin.
It is a superantiogen , that are responsible for a form of epidermolysis
Enterotoxins
Food poisoning symptoms can be caused by eight serologically differentiated enterotoxins (A-E, H, G, and I). These proteins (MW: 35 kDa) are not inactivated by heating to 100 8C for 15–30 minutes. Staphylococcus enterotoxins , are superantigens
Pathogenesis and clinical pictures.
The pathogenesis and symptoms of
S. aureus infections take one of three distinct courses:
Invasive infections. In this type of infection, the pathogens tend to remain
in situ after penetrating through the derma or mucosa and to cause local infections characterized by purulence. Examples include furuncles
,wound infections, sinusitis, otitis media, and mastitis puerperalis

Toxicoses. Food poisoning results from ingestion of food contaminated
with enterotoxins. The onset a few hours after ingestion takes the form of
nausea, vomiting, and massive diarrhea
Mixed forms. Dermatitis exfoliativa (staphylococcal scalded skin syndrome, Ritter disease) impetigo are caused by exfoliatin-producing strains that infect the skin surface. Toxic shock syndrome (TSS) is caused by strains that produce TSST-1. These strains can cause invasive infections, but may also only colonize mucosa.

Coagulase-Negative Staphylococci (CNS)
CNS are the normal flora of human skin and mucosa. They are classic opportunists that only cause infections in immunocompromised host
S. epidermidis. This is the pathogen most frequently encountered in CNS infections (70–80% of cases). CNS cause mainly foreign body infections. Examples of the foreign bodies involved are intravasal catheters.
when these organism adheres on the living or non living surfaces form biofilm The staphylococci within the biofilm are protected from antibiotics and the immune system .
S. saprophyticus is responsible for 10–20% of acute urinary tract infections, in particular dysuria in young women, and for a small proportion of cases of nonspecific urethritis in sexually active men