The Gram –Positive Cocci
Pyogenic or Pus Forming Cocci (Staphylococci and Streptococci)
The Staphylococci
Sir Alexander Ogston, a Scottish surgeon, first showed in 1880 that a numbers of human pyogenic diseases were associated with a cluster-forming micro-organism. He introduced the name ’Staphylococcus (Greek: staphyl= bunch of grapes; kokkos=grain or berry), now used as the genus name for a group of facultatively anaerobic, catalase –positive, Grame –positive cocci. Staphylococci are resistant to dry conditions and high salt concentrations, and are well suited to their ecological niche, which is the skin. They may also be found as part of the normal flora of other sites such as the upper respiratory tract, and are commonly present on animals.
The major pathogen within the genus, Staphylococcus aureus, cause a wide rang of major and minor infections in man and animals will discus later, and is characterized by its ability to clot blood plasma by action of the enzyme coagulase. There are at least 30 other species of Staphylococci, all of which lake this enzyme , so called coagulase –negative Staphylococcus which are skin comensals can cause opportunistic infections associated with prostheses or foreign bodies( usually due to Staph.epidermidis) ,and urinary tract infections (Staph. Saprophyticus).The presence of methicillin-resistant Staphylococcus aureus (MRSA) in many hospitals has become a major public health issue, with concern expressed by patients and members of the public about the clinical implications.
Diseases caused by Staphylococcus aureus
Toxin –mediated diseases
Food poisoning due to enterotoxin (A-E): individual presented with sever nausea, vomiting, and diarrhea developing 4 hours after eating poisoned food.
Staphylococcocal Scalded Skin Syndrome (SSSS) due to exfoliative: affected usually new born or young child presented with wide spread blister-like lesions which soon after ruptured leaving large areas of desquamated epithelium but no scarring left.
Toxic shock syndrome due to TSST-1: young women in shock with generalized flushing of the skin and mucus membranes. Examination shows a tampon lodged in her vagina.
Toxic shock syndrome due to TSST-1: Child or adolescent develops shock with multiorgan failure and generalized flushing of skin and mucus membranes within days of sustaining a deep wound.
Suppurative infections:
Skin and soft tissue infections: Furuncles, carbuncles, paronychia (infections around the nail), cellulitis, impetigo (also caused by Streptococci)
Bacteremia (frequently occur with metastatic abscesses)
Endocarditis, Central nervous system infections (brain abscess, meningitis rare, epidural abscess).
Pulmonary infections (embolic, aspiration).
Staphylococcus aureus
Descriptions: Staphylococcus aureus is a Gram-positive coccus about 1M? in diameter. The coccus is usually arranged in grape-like cluster. The organisms are non-spoor forming, non-motile and usually non-capsulate. When grown on many types of agar for 24h at 37?C , individual colonies are circular,2-3mm in diameter, with smooth, shiny surface ;colonies appear opaque and are often pigmented (golden yellow).Staphylococci are relatively resistant to drying
, heat (they with stand 50?C for 30 minutes), and 9% sodium chloride, but are readily inhibited by certain chemical, eg, 3% hexachlorophene. The Staphylococci produce catalase (which reduces phagocytic killing by converting H2O2 to H2O, which differentiates them from Streptococci. Staphylococci slowly ferment many carbohydrates, producing lactic acids but not gas. Pathogenic Staphylococci produce many extracellular substances such as:* coagulase which converts plasma fibrinogen into fibrin, aided by an activator present in plasma ,so helps encase infection by forming fibrin layer around abscess.*production of thermostable nuclease that break down DNA. *production of a surface associated protein known as clumping factor or bound coagulase that reacts with fibrinogen.
Virulence factors
Staphylococcus aureus possess a large number of cell-associated and extracellular factors, some of which contribute to the ability of the organism to overcome the body s defences and to invade, survive in and colonize the tissues. Although the role of each factor is not fully understood individually, it is likely that they are responsible for the establishment of infection, enabling the organism to bind to the connective tissue, opposing destruction by the bactericidal activities of humeral factors such as complement, and overcoming uptake and intracellular killing by phagocytes.
Animal experiments support the view that no single virulence factor is pre-eminent in overcoming host resistance and establishing a focus of staphylococcal infection. However, particular exotoxins are responsible for the symptoms of certain syndromes.
Virulence factor Activity
Cell wall polymers
Inhibits inflammatory response ; endotoxin –like activity Peptidoglycan
Teichoic acid phage adsorption; reservoir of bound divalent cations
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Cell surface proteins
Protein A Reacts with Fc region of IgG
Clumping factor binds to fibrinogen
Fibronectin-binding protein binds tofibrinonectin
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Exoproteins
Lysin -?
?- Lysin impairment of membrane
- Lysin permeability; cytotoxic effects on phagocytic and tissue cells ?
?- Lysin
Panton-valentine leucocidin dermo-necrotic
Epidermolytic toxins cause blistering of skin
Toxic shock syndrome toxin induce multisystem effects;
Superantigen effects
Enterotoxins induce vomiting and diarrhea;
Superantigen effects
Coagulase converts fibrinogen to fibrin in plasma
Staphylokinase degrades fibrin
Lipase degrades lipid
Deoxyribonuclease degrades DNA
Table of some Virulence factors of Staphylococcus aureus
Enterotoxins: Enterotoxins types A-E, G, H, I and J, are commonly produced by up to 65% of strains of Staph.aureus, sometimes singly and sometimes in combinations. These toxic proteins withstand exposure to 100?C for several minutes. When ingested as performed toxins in contaminated food, microgram amount of toxin can within a few hours, induce the symptoms of staphylococcal food poisoning: nausea, vomiting and diarrhea.
Toxic shock syndrome toxin (TSST-1). This was discovered in the early 1980s as a result of epidemiological and microbiological investigations in the USA of toxic shock syndrome, a multi-system disease caused by staphylococcal TSST-1 or enterotoxin, or both. A link was established with the use of highly absorbent tampons in menstruating women, although non-menstrual cases are now as common. The absence of circulating antibodies to TSST-1 is a factor in the pathogenesis of this syndrome.
TSST-1 and the enterotoxins are now recognized as superantigens, that is, they are potent activators of T lymphocytes resulting in the liberation of cytokines such as tumor necrosis factor, and they bind with high affinity to mononuclear cells. These characteristics partly explain the florid and multi-system nature of the clinical conditions associated with these toxins.
Epidermolytic toxins. Two kinds of epidermolytic toxin (type A and B) are commonly produced by strains belonging mainly to phage group ?, that cause blistering diseases, these toxins induce intraepidermal blister at the granular cell layer. Such blisters range in severity from trivial to the distended blisters of pemphigus neonatorum. The most dramatic manifestation of epidermolytic toxin is the staphylococcal scalded skin syndrome in small children, where the toxin spread systemically in individuals who lack neutralizing antitoxin. Extensive areas of skin are affected, which, after the development of a painful rash, slough off; the skin surface resembles scalding.
Epidemiology
Sources of infection
Infected lesions. large numbers of Staphylococci are disseminated in pus and dried exudates discharged from large infected wounds, burns, and secondarily infected skin lesions, and in sputum coughed from the lung of patients with bronchopneumonia. Direct contact is the most important mode of spread, but air-born dissemination may also occur. Cross-infection is an important method of spread of staphylococcal disease particularly in hospitals, and perfect hand washing is essential in preventing spread. Food handlers may similarly induce enterotoxin-producing food poisoning strains into food.
Healthy carriers. Staph.aureus grows harmlessly on the moist skin of the nostrils in about 30% of healthy persons, and the perineum is also commonly colonized. Organisms are spread from these sites into the environment by the hands, handkerchiefs, clothing, and dust consisting of skin squamous and cloth fibers. Some carriers, called shedders, disseminate exceptionally large numbers of staphylococci.
During the first day or two of life most babies become colonized in the nose and skin by staphylococci, and transmission from babies to nursing mothers, who then develop mastitis, is well described.
Animals. Animals may disseminate Staph.aureus and so cause human infection, for example milk from a dairy cow with mastitis, causing staphylococcal food poisoning.
Modes of infection
Acquisition of infection may be exogenous (from an external source) or endogenous (from a carriage site, or minor lesion elsewhere in the body). It is important to remember that the body surface of human beings and animals are the main reservoir. although no spore forming , staphylococci may remain alive in a dormant state for several months when dried in pus , sputum, bed cloths, or dust or on, inanimate surface such as floors, they are fairly readily killed by heat (e.g. moist heat at 65?C for 30minuts), by exposure to light ,and by common disinfectants.
Methicillin-resistant Staph.aureus (MRSA)
MRSA is endemic in many hospitals throughout the world and particularly affects vulnerable patients, such as those who have undergone major surgery and patients in the intensive care unit. Although 50-60% of patients with MRSA are merely colonized (i.e. they carry the bacteria but do not have symptoms or an illness), serious infections such as those involving the blood stream, respiratory tract and bones or joints do occur. These infections are then more difficult to treat than infections caused by methicillin-suceptible isolates and MRSA can spread easily among patients in hospital. Methicillin resistance is mediated through mecA gene, which encodes unique penicillin –binding protein.
Commonly –acquired strains have been described that can cause soft tissue infections; these strains often produce the Panton-Valentine leucocidin. They can be distinguished from endemic hospital strains from which it is believed that they have arisen.
The control and prevention of MRSA involves early and reliable detection in the laboratory through surveillance, patient isolation when admitted to hospital, good professional practice by all health-care workers (including complains with hand hygiene guidelines), effective hospital hyiegene programs and sensible use of antibiotics. Such measures have been very successful in Scandinavia and some other countries, but the prevalence of MRSA elsewhere, including UK, Ireland and Southern Europe, is much higher.
Laboratory diagnosis
One or more of the following specimens should be collected to confirm a diagnosis:
*pus from abscesses, wound, burn, etc. is much preferred to swabs.
*sputum from patients with pneumonia (e.g. post-influenzal or ventilator-associated pneumonia); bronchoscopic specimens are increasingly used in critically ill patients.
*faeces or vomit from patients with suspected food poisoning, or the remains implicated foods,
*blood fro patients with suspected blood stream infection (bacteraemia), such as septic shock, osteomylitis or endocarditis.
*mid-stream urine from patients with suspected cystitis or pylonephritis.
* Anterior nasal and perineal swabs (moistened in saline or sterile water) from suspected carrier; nasal swabs should be rubbed in turn over the anterior walls of both nostrils.
The characteristic clusters of Gram-positive cocci can often be demonstrated by microscopy, and the organisms cultured readily on blood agar and most other media. The tube or slide coagulase test is performed to distinguish Staph.aureus from coagulase –negative species. Molecular methods such as the polymerase chain reaction (PCR) have been developed, but are still being evaluated to determine their role in routine laboratory practice.
Typing
Most staphylococcal infections are sporadic, but out-break occur, especially in hospitals. The identification of an outbreak strain, by determining whether all the isolates are of the same type, is an important aspect in the investigation of a source. Strains of Staph.aureus can be differentiated into different phage types by observation of their pattern of susceptibility to lysis by a standard set of Staph.aureus bacteriophages (virus that infect bacteria).Virulent phages cause lysis of staphylococci and thus produce a clearing in the lawn of growth. Phage types are designated according to the phage able to cause this effect, and there is international agreement on the interpretation of results. Thus a strain type 3B/3C/55 is one that is lysed only by phage 3B, 3C and 55.
Many strains of MRSA are non-typable with standard and additional or experimental phage. Consequently, phage typing is being supplemented by genotypic methods such as PCR, pulsed-sequencing. Unfortunately, there are as yet no internationally agreed criteria for assessing the results in the same way as there is for phage typing.
Treatment
Sensitivity to antibiotics
Staph.aureus and other staphylococci are inherently sensitive to many antimicrobial agents. Among the most active is benzyl penicillin, but about 90% of strains found in hospitals are now resistant. Resistance to penicillin depends on production of the enzyme penicillinase, a ?-lactamase that opens the ?-lactam ring. Penicillinase also inactivates most of the other penicillins, although a few, including methicillin (used for laboratory testing and not for therapy), oxacillin, cloxacillin and flucloxacillin, are stable to the enzyme. Cephalosporins and
Lactamase inhibitors are also stable to penicillinase. ?-
MRSA strains are resistant to all ?-lactam agents, and often to other agents such as the aminoglycosides and fluoroquinolones. Glycopeptides (vancomycin or teicoplanin) are the agents of choice in the treatment of systemic infection with MRSA, but these agents are expensive and may be toxic. Isolates of MRSA with reduced susceptibility or full resistance to glycopeptides antibiotics are uncommon, but have been detected sporadically. These isolates have either thickened cell walls (reduced susceptibility) or the vanA gene (fully resistant), and can be difficult to detect in routine diagnostic laboratory.
Choice of antibiotic for therapy
Pending receipt of susceptibility test results, the treatment of sever infections suspected to be caused by Staph.aureus should be started with flucloxacillin unless MRSA is endemic locally, in which case a glycopeptides such as vancomycin is indicated. Erythromycin, clindamycin or vancomycin (or teicoplanin) is indicated if the patient is allergic to penicillin. Fusidic acid and rifampicin are not used alone in serious infections, because mutation to resistance arise readily. It is usually necessary to remove an infected source, such as an intravascular catheter, or drain an abscess as part of the treatment.
Infections caused by bacteria that exhibit reduced susceptibility to glycopeptides may be treated (if susceptible) with other antistaphylocaccal agents. Newer agents such as linezolid and daptomycin are promising, but experience is limited.
Life-threatening toxin-mediated disease, such as toxic shock syndrome, requires major medical support such as intravenous fluids to prevent multi-organ failure, often best provide in the intensive care unit.
Coagulase –negative Staphylococci
Coagulase –negative Staphylococci comprise a large group of related species commonly found on the surface of healthy persons, in whom they are rarely the cause of infection. More than 30 species are recognized, but few are commonly incriminated in human infection Staph.epidermidis accounts for about 75% of all clinical isolates, probably reflecting its preponderance in the normal skin flora. Other species include Staph.hominis. Staph.Capitis and Staph Saprophyticus (a cause of urinary tract infection in young women). The emergence of Coagulase –negative Staphylococci as major pathogens reflects the increased use of implants such as cerebrospinal fluid shunts, intravascular lines and cannulae, cardiac valves, pacemakers, artificial joints, vascular grafts and urinary catheters, and the increasing numbers of severely debilitated patients in hospitals.
Description
Coagulase –negative Staphylococci are morphologically similar to Staph.aureus, and the methods for isolation are the same. Colonies can be distinguished from Staph.aureus by their failure to coagulate plasma and by their lack of clumping factor and deoxyribonuclease. Because Staph.epidermidis may contaminate clinical specimens, care has to be exercised in assessing its significance, especially from superficial sites. When isolated from sites such as blood or cerebrospinal fluid, further specimens should be obtained to confirm its clinical significance. Coagulase –negative Staphylococci are opportunistic pathogens that cause infection in debilitated or compromised patients such as premature neonates and oncology patients, often by colonizing biomedical devices such as prostheses, implants and intravascular lines. They cause particular problems in:
*cardiac surgery (prosthetic valve endocarditis).
*patients fitted with cerebrospinal fluid shunt (meningitis).
*continuous ambulatory peritoneal dialysis (peritonitis).
*immunocompromised patients (e.g. blood stream infection).
*women during the reproductive years (urinary tract infection with Staph Saprophyticus).
Pathogenesis
Production of an exopolysaccharide, allowing adherence and subsequent formation of a multilayered biofilm, appears to be essential for the pathogenesis of device-related Staph.epidermidis infection. A complex array of inter-related chemical messengers controls expression of polysaccharide and drives intercellular adhesion and biofilmformation. Attachment is enhanced by the presence of matrix proteins, such as fibronectin and fibrinogen. There is considerable interest in the development of implantable device such as prosthetic heart valves or cerebrospinal shunts that are less prone to adherence by Staph.epidermidis and subsequent biofilm formation. This may be achieved by altering the structure of the polymer or by incorporating antibacterial agents on the surface of the device.
Treatment Antibiotic treatment of Coagulase –negative Staphylococcal infections is complicated because susceptibility is generally unpredictable. Strains resistant to penicillin, penicillinase-stable penicillins,gentamycin, erythromycin and chloramphenicol are common. If a strain is the cause of systemic infection, vancomycin or teicoplanin should be used. Rifampicin in combination with glycopeptides is occasionally useful in treating central nervous system infections. Vancomycin resistant is rare, but one species, staph.haemolyticus, is often resistant to teicoplanin. Uncomplicated urinary tract infection caused by Staph Saprophyticus usually responds to trimethoprim or one of the fluoroquinolone.
قال الله عز وجل ( فأما ألزبد فيذهب جفاء وأما ماينفع الناس فيمكث في ألأرض كذلك يضرب الله ألأمثال) .إن ألله تبارك وتعالى شبه أحوال الدنيا بالسيل الذي يجرف في طريقه كثيرا من ألأشياء الخفيفه كالشوك ونشارة الخشب والتبن وما شابه ذلك. ولأن هذه ألأشياء خفيفه فهي ترى على سطح الماء طافيه ولكن بعد مده تزول وتختفي وتبقى ألأشياء النافعه والمفيده.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .