Classification

1. Facultatively anaerobic - do not require oxygen for growth but do grow better in its presence
2. Catalase positive
3. Gram positive cocci
4. Hardy,being resistant to dry conditions and high salt concentration

The main species of staphylococcus is Staphylococcus aureus which is a pathogen of both humans and animals. It is characterized by its ability to clot blood plasma by action of the enzyme coagulase

There are about 30 other staphylococcus species but all lack the enzyme coagulase. Thus there are classified as Coagulase negative staphylococcus

a) Staphylococcus aureus

• 1 micrometre
• Gram positive
• Grape-like cluster but some occur as single or pair of cells
• Non-sporing,non-motile and usually non-capsulate
• Colonies are circular, 2-3 nm in diameter with a smooth shiny surface when grown on nutrient agar,milk agar or blood agar for 24 h at 37 degrees
• Colonies are often pigmented,though a few strains are unpigmented
• Salt-tolerant

Main distinctive :-

1. Production of an extracellular enzyme,coagulase,that converts fibrinogen in citrated human and rabbit plasma to fibrin,aided by an activator present in plasma. Add a drop of fresh young broth culture into a tube containing 0.5 mL of citrated plasma diluted 1 in 10. A +ve result is seen within a few hours as a distinct clot
2. Production of thermostable nucleases that breakdown DNA. This activity is detected by the ability of a boiled broth culture to degrade DNA in an agar diffusion test
3. Production of a surface-associated protein known as clumping factor or bound coagulase that reacts with fibrinogen. Clumping factor is easily detected within a few seconds by adding undiluted plasma to a saline suspension of the organism on a microscope slide

b) Pathogenesis of Staphylococcus aureus

• present in the nose of 30% of healthy people and may be found on the skin
• causes infection at sites of lowered host resistance. Eg:- damaged skin or mucous membranes

i) Virulence factors

Staphylococcis aureus has various virulence factors of which some are abled to overcome the body's defences and to invade,survive in and colonize the tissue. Below is a list of the virulence factors :-

1. Cell wall polymers - peptidoglycan (inhibits inflammatory response) and teichoic acid
2. Cell surface proteins - Protein A,Clumping factor,Fibronectin-binding protein and Collagen-binding protein
3. Exoproteins - alpha,beta gamma,sigma lysin,Panton-valentine,leucocidin,epidermolytic toxins,toxic shock syndrome toxin (TSST-1),enterotoxins,coagulase,hyaluronidase - degrades hyaluronic acid in connective tissue,staphylokinase - degrades fibrin,lipase - degrades lipid,phospholipases - degrade phospholipids,deoxyribonucleases - degrade DNA,proteases - cause proteolysis. There are five types of Enterotoxins (type A - E). These are heat stable which can withstand explosure to 100 degrees fo minutes. When ingested as preformed toxins in contaminated food,microgram amounts of toxin can induce within a few hrs the symptoms of staphylococcal food poisoning,nausea,vomiting and diarrhoea. There are two types of epidermolytic toxins (type A and B) which both causes blistering disease. These toxins induxe intra-epidermal blisters at the granular cell layer. Such blister range in severity from the trivial (little importance) to the distendal blisters of Pemphigus neonatorum. The most dramatic manifestation (make clear) of epidermolytic toxin is the scalded skin syndrome in which toxin spreads 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. Toxic Shock Syndrome Toxin (TSST-1) is recognized as superantigen as they are potent activators of T-lymphocyte resulting in the liberation of cytokines such as Tumour Necrosis Factor (TNF) and they bind with high affinity to mononuclear cells. The absence of circulating antibodies to TSST-1 is a factor in the pathogenesis of the syndrome

c) Epidemiology of Staphylococcus aureus

1) Sources of Infection

i) Infected Lesions :

• large no. of staphylococci are disseminated (spread widely) in pus and dried exudate discharged from large infected wounds,burns,secondarily infected skin lesions and in sputum coughed from the lung of a patient with bronchopneumonia
• direct contact is the most important mode of spread,but air-borne dissemination may also occur

ii) Healthy Carriers :

• spread into environment by the hands,handkerchiefs,clothing and dust consisting of skin squames and cloth fibres of healthy carriers. Some carriers,called shedders,disseminate exceptionally large no. of cocci,comparable to the no. disseminated by patients with large superficial lesions or lower respiratory tract infections

iii) Animals :

• domesticated and some wild species may disseminate staphylococcus aureus from infected lesions or carriage sites and so cause infections in humans

2) Modes of Infection

• mode of infection may be either exogenous (from an external source) or endogenous (from a carriage site or minor lesion,elsewhere in the patient's own body)
• staphylococci do not grow outside the body except occasionally in moist nutrient materials such as meat,milk and dirty water
• although not spore-forming,they may remain alive in a dormant state for several months when dried in pus,sputum,bed clothes or dust
• They are fairly readily killed by heat (65 degrees for 30 min),by exposure to light and by common disinfectants

d) Laboratory Diagnosis of Staphylococcus aureus

One or more of the following specimens should be collected to confirm a diagnosis

1. pus - abscesses,wounds,burns
2. sputum - lower respiratory tract infections
3. faeces & vomit - patients withn food poisoning
4. blood - patients with suspected bacteraemic
5. mid-stream urine - from patients with suspected cystitis or pyelonephritis
6. anterior nasal and perineal swabs - nasal swabs should be rubbed in turn over the anterior walls of both nostrils

e) Treatment of Staphylococcus aureus

1. Sensitivity to antibiotics

Staphylococcus aureus and other staphylococci are inherently sensitive to many antimicrobial agents with benzylpenicillin the most active

However,after a period of time,staphylococcis aureus and other staphylococci will be resistant to antibiotics and the various mechanisms involved in this resistance includes :-

antibiotics	mechanisms
penicillins	beta-lactamase
methicillin	altered binding protein
chloramphenicol	acetyltransferase
tetracyclines	reduced accumulation
erythromycin	methylation of ribosome
streptomycin	altered ribosomal protein
other amino-glycosides	enzymic modification
fusidic acid	altered factor G
rifampicin	altered RNA polymerase

2. Choice of antibiotic for therapy

Based on the results of sensitivity tests made on a culture of the strain isolated from the patient

Coagulase-Negative Staphylococci

• found on the surface of healthy persons in whom they are rarely the course of infection
• more than 30 species
• the emergence of Coagulase-Negative Staphylococcus as major pathogens reflects the increased use of implants and the increasing no. of severely debilitated patients in hospitals

a) Description

• morphologically similar to Staphy. aureus
• colonies are usually non-pigmented (white)
• distinguished from Staphy. aureus by their failure to coagulate plasma and by their lack of clumping factor and deoxyribonuclease
• opportunistic pathogens that causes infection in debilitated or com-promised patients sych as premature neonates and oncology patients
• cause problem after cardiac surgery,in patients fitted with cerebiospiral fluid shunts,in continuous ambulatory peritoneal dialysis and in immunocomprised patients

b) Pathogenesis

• produce few,if any,toxins and their pathogenicity is believed to be related to their ability to adhere to biomaterials consisting synthetic polymers and production of extracellular 'slime'
• adherence is mediated by polysaccharides and is enhanced by the presence of fibrinogen
• 'slime',which largely consists of teichoic acid,inhibits the proliferation of human peripheral mononuclear cells and may be partly responsible for the failure of treatment with antimicrobial agents because of difficulties in drug penetration

c) Treatment

• if the cause of systemic infection,vancomycin or teicoplanin should be used
• rifampicin plus glycopeptide is useful in treating Central Nervous System
• trimethoprin or one of the fluroquinolones such as norfloxacin is useful for uncomplicated urinary tract infection


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