Staphylococci PDF

Made with Xodo PDF Reader and Editor Introduction The Staphylococci Staphylococci are Gram-positive bacteria, with diameters of 0.5 – 1.5 µm and characterised by individual cocci, which divide in more than one plane to form grape-like clusters. To date, there are 32 species and eight sub-species i...

Made with Xodo PDF Reader and Editor Introduction The Staphylococci Staphylococci are Gram-positive bacteria, with diameters of 0.5 – 1.5 µm and characterised by individual cocci, which divide in more than one plane to form grape-like clusters. To date, there are 32 species and eight sub-species in the genus Staphylococcus, many of which preferentially colonise the human body (Kloos and Bannerman, 1994), however Staphylococcus aureus and Staphylococcus epidermidis are the two most characterised and studied strains The staphylococci are non-motile, non-spore forming facultative anaerobes that grow by aerobic respiration or by fermentation Most species have a relative complex nutritional requirement, however, in general they require an organic source of nitrogen, supplied by 5 to 12 essential amino acids, e.g. arginine, valine, and B vitamins, including thiamine and nicotinamide (Kloos and Schleifer, 1986; Wilkinson, 1997). Members of this genus are catalase-positive and oxidase-negative, distinguishing them from the genus streptococci, which are catalase-negative, and have a different cell wall composition to staphylococci (Wilkinson, 1997). Staphylococci are tolerant to high concentrations of salt (Wilkinson, 1997) and show resistance to heat (Kloos and Lambe 1991). Pathogenic staphylococci are commonly identified by their ability to produce coagulase, and thus clot blood (Kloos and Musselwhite, 1975)  Made with Xodo PDF Reader and Editor This distinguishes the coagulase positive strains, S. aureus (a human pathogen), and S. intermedius and S. hyicus (two animal pathogens), from the other staphylococcal species such as S. epidermidis, that are coagulase-negative (CoNS). SPECIES OF STAPHYLOCOCCUS Currently 32 species of Staphylococcus are recognized of which three are of medical importance Staphylococcus aureus Staphylococcus epidermidis Staphylococcus saprophyticus Identification tree for Gram-positive cocci of medical importance is given in Figure 27.1. Various tests are now available to differentiate between three species of Staphylococcus Characters distinguishing species of Staphylococcu  Made with Xodo PDF Reader and Editor STAPHYLOCOCCUS AUREUS Staphylococci are associated with skin, skin glands and mucous membranes of almost all the warm blooded animals. This bacterium is also widely present in the environment. Nearly one-third of the human population supports the colonization of Staphylococcus aureus and are designated as carriersNasal carriage of Staph. aureus occurs in 40-50% of humans. Major habitat in man is anterior nares or on skin elsewhere. Hospitalised patients as well as medical and paramedical staff show higher incidence of carriage of Staph.aureus. Several coagulase negative species of staphylococci are present on the skin as commensal normal flora.  Made with Xodo PDF Reader and Editor Morphology These are nonmotile, nonspore forming, Gram positive cocci which measure around 0.7 to 1.2 μm in diameter. After phagocytosis or in old cultures, these may appear as gram-negative Young cocci stain strongly Grampositive. They are non-motile and don’t form spores. These are characteristically grouped in irregular clusters that resemble bunches of grapes (Fig. 27.2). This appearance is because of incomplete separation of cells after successive cell divisions which take place in perpendicular planes. Instead of residual attachment along the plane of division, the attachment point is usually eccentric to the plane of division. Cell Wall The cell wall of staphylococci is rich in peptidoglycan (Fig. 27.3) which is characterised by unique pentaglycine bridges that link the tetrapeptides attached to the muramic acid residues. Peptidoglycan is destroyed by strong acid or exposure to lysozyme. It elicits production of Interleukin-1 (endogenous pyrogen). Characteristics These are easy to grow organisms. They prefer aerobic environment but can also grow in the absence of oxygen; range of temperature for growth is 6-44°C (optimum 37°C) and the range of pH is 4.2-9.3 (optimum 7). The organisms grow easily in simple liquid media especially nutrient broth and peptone water. In both these media the growth becomes visible as uniform turbidity. They grow best at 37oC but form pigment best at room temperature (20-25oC).  Made with Xodo PDF Reader and Editor Mechanism of action of Staphylococcus aureus toxin Staphylococcus aureus produces several toxins that play a major role in the ability of this bacterium to cause disease. The toxins secreted by S. aureus can have direct toxic effects on human cells and can cause diseases such as food poisoning, dermatitis, and blood poisoning. Some of the toxins it produces include: 1. Neurotoxin: S. aureus produces neurotoxins such as α-toxin and β-toxin that cause destruction of muscle and nerve cells. α-toxin, for example, interacts with the cell membrane and forms channels in white blood cells, leading to cell lysis. 2. Enterotoxin: S. aureus also produces toxins called enterotoxins that bind to specific receptors in the intestine. These toxins are the main causes of food poisoning. When toxins enter the digestive tract, they cause tissue irritation, leading to symptoms such as nausea, vomiting, and diarrhea. 3. Exotoxins: Some toxins, such as TSST-1 (Toxic Shock Syndrome Toxin-1), can cause a severe toxic state known as toxic shock syndrome (TSS). These toxins are thought to interact with the immune system and trigger the release of large amounts of cytokines, leading to a broad inflammatory response that can lead to shock. 4. Pore-forming toxins: As mentioned earlier, many of the toxins secreted by S. aureus work by forming channels in cell membranes, allowing ions and water to leak into the cells, ultimately destroying the cells. Through these different toxins, S. aureus can infect a wide range of tissues and organs, leading to acute and chronic infections. Made with Xodo PDF Reader and Editor Made with Xodo PDF Reader and Editor Made with Xodo PDF Reader and Editor Made with Xodo PDF Reader and Editor  Made with Xodo PDF Reader and Editor Enterotoxin This is produced by one-third of all the strains of Staph. aureus. In a protein rich food contaminated with enterotoxigenic Staph. aureus sufficient enterotoxin is formed at room temperature within four hours to cause clinical features of food poisoning in man. Patient does not die of this poisoning but may wish he had because the reactions are so violent. Some of the salient characteristics of staphylococcal enterotoxin are mentioned in Table 27.5.  Made with Xodo PDF Reader and Editor Pathogenesis and Clinical Features Two types of diseases are produced by Staph. aureus: invasive and toxigenic (Table 27.7). The invasive lesions are suppurative whereas toxinoses are nonsuppurative. Laboratory Diagnosis a. Specimens: Clinical sample as per the site of lesion is collected. These could be pus, blood, surface swab, CSF or tracheal fluid, etc. b. Smears: The diagnosis of staphylococcal disease is suggested by the finding of Gram positive bacteria in clumps in the pathological material such as cerebrospinal fluid, but final diagnosis can be achieved by culture and appropriate biochemical test c. Culture: Pus, purulent fluid, sputum and urine can be directly streaked onto the blood agar. Specimens from patients who have been administered penicillin should be treated first with penicillinase to inactivate residual penicillin in that specimen.  Made with Xodo PDF Reader and Editor Treatment When penicillin was introduced almost all the strains of Staph. aureus were sensitive to this drug. Now more than 50% isolates show resistance to it. The resistance is due to the production of penicillinase (beta lactamase) by the organism which inactivates penicillin. Penicillinase resistant penicillins such as methicillin, oxacillin and cloxacillin can be used in treating patients having infection with penicillinase producing strains. Cephalosporins are the drugs of choice in those who are hypersensitive to penicillin.  Made with Xodo PDF Reader and Editor References Ames GF, Nikaido, K (1976) Two-dimensional gel electrophoresis of membrane proteins. Biochemistry 15: 616-623. Arens S, Schlegel U, Printzen G, Ziegler WJ, Perren SM, Hansis M (1996) Influence of materials for fixation infections associated with intravascular devices. Antimicrob Agents Chemother. 33: 597-601. Eickhoff TC (1972) Therapy of staphylococcal infection. In: Cohen JO, ed. The Staphylococci. Wiley, New York. pp. 517-541.