Enseñanza de las leyes de Newton

The virulence of P .aeruginosa is multi-factorial and is dependant on both host and bacterial variables. Colonization of the lungs with virtually any P .aeruginosa strain induces a strong antibody response in serum, saliva and pulmonary secretions. In patients with cystic fibrosis increased antipseudomonal antibodies titres may precede the isolation of the pathogen from the respiratory tract (Brett et al 1992) . These antibodies are raised against a number of antigens including proteases and mucoid exopoly­ saccharide, LPS and OMP. However once colonisation is established the bacteria is rarely eliminated, despite antibiotic treatment. The failure to eliminate P .aeruginosa despite the immune response is a result of the immunoevasive activities of P .aeruginosa (Wilson et al 1985; Buret and Cripps 1993) often combined with a pre­ existing weakness in the host defense.

1.7.1.1 Adhesins

P .aeruginosa pili, mucoid exopolysaccharide, haem- agglutinins, and possibly OMP mediate adherence of P .aeruginosa to the respiratory tract (Hata and Pick 1991; Woods et al 1990) . Using non-mucoid P . aeruginosa it has been demonstrated that tracheal cells bind the organism more avidly than buccal cells and that tracheal cells from tracheostomised patients bind more bacteria than cells from normal subjects (Niederman et al 1994). Other variables which influence the number of epithelial cell receptors for

p .aerucrinosa include the density of cilia, structure of surface mucins and glycoproteins, and the amount of damaged epithelium. Bacterial elastase, alkaline phosphase and phospholipase C actively enhance the process of adherence, possibly by unmasking a potential receptor for P .aeruginosa

(Nicas and Iglewski 1986; Saiman et al 1990) .

P .aeruginosa pili are composed of pilin protein subunits which mediate adherence of non-mucoid P .aeruginosa to mucin (Ramphal 1987) and to damaged epithelial cells (Ramphal et al 1984) adherence may be blocked by pre-incubation of respiratory epithelial cells with purified pili (Woods et al 1980). In contrast the adhesion of mucoid P .aeruginosa to mucin has been reported to be mediated by the mucoid exopolysaccharide (Ramphal et al 1987).

1.7.1.2 Pigments

P .aeruginosa produces at least four distinct pigments and several precursors. The four distinct pigments are known as pyocyanin, pyoverdin, pyorubin and pyomelanin.

The distinctive blue pigment of P .aeruginosa was first noted as a blue/green discolouration of surgical dressings (Gessard 1882). Fordos first isolated a crystallised substance (Fordos 1860) and discovered the indicator and redox properties of the compound. Pyocyanin has been shown to cause slowing and dyskinesia of the beating of human cilia (Wilson et al 1987) and to inhibit epidermal cell

growth and lymphocyte proliferation (Cruikshank and Lowbury 1953; Sorenson et al 1983) . Pyocyanin is also known to inactivate both the production of IL2 and the expression of its receptor on the T cell membrane (Nutman et al 1987; Muhlradt et al 1986). Pyocyanin is a redox compound

(Hassett 1992) which has been previously shown to stimulate redox cycling in bacteria, liver cells and a human epithelial cell line (Armstrong et al 1971; Cohen et al 1990; Hassan and Fridovich 1980). Free radical generation has also been proposed as a mechanism of its antibacterial action (Hassat and Cohen 1989) .

Pyocyanin slowly decomposes to a yellow substance 1-HP (Fordos, 1863) . Synthetic 1-HP (>10/zM) immediately slows and disorganises the beating of human cilia (Wilson et al 1987) and causes immediate slowing and then recovery of the tracheal transport velocity of mucus in the guinea pig (Munro et al 1989). Synthetic pyocyanin (>10^M) however causes progressive ciliary slowing of human cilia after incubation for 1 hour and slowing of tracheal transport velocity of mucus after 2 hours. 1-HP has also been shown to inhibit mammalian cell respiration (Stewart-Tull and Armstrong 1972). Pyocyanin and 1-HP are detectable in the sputum of patients with bronchiectasis at levels as high as 27.3^g/ml and 3.5/zg/ml respectively (Wilson et al 1988). Therefore in vivo these toxins are found in concentrations above that required to cause ciliary slowing and epithelial disruption in vitro.

1.7.1.3 Haemolysins

Two classes of haemolytic substance are produced by P .aeruginosa. Heat stable glycolipids (rhamnolipids) and heat labile phospholipase C (lecithinase). Glycolipids are produced by clinical isolates during the stationary phase of growth. They have a detergent like structure with a polar head and a non-polar tail and their surfactant like properties may account for their known haemolytic activity. They cause ciliary stasis associated with altered ciliary membranes in rabbit tracheal epithelium (Hingley et al

1986), and interfere with epithelial ion transport in vitro in sheep in a dose dependant manner (Graham et al 1993) . At pathophysiological concentrations they act as mucus secretagogues in the cat in vivo (Somerville et al 1992) , slow CBF and disrupt the epithelial integrity of human epithelium in vitro (Read et al 1992).

Phospholipase C is a lecithinase which liberates phosphorylocholine from lecithin. Phosphorylocholine causes in vitro platelet aggregation (Coutinho et al 1988), the release of inflammatory mediators from human granulocytes in vitro (Bergmann et al 1989) and the accumulation of granulocytes in the peritoneal cavity of mice injected with phosphorylocholine (Meyers and Berk 1990). They have been

shown to damage ciliated epithelium.

1.7.1.4 Proteases

secretion from animal tissue in vitro (Adler et al 1986), in vivo in the cat (Somerville et al 1991) and in human tissue in vitro (Somerville et al 1991) . This may enhance bacterial clearance since an increase in mucus load may stimulate ciliary beating. However this host defense may be ineffective if the stimulated mucus possesses abnormal rheological properties (Puchelle et al 1980) or if the increase output is sufficient to uncouple cilia from the mucus layer (Sleigh et al 1988) . Other bacterial products including pyocyanin, rhamnolipid and 1-HP may join with proteases to disrupt and slow normal ciliary beating (Wilson et al 1987) . Proteases of P .aerucrinosa also play an important role in modulating the humoral and cellular response to infection.