Enteric bacteria membrane systems

Accordingly, this chapter deals mainly with some of the very recent advances in knowledge of iron transport systems in enteric bacteria. I review critically and place in historic perspective current understanding of the competition phenomenon between phages, colicins, and sidero-phores for outer membrane receptors, the resistance to certain colicins imparted by iron, and, finally, the mechanism of siderophore-mediated iron uptake. 

The existence of ferrichrome and ferric enterobactin receptors in the outer membrane of enteric bacteria confirms the discovery, first reported for vitamin Bi2 (34), for a genuine transport role for this segment of the cell envelope. The properties of the four analogous systems known at the present time are shown in Table IV. 

Figure 13 summarizes the present state of knowledge regarding interactions of siderophores, phages, and colicins with membrane systems in the enteric bacteria. Over the eons of evolutionary time, noxious agents have acquired the capacity to exploit these receptors as a means of penetrating the cell envlope. It would be important to ascertain if this analogy extends to plant and animal virus receptors. 

This study shows that K. pneumoniae when growing under aerobic or semiaerobic conditions, transports NO3" into the cells. Therefore assimilatory or aerobic NOa" reduction probably occurs inside the cells. The observation that NO2" is transiently excreted into the mediiun from which it then disappears suggests the presence of both a NO2" export system (possibly a consequence of NO3" reduction by a membrane-associated enzyme) and a NO2" uptake system. The presence of a NO2" uptake capability may simply reflect the permeability of membranes to weak acids in the presence of a transmembrane pH gradient (19-23) or may indicate the presence of carrier-mediated transport systems. If the N02 uptake is carrier-mediated, the transient nature of the accumulation of N02 outside the cells suggests that uptake and/or reduction may be activated shortly after the cells are exposed to N03 . It may also be possible that simultaneous transport of nitrite across the cell membrane and reduction are mediated by one enzyme system. While we have not attempted to demonstrate a membrane-associated nitrite reductase, Jones and Garland (3) and Coleman et al. (22) have found such activities in other enteric bacteria. 

Virases are much simpler organisms than bacteria, and they are made from protein substances and nucleic acid. A single nucleoprotein molecule formed from molecules of nucleic acid that are chemically bound to a bulky protein molecule can be considered a simple viral particle. The protein molecule plays the role of a protective membrane. Thus the virus can be schematically described as a nucleic acid insert that is protected by a protein covering. A virus can contain either ribonucleic acid or deoxyribonucleic acid, but it never contains both of them together. The type of nucleic acid is the basis of one of the classifications of viruses. Viruses are obligatory intracellular parasites, which, upon entering a cell (i.e. after being infected) use many biochemical systems of the host cell. 

Gram-negative bacteria contain lipopolysaccha-rides (endotoxins) in their outer cell membranes (Chapter 19) these can remain in an active condition in products even after cell death and some can survive moist heat sterilization. Although inactive by the oral route, endotoxins can induce a number of physiological effects if they enter the bloodstream via contaminated infusion fluids, even in nanogram quantities, or via diffusion across membranes from contaminated haemodialysis solutions. Such effects may include fever, activation of the cytokine system, endothelial cell damage, all leading to septic and often fatal febrile shock. 

Organometallic compounds may enter the environment directly as pollutant industrial chemicals and some are synthesized biologically by bacteria. Some of these compounds are toxic because of their mobility in living systems and ability to cross cell membranes. 

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