Antibiotic resistance, transfer

Bacteria produce chromosomady and R-plasmid (resistance factor) mediated P-lactamases. The plasmid-mediated enzymes can cross interspecific and intergeneric boundaries. This transfer of resistance via plasmid transfer between strains and even species has enhanced the problems of P-lactam antibiotic resistance. Many species previously controded by P-lactam antibiotics are now resistant. The chromosomal P-lactamases are species specific, but can be broadly classified by substrate profile, sensitivity to inhibitors, analytical isoelectric focusing, immunological studies, and molecular weight deterrnination. Individual enzymes may inactivate primarily penicillins, cephalosporins, or both, and the substrate specificity predeterrnines the antibiotic resistance of the producing strain. Some P-lactamases are produced only in the presence of the P-lactam antibiotic (inducible) and others are produced continuously (constitutive). 

Plasmids have the ability to transfer within and between species and can therefore be acquired from other bacteria as well as a consequence of cell division. This property makes plasmid-acquired resistance much more threatening in terms ofthe spread of antibiotic resistance than resistance acquired due to chromosomal mutation. Plasmids also harbour transposons (section 2.1.3), which enhances their ability to transfer antibiotic resistance genes. 

More recently, a range of other quality and safety issues have been recognised by consumers and now influence poultry meat and egg buying patterns and behaviour. Most importantly this includes (i) the routine use of antibiotics as growth promoters and curative medicines and the potential for development of transferable antibiotic resistance, (ii) risk associated with enteric pathogen (e.g. Salmonella and Campylobacter) and toxin (e.g. dioxin) contaminants of poultry products, (iii) the environmental impact of poultry production and (iv) the sensory and nutritional quality of eggs and poultry meat (Menzi et al., 1997 Hamm et al., 2002 Rodenburg et al., 2004 Horsted et al., 2005). 

Preston-Mafham J, Boddy L, Randerson PF (2002) Analysis of microbial community functional diversity using sole-carbon-source utilisation profiles -a critique. FEMS Microbiol Ecol 42 1-14 Quentmeier A, Friedrich CG (1994) Transfer and expression of degradative and antibiotic resistance plasmids in acidophilic bacteria. Appl Environ Microbiol 60 973-978... 

Studies by Levine et al. (1983) have addressed the issue of plasmid mobilizations, the movement of plasmids between different host cells. Human volunteers fed tetracycline along with E. coli HS-4 (typical of the normal intestinal flora of humans) bearing highly mobilized plasmids (e.g., pJBK5) that carried resistance to chloramphenicol and tetracycline became co-colonized with E. coli HS-4 bearing the antibiotic-resistant plasmid. However, the use of a poorly mobilizable plasmid (pBR325) did not result in plasmid transfer. 

Fig. 1 Mechanisms of antibiotic resistance acquisition (a) and spreading in a community over time (b). Arrows indicate horizontal gene transfer processes and R acquired antibiotic resistance gene...

In comparison with culture-dependent procedures, culture-independent methods are more sensitive and have an increased potential to survey the diversity of antibiotic resistance genes in the environment. A weakness of these methods is the impossibility to elucidate about the bacteria in the community that host specific resistance determinants. On the other hand, the possibility to explore the genetic environment (mobile element, associated genes, promoter, etc.) in which the resistance determinant is integrated offers relevant clues about the gene transfer potential and gene acquisition history. 

There has been great concern that the feeding of low levels of antibiotics that are also used in human medicine could lead to serious human health problems. There is no question that bacteria develop resistance to these antibiotics, and that they can transfer their resistance to other bacteria, even to other species. There is also no question that antibiotic resistance has become a serious problem in human medicine. However, the extent to which the feeding of antibiotics to animals has contributed to the human health problem is still unclear and a source of great controversy. 

These two herds have provided information on development, persistency and transfer of antibacterial resistance and. furthermore, they have provided information regarding the impact the previously proposed restrictions (Fed. Reg. 42 43770 and 42 52645. 1977) of antibiotic usage would have on antibiotic resistant bacteria of animal origin as a health hazard to humans. 

Antibiotic resistance and its transfer Feed Management June. 1979. pp 32. 34 and 37. 

The presence of antibiotic resistant intestinal organisms resulting from the use of antibiotics in feeds is well established (, 5, ). Wanatabe ] ) reported on the transferability of this trait and Anderson and Lewis ( ) showed the transfer of antibiotic resistance between species involving Salmonella typhimurium. 

Long-term, low-level feeding of penicillin and the tetracyclines promotes, by natural selection from the pool of normal intestinal flora, those enteric (gut) bacteria that contain R-plasmids. R-plasmids, also known as R-factors, are extrachromosomal genetic material which confer antibiotic resistance to host bacteria. These plamids can be transferred between various kinds of bacteria through cell-to-cell contact (conjugation). Simultaneous resistance to several unrelated antibiotics is commonly carried on a single plasmid and therefore is simultaneously transferred from one bacterium to another. 

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