Antibiotic resistance, spread

Mellon AA, Hassani L (1999) Antibiotic resistance of Salmonella strains isolated from children living in the wastewater-spreading field of Marrakesh city (Morocco). World J Microbiol Biotechnol 15 81-85... 

Acquired resistance. This occurs when bacteria which were previously susceptible become resistant, usually, but not always, after exposure to the antibiotic concerned. Intrirrsic resistance is always chromosomally mediated, whereas acquired resistance may occirr by mutations in the chromosome or by the acquisition of genes coding for resistance ftom an external source normally via a plasmid or transposon. Both types are clinically important and can result in treatment failure, although acquired resistance is more of a threat in the spread of antibiotic resistance (Russell Chopra 1996). 

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. 

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...

Examples of sites with the characteristics mentioned above and, thus, potential reservoirs of antibiotic resistance are common in areas subjected to human activities (Fig. 2). Particularly, sites in which the use or disposal of antimicrobials is frequent, such as animal farms or municipal waste water treatment plants, are likely habitats to favour the spreading of antibiotic resistance. In Europe, the use of antibiotics as growing factors in animal husbandry is prohibited since 2006 [47]. However, animal farming still represents a critical point for antibiotic resistance development and dissemination. Among the major main routes of dispersal are manure, which sometimes is used directly or after composting in agriculture as fertilizers [48, 49], and animal farms waste water [50-53]. 

The dramatic increase of severe or lethal infections caused by antibiotic-resistant bacteria triggered numerous studies on antibiotic resistance, not only from clinical but also from environmental sources. Nowadays it is clear that environment, and water in particular, plays a central role on antibiotic resistance dispersion to and from clinical settings. However, the current state of the art clearly suggests that only a small fraction of the environmental resistome is known. The modes and mechanisms of emergence, evolution and transmission of resistance determinants are still not very well understood. Although environmental pollution is recognized to play an important role on antibiotic resistance evolution and spreading, it is still very difficult to draw cause-effect relationships, which sometimes seems to be strain/species dependent. 

A number of characteristics of plasmids are significant in relation to the development of bacterial resistance. These include the encoding of resistance capability for as many as six unrelated antibiotics in the same DNA material, the capacity to transfer from one cell to another and thus disseminate the resistance, and the mobilization by which ordinarily nontransferable gene fragments can be transferred by the plasmid from one bacterial cell to others. Plasmids are of varying size and have been identified in most bacteria. Their majority carries resistance determinants for two or more antibiotics not of the same chemical class. It appears that two identical plasmids cannot coexist in one cell, but plasmids of different groups can occur, increasing even further the possibilities for resistance spread. The transfer or acquisition of other plasmid-mediated characteristics, such... 

The phenomenon of antibiotic resistance gives the public a significant stake in whether an individual completes or complies with therapy. For example, in the case of tuberculosis, an individual s failure to comply with the lengthy therapeutic regime can give rise to the development of multi-drug resistant strains (MDR-TB) of the disease, which are difficult and expensive to treat (Kim 2003). MDR-TB can spread... 

Bacteria have two classes of transposons. Insertion sequences (simple transposons) contain only the sequences required for transposition and the genes for proteins (transposases) that promote the process. Complex transposons contain one or more genes in addition to those needed for transposition. These extra genes might, for example, confer resistance to antibiotics and thus enhance the survival chances of the host cell. The spread of antibiotic-resistance elements among disease-causing bacterial populations that is rendering some antibiotics ineffectual (pp. 925-926) is mediated in part by transposition. 

Several classes of (3-lactamases, often encoded in transmissible plasmids, have spread worldwide rapidly among bacteria, seriously decreasing the effectivenss of penicillins and other (3-lactam anti-biotics.t y Most (3-lactamases (classes A and C) contain an active site serine and are thought to have evolved from the dd transpeptidases, but the B typey has a catalytic Zn2+. The latter, as well as a recently discovered type A enzyme,2 hydrolyze imipenem, currently one of the antibiotics of last resort used to treat infections by penicillin-resistant bacteria. Some (3-lactam antibiotics are also powerful inhibitors of (3-lactamases.U/aa/bb These antibiotics may also have uses in inhibition of serine proteasesCC/dd such as elastase. Some antibiotic-resistant staphylococci produce an extra penicillin-binding protein that protects them from beta lactams.ee Because of antibiotic resistance the isolation of antibiotics from mixed populations of microbes from soil, swamps, and lakes continues. Renewed efforts are being... 

Goode J. and D.J. Chadwick Antibiotic Resistance, Origins, Evolution. Selection and Spread. Vol. 207, John Wiley Sons. Inc.. New York, NY, 1997. 

There is no known mechanism by which a gene can be transferred from a plant chromosome to a microbe. Thus, the possibility of that such transfer would generate new resistant organisms is very small, especially when compared to the high rate of spread of resistance through known mechanisms of microbe to microbe transfer to antibiotic resistance. FDA believes that the use of marker genes that encode resistance 10 other clinically useful antibiotics can be evaluated by similar criteria that were used for kanamycin resistance. 

Gootz TD. The forgotten Gram-negative bacilli what genetic determinants are telling us about the spread of antibiotic resistance. Biochem Pharmacol. 2006 71 1073-1084. 

---