Antibiotic-Resistant Mutants

Figure 7. Antagonism between ferrichrome and albomycin for membrane receptor in Salmonella typhimurium and the appearance of mutants resistant to the antibiotic (2). Several of these mutants owe their resistance to a defect in ferrichrome transport. The nature of this effect as competition for a surface site was elucidated by Zimmerman and Knusel (32).

Grivell, L.A., Netter, P., Borst, P., Slonimski, P., Mitochondrial antibiotic resistance in yeast ribosomal mutants resistant to chloramphenicol, erythromycin and spiramycin. Biochim. Biophys. Acta 1973, 312, 358-367. 

According to Nissen et al. [27], proteins L4, L22, and L39e (the letter e represents a protein in a bacterial SOS ribosomal subunit, a protein that belongs to one of the homologs in eukaryotic 60S ribosomal subunit) have been shown to be present in the polypeptide exit tunnel [average diameter, about IS A length of the tunnel, 100 A (Fig. 3B)] present in the SOS ribosomal subunit from H. maris-mortui. Six other proteins (L19, L22, L23, L24, L29, and L31e) are known to be located in the exit area of the polypeptide tunnel [26, 27]. If these proteins are involved in the inhibitory actions of macrolide antibiotics, mutant bacteria resistant to macrolides will develop in the future. 

The selected organism may further be improved through mutation. Table 2 illustrates the type of increases reported in published work. Techniques have been described for obtaining (a) mutants resistant to catabolite repression, (b) constitutive mutants which produce enzyme in the presence of repressor, and (c) constitutive mutants which form enzymes without the addition of inducer (Demain, 1968). The aim of most work along these lines has been the increased production of relatively simple compounds, e.g. aminoacids and antibiotics. The procedures involve resistance to antimetabolites (analogs of the desired product) as a means of selecting mutants. For the production of hydrolytic enzymes, there seems to be no similar simple means for a preliminary selection of the desired mutant, unless en me production happens... 

Strains of P. oryzae resistant to blasticidin S have been easily selected on agar media. Preparations of cell-free systems from sensitive and resistant strains proved to be equally inhibited by the antibiotic and it has been shown that resistance to the antibiotic was reduced due to decreased permeability of the plasma-lemma for the antibiotic [10]. Resistant mutants obtained in laboratory experiments displayed a decreased pathogenicity [12]. No emergence of resistance in... 

The bacterial cell wall is the first line of defense against environmental stress in gram-positive bacteria. Thus, selection schemes that change the cell wall structure can potentially make cells more resistant to environmental stress. D-cycloserine is an antibiotic that inhibits alanine racemase and D-alanine D-alanine ligase, both involved in cell wall biosynthesis. Mutants resistant to this antibiotic have been isolated in a number of gram-positive species and shown to have differences in surface properties (Clark Young, 1977 Reitz, Slade, Neuhaus, 1967). We have used D-cycloserine as a selective agent to directly select mutants in a variety of strains and found that the mutants obtained have properties useful in industrial fermentation processes. 

Hie replicase of virus Q() (an RNA-phage) consists of four subunits one is encoded by the vM genome, wliile tlie other three are the host proteins EFTu, EFTs and R.p. SI. The Fig. shows a three dimensional model for the arrangement of R.p. in the SOS-subunit. Many antibiotics act by combining with R.p., e.g. streptomycin interacts with S12, and bacterial mutants resistant to streptomycin have been shown to liave an altered S12. Erythromycin interacts with L22, spiramycin with LA. 

Antibiotics may be defined as secondary metabolites of micro-organisms. In contrary to primary metabolites (proteins, carbohydrates, nucleic acids, lipids) which play an essential role in the growth and multiplication of cells secondary metabolites are of no importance in that respect. Antibiotics dispose of a relative low molecular mass and the ability to exhibit microbistatic or microbicidal efficacy in/on other microbe species by impairing the cell wall biosynthesis, the cytoplasmic membrane, the oxidative phophorylation. Because of there extremely high antimicrobial activity antibiotics are mainly used as chemotherapeuticals however, some antibiotics are also used in the food industry for the protection of food against deterioration e.g. Nisin (20.11.1.), Pimaricin (20.11.2.). But these applications will be more and more restricted or even completely banned as microbes may acquire resistance which represents a severe problem in chemotherapy with antibiotics. Acquired resistance is a consequence of the selection pressure on a microbe population in the presence of microbicides. Chemotherapy with an antibiotic the application of which has led to the selection of mutant resistant organisms is no longer successful. 

Studies of laboratory-derived (single-step) resistant mutants have proved to be of considerable biochemical utility. For example, streptomycin-resistant mutants have an altered receptor site due to a change in a protein of the small ribosomal subunit, while erythromycin-resistant mutants have a change in a protein of the large ribosomal subunit 7. Altered RNA polymerases are found in mutants resistant to rifampicin and streptol-ydigin 9. Fusidic acid resistance occurs as a result of mutations which alter a factor required in protein synthesis O. Such forms of antibiotic resistance have been useful in determining the mode of action of the antibiotic and in studies of macromolecular structure and function. By contrast, in "natural" resistance, the resistance mechanism is, in most cases, unrelated to the biochemical mode of action of the drug. The antibiotics with known plasmid-determined resistance are listed in Table 2. 

These systems use selective conditions for the detection of forward mutations. The easiest method is to look for the induction of a mutation which renders the mutants resistant to a specific compound, for example an antibiotic or antimetabolite, although other possibilities exist. 

Furthermore, if the antibiotic passes membranes through a specific port of entry, its mutational loss leads to resistance. The lack of the outer membrane protein OprD in P. aeruginosa causes resistance to the (3-lactam antibiotic imipenem. Fosfomycin passes the cytoplasmic membrane via an L-a-glycerol phosphate permease. This transport system is not essential for bacterial growth and therefore mutants with a reduced expression are frequently selected under therapy. 

Pillai SD, Pepper IL (1991) Transposon Tn5 as an identifiable marker in rhizobia survival and genetic stability of Tn5 mutant bean rhizobia under temperature stressed conditions in desert soils. Microbial Ecol 21 21-33 Pote J, Ceccherini MT, Van VT, Rosselli W, Wildi W, Simonet P, Vogel TM (2003) Fate and transport of antibiotic resistance genes in saturated soil columns. Eur J Soil Biol 39 65-71... 

Since rifamycins are important drugs for the treatment of M. tuberculosis infection [36, 86, 87] the activity of rifaximin on and interference with this bacterium have been carefully studied. Indeed, a potential problem of the treatment with this antibiotic is represented by the possibility that even very low blood levels achieved by oral administration might be able to select mutants, cross-resistant to rifamycins [85], in patients treated for GI infections and harboring M. tuberculosis. 

The strategy for development of /3-lactamase-resistant /3-lactams has some limitations. Indeed, it has often been found that the more-resistant compounds are less-efficient antibiotics. Furthermore, the natural weapons wielded by bacteria mutation, gene transfer, and natural selection, combine to counter /3-lactamase resistance. Thus, /3-lactamase mutants have emerged that efficiently hydrolyze compounds that were previously considered /3-lactamase-resistant [37-41], The overproduction of enzymes - either PBPs or the original /3-lactamases - as well as a decrease in the permeability of the bacterial membrane to antibiotics - are other defense strategies of the bacteria [42] [43],... 

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