Development Of Resistant Mutants Research Articles

Metalaxyl resistance in Phytophthora infestans: An overview

Late blight is one of the major destructive diseases of two most important crops –potato and tomato. The disease is known to cause immense loss worldwide. The causal agent Phytophthora infestans, due to various reasons, developed resistance against Metalaxyl fungicide which was very effective against the pathogen, in its initial years of introduction. A number of possibilities have been estimated for the occurrence and development of resistance in the fungal pathogen isolates: existence of resistant isolates before the introduction of the fungicide, development of resistant mutants, presence of both the mating types A1 and A2 of pathogen in a locality leading to the possibilities of recombinants, migration of resistant isolates, excessive use of site specific fungicide etc. Since development of resistance in pathogen population against fungicide is an issue of serious concern and can pose a great threat to the potato and tomato production thus various anti resistance strategies need to be practiced.

Mutant prevention concentrations of ciprofloxacin against urinary isolates of Escherichia coli and Klebsiella pneumoniae

The aim of this work was to study the ability of ciprofloxacin to restrict the development of resistant mutants of Escherichia coli and Klebsiella pneumoniae through determination of the Mutant Prevention Concentration (MPC). We studied 140 strains of E. coli and 86 strains of K. pneumoniae with different profiles of sensitivity to fluoroquinolones and extended spectrum beta lactamase (ESBL) production. The MPCs were determined using an inoculum of 1010 CFU/ml in Mueller-Hinton agar plates with serial concentrations of ciprofloxacin. Ciprofloxacin-susceptible ESBL-producing strains showed a higher MPC for ciprofloxacin (P <0.001) than ciprofloxacin-susceptible non ESBL-producing strains, while ciprofloxacin-resistant ESBL-producing and non ESBL-producing strains did not significantly differ. The presence of qnr variants was associated with elevated MPCs. This was observed for both tested organisms. Our study helps to explain the frequent finding of resistance to fluoroquinolones in ESBL-producing strains. Consequently, the use of concentrations of ciprofloxacin higher than the MIC in order to prevent the recovery and growth of resistant mutants is recommended.

Editorial Commentary: Rifampicin Plus Colistin in the Era of Extensively Drug-Resistant Acinetobacter baumannii Infections

employthistherapyremainsunknownand resistance may develop on therapy [4, 5]. Approaching the dilemma from an alternate angle, experimental models of infection reveal that rifampicin monotherapy has activity versus XDR Acb, although high-level resistance develops early during monotherapy, presumably through overexpression of efflux pumps or rpoB gene mutation. The combination of colistin plus rifampicin has been shown to prevent the development of resistant mutants and to be synergistic versus a number of Acb isolates, with available in vitro and in vivo datasuggestingsafetyand efficacy.Whether enhanced bacterial killing translates into improved outcomes for critically ill patients infected with XDR Acb nonsusceptible to carbapenems is the focus of the trial by Durante-Mangoni et al in this issue of Clinical Infectious Diseases [6].

Can long term therapy with oral nucleos(t)ide analogs influence the prognosis of chronic hepatitis B‐related complications?

1 the majority of whom acquire infection at birth or in early childhood. 2 It is estimated that 50% of male and 14% of female carriers will die from CHB-related com- plications such as hepatocellular carcinoma (HCC) or cirrhosis. 3 Since the global introduction of routine vaccination against hepa- titis B virus (HBV) in newborns in countries where the incidence of HBV-related complications are high, such as in theAsia-Pacific region, there has been a significant and gratifying decline in rates of CHB and HCC. 3,4 In the last decade, we have developed a better appreciation of the natural history of CHB—it appears that the progression to cirrhosis and HCC in these patients is associated with persistently high serum HBV DNA levels over time. The other major risk predictors of progression to HCC and cirrhosis include age, male gender, hepatitis B e antigen (HBeAg) status, serum alanine ami- notransferase (ALT) levels and HBV genotype. 5 Two recent land- mark clinical trials in Asia (Taiwan) clearly demonstrate the dramatic improvement in survival, reduced risk of HCC and advanced fibrosis/cirrhosis with the use of antiviral therapy. 5,6 Where treatment options are concerned, we have come a long way in the last decade—complete suppression, or elimination of HBV is now eminently feasible and well-tolerated, and should be the key management goal in the treatment of CHB. The major strides in therapy lie in the introduction of newer oral nucleos(t)ide agents with higher genetic barrier to drug resistance. 7

RNA Interference: A Promising Approach for the Treatment of Viral Hepatitis

Hepatitis viruses are the leading cause of liver cirrhosis and hepatocellular carcinoma (HCC) worldwide. The hepatitis C virus (HCV), together with the hepatitis B virus (HBV), accounts for 75% of all cases of liver disease around the world, with chronic hepatitis, cirrhosis, and HCC causing 500,000 to 1.2 million deaths per year [1][2][3]. More than 170 million people worldwide have chronic HCV infection. According to the 2002 WHO report, chronic liver diseases were responsible for 1.4 million deaths, including 796,000 deaths due to cirrhosis and 616,000 deaths due to primary liver cancer. At least 20% of these deaths (more than 280,000 deaths) are probably attributable to HCV infection [4]. With the WHO officially recognizing hepatitis as a global health issue, more research needs to be conducted to develop new therapeutic interventions. Current antiviral therapies for chronic viral hepatitis are effective only in approximately half of the patients [5]. The most widely available agents for the treatment of chronic hepatitis are interferon-α (IFN-α) and nucleoside analogs such as lamivudine or adefovir [6][7]. However, treatment with these agents has some disadvantages, including possible serious adverse effects in the case of interferon treatment or recurrence of viremia after discontinuation of therapy and development of resistant mutants after prolonged lamivudine treatment [8][9][10][11]. Moreover, nucleoside analogs such as 3TC-lamivudine only interfere with viral replication and do not induce cessation of the process. The low efficacy of these agents, their adverse effects, and development of resistant viral mutations are major impediments to the clinical application of these agents for the treatment of viral hepatitis [6]. These shortcomings necessitate the development of alternative treatment strategies. Sequence-specific gene silencing using RNA interference (RNAi) is a Nobel prize-winning technology that represents a promising new approach to overcome viral infections [5][12][13][14]. RNA interference is an evolutionary mechanism for protecting the genome against invasion by mobile genetic elements such as transposons and viruses [6]. It is a process by which small interfering RNA (siRNA) with specific target sequences induce silencing of homologous genes by binding to their complementary mRNA and inducing the elimination of the mRNA molecule [15]. This process occurs post-transcriptionally in the cytoplasm and is mediated by small RNA molecules (21 to 25 nucleotides in length) that bind to their complementary mRNA targets and silence the expression of these targets [6]. The phenomenon of RNA interference (RNAi) was first described by Fire et al. They observed that in the nematode Caenorhabditis elegans, the presence of double-stranded RNA (dsRNA) resulted in sequence-specific gene silencing at the post-transcriptional level. The RNAi pathway has since been recognized as a conserved biological pathway, and several experimental models have contributed to the understanding of this process [15][16].

Antiviral interactions of an HCV polymerase inhibitor with an HCV protease inhibitor or interferon in vitro

The combinations of Abbott Hepatitis C virus (HCV) polymerase A-782759 with either Boehringer Ingelheim HCV NS3 protease inhibitor BILN-2061 or interferon (IFN) displayed additive to synergistic relationships over a range of concentrations of two-drug combination. Treatment of HCV replicon with A-782759, IFN or BILN-2061 for about 16 days resulted in dramatic reductions in HCV RNA (5.1, 3.0 and 3.9log10 RNA copies, respectively). However, none of the compounds tested alone lead to replicon RNA reduction to undetectable levels. Ongoing replication in the presence of A-782759 or BILN-2061 was associated with the appearance of resistant mutations M414T in NS5B and D168V in NS3, respectively. In contrast, a combination of A-782759 with BILN-2061 resulted in greater than 7logs RNA reduction leading to undetectable replicon RNA after 16 days of treatment. Our findings suggest that a monotherapy with either drug alone is likely to result in development of resistant mutants. However, a combination therapy with polymerase inhibitor has the potential to improve the efficacy of IFN or a protease inhibitor alone in vivo, due to the lower likelihood of resistance development.

Molecular Virology and the Development of Resistant Mutants: Implications for Therapy

Two of the key events in the viral life cycle of the hepatitis B virus (HBV) involve (1) generation from genomic DNA of the covalently closed circular DNA transcriptional template and (2) reverse transcription of the viral pregenomic RNA to form the HBV DNA genome. Because the virus employs reverse transcription to copy its genome, mutant viral genomes are found frequently. Particular selection pressures, both endogenous (host immune clearance) and exogenous (vaccines and antivirals), readily select out these escape mutants. The introduction of nucleoside/nucleotide analogue therapy has seen the emergence of drug resistance as the major factor limiting drug efficacy. The development of drug resistance is not unexpected if viral replication continues in the setting of ongoing treatment, especially monotherapy. Thus, prevention of resistance requires the adoption of strategies that effectively control virus replication.

In vitro pharmacodynamic modelling simulating free serum concentrations of fluoroquinolones against multidrug-resistant Streptococcus pneumoniae.

An in vitro pharmacodynamic model was used to compare bacterial killing and the development of resistant mutants in the presence of respiratory fluoroquinolones and ciprofloxacin. Free (protein unbound) serum concentrations were simulated using peak serum concentrations and AUC(24), achieved after standard oral doses for treatment of community-acquired respiratory infections. Respiratory fluoroquinolones reduced the inoculum of isolates of multidrug-resistant Streptococcus pneumoniae below the level of detection in the model at 12 h and did not allow regrowth to occur over 48 h. In contrast, ciprofloxacin had a bacteriostatic (<3 log(10) reduction of the initial inoculum) effect, with rapid regrowth occurring over 48 h. Bacterial regrowth and the development of resistant mutants did not occur with any of the respiratory fluoroquinolones. Rapid regrowth and the development of resistant mutants, with MICs two- to eight-fold higher than the MIC before treatment, occurred with ciprofloxacin. These data suggest that the new respiratory fluoroquinolones with a free AUC(24)/MIC of 35-63 reduce the inoculum of multidrug-resistant S. pneumoniae below the level of detection without regrowth or development of resistance over 48 h. This study also demonstrated the poor activity of ciprofloxacin against S. pneumoniae.

Anti-HIV Type 1 Activity of 3′-Fluoro-3′-Deoxythymidine for Several Different Multidrug-Resistant Mutants

The objective of this work was to test the antiviral activity of a potent nucleoside reverse transcriptase inhibitor, 3'-fluoro-3'-deoxythymidine (FLT), on both a wild-type human immunodeficiency virus (HIV-1) isolate and multidrug-resistant HIV-1 patient isolates. Drug-resistant viral isolates were selected on the basis of four different categories of well-characterized and representative multidrug-resistant mutants. The isolates included three variants containing 151M alone or in combination; three variants containing 215Y and 41L, 67N, 184V, 210W, and 219N in combination; two insertion mutant viruses (69 + EA and 69 + SA); and two deletion mutant viruses (del67NG and del67GS), the latter two groups both also containing other significant mutations. The activity of FLT and AZT against these isolates was determined by drug susceptibility assays and by measuring viral antigen p24 by ELISA. The cytotoxicity of FLT and AZT was assessed in PHA-stimulated PBMCs. Development of resistant mutants under FLT pressure was attempted by passaging HIV-1 isolates in SupT1 cells and stepwise increasing the concentration of FLT. The multidrug-resistant mutant HIV-1 isolates exhibited 7-fold to >100-fold increased resistance to AZT, but showed IC(50) values for FLT of 0.0014-0.0168 microM, which were lower than or similar to that of wild type (0.0075 microM). The cellular cytotoxicities of FLT and AZT fell into a similar range in PBMCs. The development of HIV mutants resistant to FLT appeared to be slower than for other RT inhibitors. HIV isolates with mutations resulting in multidrug resistance had no evidence of resistance to FLT. FLT may be useful in salvage therapies for patients harboring resistant strains and a reassessment of its therapeutic potential seems required.

Synergism of Polyene Antibiotics with 5-Fluorocytosine

The synergism of 5-FC with 4 polyene antibiotics, amphotericin B (amph. B), candicidin, trichomycin and nystatin were investigated in 3 in vitro models measuring the fungistatic and fungicidal activity as well as the development of resistant mutants. In these 3 models candicidin and trichomycin exerted a higher synergistic effect than amph. B or nystatin. In vivo (systemic treatment of septicemic candidiasis of the mouse) the combination of 5-FC with amph. B was, however, the most effective. Only with this combination a complete cure (culturally negative) was observed. For topical treatment of Candida vaginitis in rats, the combination of 5-FC with candicidin proved the most active. As far as the biochemical basis of synergism is concerned in Candida albicans in the presence of polyene antibiotics, the incorporation of fluorinated pyrimidines was increased and the reduction of uptake of histidine by 5-FC alone, significantly enhanced. These effects could not be observed in Cryptococcus neoformans. The release of amino acids, phosphate and potassium caused by polyenes was significantly more pronounced in cells pretreated with 5-FC. Thus, the interaction between 5-FC and the polyenes may, in fact be, mutual.